Power

Module 07 ended at a tap: a circuit with headroom, read at the panel, carrying a small load. That answer holds only where a service exists and only as far as its headroom reaches, and the load it sized was a Pi drawing under a watt. The sites this series builds toward carry more. Volume I states the standing requirement for a remote comms site: via solar power and auxiliary batteries, the system will remain on the air in the event of failure of the electrical grid.1 Volume III states the same limit from the drone side: power supply is one of the most significant limitations of sustained operation, and the regeneration methods it names are larger batteries, like a car battery or LiFePO4, generators, and the outlet.2

The outlet is Module 07's tap, and it needs a live service you may not have. A generator makes power on demand, and it also makes noise, heat, and exhaust that carry to anyone nearby. A battery bank is silent. It gives off no light and no exhaust, and it fits inside a case or under a bench. The standard this module grades against is Capable and Quiet. Capable means banked to the real load, with the runtime computed and proven. Quiet means no generator hum, no heat or light tell, and a build that can be concealed.

The working frame for the module is a notional field trauma site: a place that must run communications, light, and small medical equipment for days with no service and no noise. The same frame scales down to a radio site or a sensor relay; the arithmetic does not change, only the size of the numbers.

One measure carries the whole module: the watt-hour (Wh), the amount of energy a battery holds or a load uses. Module 02 gave you watts, volts times amps; Module 03 gave you capacity in amp-hours. A battery's energy is its voltage times its amp-hours, so a 12-volt battery rated at 100 amp-hours holds 12 × 100 = 1,200 Wh, and a load's use is its watts times its hours, so a 10-watt light burning 5 hours uses 10 × 5 = 50 Wh.3 Watt-hours compare any battery against any load, whatever the voltage or chemistry, and every sizing decision below is that one comparison.

Every sizing decision downstream hangs off one number: what the equipment actually draws. You do not take that number from a label, a manual, or a forum table. A label states a maximum the power supply was designed for, not the running draw, and the running draw of a medical device, a radio, or a terminal moves with what the device is doing. The number comes from your own meter, on your own equipment, while it runs.

Set the meter to DC amps, put it inline on the load's positive lead, and read the current while the device does its real job. Watts is that current times the system voltage: V × I = W. A device that reads 0.7 A on a 12-volt bank draws 12 × 0.7 = 8.4 W. A radio needs two readings, because it draws a fraction of an amp receiving and several amps transmitting; weight the two by how much of the hour it actually transmits. A device with a surge at start, a compressor or a suction unit, gets read at start and at steady state, and the bank must carry the surge.

1. Read each load's running current with the meter inline, and multiply by the voltage for watts.3
2. Turn watts into a daily budget. Multiply each load's watts by the hours a day it runs, then add the loads up.3
3. Add a quarter for margin, to cover conversion losses and the loads you add later: budget times 1.25. That number is what the bank must carry per day.

One clinic-scale device publishes its input spec, and it anchors the tier this module builds at. The Starlink Standard dish's own DC power supply takes 12 to 48 volts DC in at up to 30 amps, feeds the dish 56 volts DC, and is weather-rated to IP66.4 It runs off a 12-volt bank directly, with no inverter anywhere in the path. Its running draw is not published. The input voltage shows it fits the bank; the meter gives its draw on the bank.

The bench below runs the sizing arithmetic on your own numbers: the current you measured, the daily run time, and the battery you found. Run your measured loads through it before Unit 03 and keep the daily watt-hour figure; the rest of the module sizes against it.

The primary battery of this module is the 12-volt lead-acid block, and the reason is sourcing: nothing in this module depends on a find you cannot make, so every battery taught here exists in volume in the discard stream, not in a specialty channel. The three finds carry three roles, ranked by how easily you reach them:

• The UPS battery, the learning find. Offices discard UPS units continuously, and a dead UPS is usually a good box with a worn battery, because the sealed battery inside has a service life of only three to five years.5 Module 04 taught the teardown that recovers it. The pull is small and often worn, which is exactly why the test procedure below exists; a good one runs the bench, and the box supplies a charger and an enclosure either way.
• The car battery: the easiest big find, drawn gently. A self-service junkyard sells one for the fee at the gate, in cash,6 and it is several times the UPS pull’s size. It is a starter build, thin plates for cranking, so it carries a site only if you keep the discharges shallow; the cycle-life figures below say what deep draws cost it.
• The deep-cycle block: the right battery, worth the search. A discarded mobility scooter carries sealed 12-volt AGM blocks built to be recharged hundreds of times,7 and a commercial floor scrubber carries deep-cycle lead-acid blocks in the same voltages.8 Scooters and scrubbers reach the stream less often than UPS units, but they are trade equipment, not a niche product, and this is the right block for the bank.

1. Two 12 V blocks. This UPS cartridge is two sealed blocks strapped as one unit; each comes out and tests separately.
2. Charge figures. The label prints the maker's own profile: cycle use 14.5 to 14.9 volts, standby 13.6 to 13.8. Unit 06 runs on these numbers.
3. The chemistry. Sealed, non-spillable lead-acid: the VRLA family, safe indoors in any orientation.
4. UPS connector. The cartridge plugs into the UPS here; cut nothing, it unplugs.

The first find: a UPS replacement battery cartridge, two sealed 12 V blocks and their connector. Photo: Rjluna2, Wikimedia Commons, CC BY-SA 4.0.

Learn on the free UPS pull, use the junkyard block when it is what you have, and build the real bank on deep-cycle or the LiFePO4 of Unit 04.

A lead-acid cell holds 2.0 volts nominal, so the 12-volt block is six cells in series, and at rest a healthy block reads about 12.3 to 12.75 volts open-circuit, 2.05 to 2.125 volts per cell depending on design.9 The sealed form most common in the discard stream is valve-regulated lead-acid, VRLA, and its common construction is the absorbent glass mat, AGM: a porous mat of microglass fibers holds the electrolyte instead of loose liquid, which lets the battery operate in different orientations without spilling.9 The cost of the chemistry is weight. Lead-acid stores about 30 to 40 Wh/kg,9 the summary tables putting a starter battery at 30 to 50 and a deep-cycle at 20 to 30 with some designs higher,10 which is a fraction of what lithium stores at the same weight. For a bank that sits in place, weight matters least.

1. Terminal post. Rises from the plate group; everything lands here.
2. Cell divider. Each compartment is one 2-volt cell; six in series make the 12-volt block.
3. Separator mat. The absorbent mat between plates holds the electrolyte, which is what makes the sealed block spill-proof.
4. Plate stack. The lead plates themselves; thick plates mark a deep-cycle build, thin a starter.

A sealed lead-acid battery cut open: the construction described above. Photo: Bullenwächter, Wikimedia Commons, CC BY-SA 3.0.

The 12 V block: two terminals, a label that names the chemistry and the amp-hours, and real weight. Read the label before you lift it, and plan on using half of what it claims.

Know which of the two builds you are holding, because they are made for opposite jobs. A starter battery, the car kind, is built with thin plates for maximum cranking current, and the battery handbook states the other job's limit: SLI batteries are not designed for deep-cycling service, and very short lives are generally obtained with such operation.9 A deep-cycle battery, the scooter, scrubber, or boat kind, is built with thick plates for repeated discharge.9 A free starter battery runs a bench if you draw it gently; when you have the choice, take the deep-cycle block.

Cycle life is a function of depth of discharge, and it is stated per depth, never as one number. Battery University's figures for a deep-cycle block: 150 to 200 cycles at full discharge, 400 to 500 cycles at 50 percent, and 1,000 or more cycles at 30 percent.11 Victron's AGM datasheet runs the same curve on its own product: 400 cycles at 80 percent discharge, 600 at 50, 1,500 at 30.12 Read the 80 percent figure for what it is: the deep extreme of a cycle-life curve, a trade of battery life for capacity per cycle, not a safety ceiling and not a free allowance. Trojan, a deep-cycle maker, states the working doctrine: 80 percent discharge is the maximum, 50 percent or less is recommended, and shallow discharges give a longer battery life.13 So you plan the bank at 50 percent: the 100 amp-hour block that holds 1,200 Wh on the label is, for planning, 1,200 × 0.5 = 600 Wh you plan to use. Draw deeper only when the mission is worth the cycles it costs.

One AGM battery, three depths of discharge: 400 cycles at 80 percent, 600 at 50, 1,500 at 30. Figures from the Victron datasheet. Shallower cycles multiply the battery's life.

Self-discharge depends on construction, so state the construction before the number. The handbook names the mechanism: open-circuit stand loss is more severe with batteries which use antimonial lead grid alloys in the positive plates, and batteries built on nonantimonial calcium-lead grids minimize the loss.9 A modern sealed block is the second kind: Victron rates its VRLA batteries, built on lead-calcium grids, at less than 2 percent per month at 20°C, doubling for every 10°C rise.12 A find that drains in weeks has antimonial grids or damage; the rate is not a property of lead-acid in general. Two handbook rules apply at the curb: never store a lead-acid battery discharged, because standing discharged promotes sulfation, which permanently costs capacity and life; and do not condemn a find too fast, because batteries discarded as dead can often be restored with a long, slow recharge, three to four days at 2 to 3 amps for a starter battery.9

Test the find before you carry it, with the meter and a clock, in this order:

1. Inspect the case. A crack, a bulged side, or a leaking post rejects the find on sight; everything after this step assumes an intact battery.
2. Read the resting voltage. A healthy 12-volt block at rest reads about 12.3 to 12.75 volts.9 A reading far below that is a discharged battery, not yet a dead one.
3. Recharge a low find slowly, ventilated. Batteries discarded as dead can often be restored with a long, slow recharge, three to four days at 2 to 3 amps for a starter battery.9 Unit 08 carries the hydrogen rule this step runs under.
4. Rest it, then read again. A resting reading is trustworthy only after the battery has sat unused: for the sealed family, within 20 percent after a day's rest and within 5 percent after five days.9 A block that will not hold its resting voltage after a full slow charge is scrap.
5. Prove the capacity with a timed load. Put a measured load on the block, log the voltage every hour, stop at your planned depth of discharge, and multiply the load's watts by the hours it ran.3 That figure, not the label, is the battery's capacity in every later plan.

Below this tier sits the 18650 cell, Module 03's territory: right for a sensor or a node, the wrong scale for hundreds of watts. Above it sits the electric-vehicle pack, and the exclusion is hazard, not capacity. Electric, hybrid, and plug-in hybrid vehicles carry high-voltage battery systems of 200 to 800 volts,14 stored energy remains after the pack is disconnected, and the safe procedures are the manufacturer's own, verified with a rated voltmeter.14 A 12-volt block cannot electrocute you across dry skin; a traction pack can. The EV pack is work for a trained high-voltage technician, and this series does not train that. Leave it in the wreck.

The second source is lithium iron phosphate, LiFePO4, and it is not Module 03's lithium. Module 03's lithium is the cobalt family, LiPo pouches and 18650 cells: 3.6 volts nominal, 4.2 volts full, energy-dense and volatile. LiFePO4 is a different lithium chemistry. Its cell runs 3.2 volts nominal and 3.65 volts full, four cells in series make the 12-volt-class pack, and its discharge curve is flat, holding near its nominal voltage across most of the discharge. It tolerates abuse the cobalt chemistries do not, and it is the lithium built for exactly this module's job: a maker's figure runs over two thousand charge cycles where an ordinary lithium cell is rated for roughly three hundred to a thousand.15 Do not carry Module 03's charging numbers over; this chemistry has its own.

It turns up in e-bike packs, solar storage packs, portable power stations, and some cordless-tool lines, in the same discard stream Module 04 taught you to work. The power station is the clearest current case: EcoFlow’s current Delta-class units state the chemistry on the spec sheet, LFP (LiFePO4 battery), with a published cycle life of 4,000 cycles to at least 80 percent capacity,27 so a discarded 2020s power station is a LiFePO4 pack in a case with its charger already around it. The market also sells the drop-in replacement, a 12-volt lead-acid-shaped case with LiFePO4 cells and a BMS inside, marketed as a straightforward upgrade from traditional lead-acid batteries and charging at 14.2 to 14.6 volts with a 13.4 to 13.8 volt float; Battle Born’s 100 amp-hour block lists at $799 in 2026.28 A new pack at that price is the argument for the salvage bench: the same chemistry comes out of a discarded power station or e-bike pack for the work of testing it. A pack that still works is worth more whole than opened. Its cells are matched and its battery management system, the BMS, already carries the protection, so you keep it intact, use its own connector, and never draw more current than its rating allows.16 A multi-cell lithium bank never runs without a BMS: the board balances the cells on charge, cuts the discharge at the first cell to reach its low-voltage limit, and trips on a short or an overload.15 And do not condemn a dead pack too fast: the BMS cuts the pack off at its own limits, so a pack that reads zero at its connector can be a tripped protection board with good cells behind it, and only testing tells you which.15

1. Carry handle. The pack detaches and carries; this is the form found in the discard stream.
2. Model label. The rating lives here: voltage, watt-hours or amp-hours, and the series count behind them.
3. Mount, contacts. The pack's own connector is at the base; testing and loading run through it, never around it.

A detachable e-bike pack, the whole-pack find of this unit: matched cells and a BMS inside one case. Photo: Stefan Bellini, Wikimedia Commons, CC0.

Test a salvaged pack the way Module 04 tests any pull, at the pack's own terminals first. Read the resting voltage and divide by the series count: a 4S LiFePO4 pack reading 13.2 volts is sitting at 3.3 volts per cell, in the healthy band; a pack reading near zero is a BMS trip or a dead pack, and only opening it tells you which. Then prove capacity the only way capacity is proven: a measured load, a clock, and the watt-hour arithmetic from Unit 01. The triage discipline below is the same one Module 03's cell tier uses, and the bench runs it as a drill: report what you see and measure, and take the verdict.

1. Read the pack at its own connector. Multimeter on DC volts, probes on the output terminals, nothing else connected.
2. Divide by the series count. A 4S LiFePO4 pack reading 13.2 volts is sitting at 3.3 volts per cell, inside the healthy band between 3.2 nominal and 3.65 full. A per-cell figure near the bottom of the band is a discharged pack; charge it on the right profile before judging it.
3. Treat a zero reading as a question, not a verdict. The BMS opens the circuit at its own limits, so zero at the connector can be a tripped board over good cells; only opening the pack settles it, and the pack stays whole until the reading forces the question.15
4. Prove the capacity through the pack's own connector. A measured load within the pack's rating, a clock, an hourly voltage log: watts times hours is the tested capacity, the same arithmetic as the lead-acid bench.3 16

The charging difference is the one that damages equipment. A LiFePO4 pack replacing lead-acid charges at about 14.4 volts absorption and 13.6 volts float for a 4S battery, and the controller charging it must be set to its lithium profile, not lead-acid, with equalization disabled and temperature compensation off; when in doubt, ask the battery's manufacturer for the recommended charge profile.17 Unit 06 carries this rule in full. Until then, a salvaged pack never hangs off a controller still set to another chemistry's profile.

Module 03 states the physics: series raises voltage, parallel raises capacity, and parallel requires matched voltages before joining. This unit applies it to whole 12-volt batteries, with five rules.

1. Pick the system voltage and hold it. Everything in this module runs at 12-volt class, because the loads take it directly: the light, the radio, the Starlink DC supply.4 Two 12-volt blocks in series make 24; nothing here needs it.
2. Parallel for capacity, matched first. Two 100 amp-hour blocks in parallel are a 200 amp-hour bank at 12 volts. Before the jumper lands, both blocks read within a tenth of a volt at rest, or the fuller block dumps current into the emptier one through nothing but the cables.
3. Never mix chemistry or age on one bank. One bank, one chemistry, one charge profile: a lead-acid block and a LiFePO4 pack cannot share a bus, because no controller setting is right for both.17 And a worn block dragged into a young bank pulls the bank down to itself; batteries of visibly different age or tested capacity run as separate banks.
4. Fuse every branch at the battery positive. The rule set is Module 07's, reused as written: conductors are protected against overcurrent in accordance with their ampacities,18 so the fuse is rated to the wire and blows before the wire overheats. The fuse maker's margin: a fuse should carry no more than about three-quarters of its rating in normal use, so a 0.7-amp branch gets 0.7 ÷ 0.75 ≈ 0.9, the next standard size up, 1 amp.19
5. Size the wire to the current, and land every negative on one bus. A couple of amps runs on 18-gauge; ten amps needs 14-gauge.20 One common ground gives the meter, the loads, and the controller a single reference.

The small loads ride the same bank. A sensor, an alarm panel, or a controller board at 5 or 9 volts does not get its own battery: it feeds from the 12-volt bus through the buck converter Module 03 taught, on its own fused branch like any other load. Only a device that must run detached from the site, a standalone sensor placed away from the bank, belongs on Module 03's cell tier instead; everything that lives at the site draws from the bank, so one charge system maintains everything.

An inline blade-fuse holder: one on each branch at the battery positive, the fuse rated below what the wire can carry.

None of this needs solder. Battery posts take ring terminals or the clamps the find came with, an inline blade-fuse holder splices into the positive wire with the crimp joints Module 03 taught, and a controller's terminals are screw clamps. Strip, land, tighten, tug-test.

Then prove the bank before it carries anything that matters. Put the measured load from Unit 02 on it, log the voltage every hour, and run it to your planned depth of discharge. The computed runtime is usable watt-hours divided by the load's watts; the log either confirms it or yields the bank's true capacity, which replaces the label in every later plan.

A bank is charged to a charge profile, a voltage pattern belonging to one chemistry, and the profile mismatch is the hazard that wrecks salvaged equipment. Lead-acid VRLA charges on a multi-step curve, a constant-current bulk phase followed by constant-voltage absorption and float phases.12 LiFePO4 replacing it charges at about 14.4 volts absorption and 13.6 volts float for a 4S pack, on the controller's lithium profile, with equalization disabled and temperature compensation off; when in doubt, ask the battery's manufacturer for the recommended charge profile.17 Equalization is a deliberate lead-acid overcharge; left enabled against a lithium pack it is an overvoltage event on schedule. Set the profile before anything is connected, every time.

Where mains power is usually up, the simplest habit is a mains charger with the right profile topping the bank whenever mains power is live, the bank carrying the site when it is not. That is the architecture of every UPS, run at bench scale with your own bank. Off-grid, the source is a panel through a charge controller, and the controller comes in two kinds. A PWM controller is in essence a switch connecting the panel to the battery, pulling the panel down to near the battery's voltage whether or not that is where the panel produces its best power; an MPPT controller adjusts its own input voltage to draw the panel's maximum power, then converts it to what the battery needs.21 Victron's worked example puts numbers on the gap: at 25°C the same 100-watt panel delivers 81 watts through PWM and the full 100 through MPPT, and MPPT is generally accepted to outperform PWM in cold to temperate climates, the two running about even in subtropical to tropical ones.21 A salvaged panel's voltage was never chosen to match your bank; an MPPT controller accepts that mismatch and still delivers the panel's full power.

1. Charging LED. Lights while the panel charges the battery: the first thing checked in sun.
2. Full LED. Lights when the controller holds the battery at float.
3. Battery pair. The bank lands here, positive through the main fuse.
4. Panel pair. The panel's two leads, polarity checked before they land.
5. Screw terminals. Stripped wire under a screw: no solder anywhere in the build.

A small PWM controller. This one carries battery and panel pairs only, so the load lands on the battery bus through its own fuse, the case Unit 06's setup step 4 covers. Photo: MAmberg, Wikimedia Commons, CC BY-SA 4.0.

The same panel, two controllers: 81 watts through PWM, 100 through MPPT at 25°C. Figures from the Victron technical note.

Size the panel from the daily budget, and look the solar figure up instead of asserting it. NREL's PVWatts calculator returns your own location's daily solar resource, its peak sun hours;22 divide the daily watt-hour budget by that figure, then by 0.7 for system losses, and the result is the panel wattage that replaces one day's use.3 Peak sun hours belong to a location; the look-up takes a minute. Size the controller from the panel's short-circuit current, Isc, printed on its label: the controller's current rating stands above the panel's Isc with margin, the working notes' rule being Isc times 1.25.23 A smaller salvaged panel than the arithmetic asks for still works; it refills less than you use, the bank drains over days instead of holding level, and your log shows exactly that.

Run the setup in this order, because the profile is a safety setting and the meter is the proof:

1. Set the chemistry profile first, with nothing connected, and check it against the battery maker's figures: for a 4S LiFePO4 pack, about 14.4 volts absorption and 13.6 float, equalization disabled.17 For the sealed lead-acid family, the maker's own absorption and float figures.12
2. Land the bank on BATT, the positive through the main fuse from Unit 05, the negative to the common bus.
3. Land the panel on PV, both leads, polarity checked with the meter before they land.
4. Land the load on LOAD, or on the battery bus through its own fuse if the controller has no load terminals.
5. Prove it is charging with the meter. In sun, the voltage across the bank climbs to the profile's absorption figure and holds, then falls back to float; a bank that never reaches absorption is a panel, wiring, or profile problem, found in that order.12

The whole charge path: panel on PV, bank through the main fuse on BATT, the load on LOAD. The controller carries the night-time reverse protection. Drawn in KiCad.

The state of charge of a resting lead-acid battery shows in its open-circuit voltage. Battery University's reference for a 12-volt starter battery: 12.65 volts at 100 percent, 12.45 at 75, 12.24 at 50, 12.06 at 25, and 11.89 at zero, read at 26°C after a 24-hour rest.29 The rest matters more than the tenth of a volt: to get accurate readings, the battery needs to rest in the open circuit state for at least four hours, and battery manufacturers recommend 24 for lead acid.29 A bank under load or fresh off the charger reads high or low of its true state, so the routine reading is taken before the day's charging starts. Trojan's own full-charge figure for its 12-volt deep-cycle blocks runs slightly higher, 12.73 volts,13 so learn your bank's own full-and-rested number during the Unit 05 proof and judge later readings against it.

• 100 percent12.65 V at rest, 26°C
• 75 percent12.45 V
• 50 percent12.24 V, the lead-acid planning floor
• 25 percent12.06 V, past the plan; recharge now
• 0 percent11.89 V, standing here costs capacity for good

Trojan tells its flooded-battery customers to check once a month until the watering pattern is known;13 this module takes the same monthly rhythm for the bank’s log, and the check is short: the resting voltage against the table above, the terminals for corrosion and tightness, the case for cracks or bulging, and the log updated with the date and the reading. A bank in storage gets the same reading on the same cadence, and Trojan's storage rule sets the trigger: a stored battery gets a boost charge when it shows 70 percent charge or less.13 A LiFePO4 pack's flat discharge curve makes voltage a poor state-of-charge gauge across the middle of its range, which is one more job its BMS already does; read the pack's own indicator where it has one, and treat the meter reading as a full-or-empty check.

Work the fault list from the symptom, with the meter and the log:

1. The bank never reaches absorption voltage. A panel, wiring, or profile problem, checked in that order: panel voltage at the controller's PV terminals, then every joint on the charge path, then the profile setting against the battery maker's figures.12 17
2. The bank charges but drains overnight with the loads off. Read the branch fuses one at a time: a branch that still draws with its device off is the leak. A bank that drains with every branch pulled is a battery problem: one worn block dragging a parallel bank down, found by resting-voltage readings on each block separately.
3. A fuse blows more than once. The fuse is doing its work: the branch draws more than its wire was sized for, from a fault in the device or a short in the run. Find the draw with the meter before any larger fuse goes in; the fuse rating protects the wire, never the convenience.19
4. A block runs warm on charge, or the sealed case bulges. Overcharge: the profile or its voltage setting is wrong for the chemistry, and a VRLA block vents only on overcharging or cell failure.12 Disconnect the charge source, re-check the profile, and re-read the block rested before it returns to service.

1. Fused lead. The accessory socket rides its own fused positive lead, the Unit 05 rule applied.
2. The label. It states the rating: 12 V, 100 Ah, deep-cycle VRLA, the bank tier of Unit 03.
3. Charge leads. Red to positive, black to negative, clamped at the posts.
4. The charger. A lead-acid charger on its lead-acid battery; the profile is checked before the clamps land.

A working bank on its charger, every Unit 05 and 06 rule visible at once. Photo: Stephan Ridgway, Wikimedia Commons, CC BY 2.0.

Charging makes hydrogen. When lead-acid batteries are being recharged, they generate hydrogen gas that is explosive in certain concentrations in air: the explosive limits are 4.1 to 72 percent hydrogen in air.24 The DOE handbook's rule is absolute: since hydrogen is highly explosive, it is necessary to provide adequate ventilation to the battery whenever charging is in progress, and no smoking, electric sparks, or open flames are allowed near a charging battery.25 The working practice: charge in a ventilated space, and make and break every connection away from the battery posts, so any spark happens away from any gas. Ventilation guidance holds the concentration below a quarter of the lower explosive limit, about 1 percent of the room's volume.24 A sealed AGM block vents only on overcharge or cell failure,12 which is one more reason the charge profile in Unit 06 is a safety setting, not a performance setting.

Never back-feed. The rule is Module 07's, unchanged: do not energize the premises wiring or the utility side from your own source. A generator connects only through transfer equipment, designed and installed so as to prevent the inadvertent interconnection of normal and alternate sources of supply.26 Your bank powers its own loads on its own wiring. The moment it feeds a wall receptacle, it is energizing circuits a lineworker believes are dead.

Voltage class is where this module stops. Everything it builds runs at 12-volt class, which cannot drive a dangerous current across dry skin. The EV traction pack runs 200 to 800 volts with stored energy remaining after disconnect,14 and it stays excluded. The remaining 12-volt hazard is current, not shock: a lead-acid block delivers hundreds of amps into a short, which is why Unit 05 fuses every branch, why nothing metallic crosses the posts, and why rings and watches come off before any work at the posts.

The signature half of the standard is quiet power. A generator marks a powered site with engine noise, exhaust, and a heat plume; a battery bank makes none of those. What a bank build can still leak: light from status LEDs and displays, which tape covers; heat from an inverter under load, one more reason this build stays DC; and switching noise from cheap converters and controllers, which rides the wiring as radio interference on the bands Module 06 teaches. Keep the DC runs short, keep converters few and away from antennas, and the site's electrical signature stays as low as its acoustic one. The bank conceals by Module 04's method: a case or a floor void, with ventilation preserved for anything lead-acid that charges there.24

The exercise is the module run once, start to finish; each step feeds the next:

1. Measure the load with the meter inline: watts, then the daily budget with its quarter of margin (Unit 02).
2. Salvage and inspect at least two batteries, and read every resting voltage (Units 03 and 04).
3. Test the true capacity of each battery under a timed, measured load (Units 03 and 04).
4. Build and fuse the bank: one chemistry, voltages matched, every branch fused at the battery positive (Unit 05).
5. Charge it on the right profile, set before anything connects, proven at the absorption voltage (Unit 06).
6. Prove the runtime and the signature: computed against observed, and the site checked Quiet from outside (Units 07 and 08).

Task: Salvage at least two batteries (lead-acid/AGM and/or a LiFePO4 pack), test each for condition and true capacity, build a bank at your system voltage, and prove it delivers a computed runtime under a measured load.
Condition: Given salvaged batteries, a multimeter and a clamp meter (or a known load), fuses and holders with wire sized to the current, and the notional med-site load or a stand-in load you can measure. Work only with batteries you have every right to take, and charge lead-acid only in a ventilated space.
Standard: The product is the tested bank plus a dated record: each battery's measured resting voltage, its tested capacity under a real load, the measured load in watts (V × I = W, from your own meter), and the computed-versus-observed runtime to your planned depth of discharge. The bank is fused at every battery-positive branch, its charge source is set to the bank's chemistry profile before connection, and the build is Quiet: no generator, no exposed light, no heat or interference tell. Solar charging is taught in Unit 06 and is not required to complete this exercise. The completed field exercise worksheet, dated and carrying every reading, is the record a later run is measured against.

Download the field exercise worksheet (PDF)

Task: Complete the Module 05 test.
Condition: Given ten questions covering units one through eight, without reference to the module units or notes.
Standard: Answer at least eight of ten questions correctly. For each question missed, reread the unit named in the feedback and retake the test until the standard is met.

AGM (absorbent glass mat)

A sealed lead-acid construction in which a porous microglass mat holds the electrolyte, so the battery works in any orientation without spilling.

BMS (battery management system)

The board inside a lithium pack that balances the cells on charge, cuts discharge at the low-voltage limit, and trips on a short or overload.

Charge profile

The voltage pattern a charger or controller applies, belonging to one battery chemistry; a mismatched profile damages the bank.

Cycle life

The number of charge-discharge cycles a battery delivers, stated per depth of discharge; shallower cycles multiply it.

Deep-cycle battery

A lead-acid build with thick plates made for repeated discharge, as against the thin-plate starter battery built for cranking bursts.

Depth of discharge (DoD)

The fraction of a battery's capacity taken out before recharge; the planning figure for lead-acid is 50 percent.

Equalization

A deliberate controlled overcharge in lead-acid maintenance; it must be disabled when a lithium pack is on the controller.

Isc (short-circuit current)

The most current a solar panel can produce, printed on its label; the controller's rating stands above it with margin.

LiFePO4

Lithium iron phosphate: a lithium chemistry at 3.2 volts nominal and 3.65 full per cell, flat discharge curve, long cycle life, distinct from the cobalt lithium of Module 03.

MPPT (maximum power point tracking)

A controller type that adjusts its input voltage to draw the panel's maximum power, then converts it to the battery's need.

Peak sun hours

A location's daily solar resource expressed as equivalent full-sun hours, looked up in NREL's PVWatts calculator.

PWM (pulse-width modulation) controller

A controller type that switches the panel onto the battery, pulling the panel to near battery voltage.

Self-discharge

Capacity lost on open-circuit stand; its rate depends on the grid construction, antimonial grids losing far faster than calcium-lead.

Sulfation

The permanent capacity loss a lead-acid battery takes from standing discharged.

VRLA (valve-regulated lead-acid)

The sealed lead-acid family, venting gas only on overcharge or cell failure; AGM and gel are its constructions.

Watt-hour (Wh)

The energy measure comparing any battery to any load: volts times amp-hours for the battery, watts times hours for the load.

1. “Mission Critical Radio and You,” Volume I, Light Fighter Manifesto (Light Fighter Library, LFM Publications): the remote-base requirement that “via solar power and auxiliary batteries, the system will remain on the air in the event of failure of the electrical grid.”
2. “Kamikaze FPV Drones,” Volume III, Light Fighter Manifesto (Light Fighter Library, LFM Publications): “Power Supply is one of the most significant limitations of sustained drone operation. Some regeneration methods include larger batteries, like a car battery or LiFePo4, generators, and the apparent outlet.”
3. Unbound Solar, “Battery Bank Sizing”: watt-hours per day = watts × hours, amp-hours = watt-hours ÷ voltage, usable depth of discharge of about 80 percent for lithium and 50 percent for lead-acid, and sizing a bank and its days of autonomy against the daily load. unboundsolar.com (verified 2026-07-08).
4. Starlink, Standard DC-DC Power Supply specification sheet: “Input 12-48VDC 30A,” “Output 56V 3.57A,” “IP66 Type 4,” operating −40 to 60°C; the Standard dish runs DC-native from a 12 V battery system through this supply, with no inverter. starlink.com (re-verified from the current spec sheet 2026-08-11; the 2026-07 support-article URL is dead).
5. Schneider Electric (APC), UPS battery service life (three to five years): a dead UPS is usually a failed sealed-lead-acid battery rather than failed electronics. apc.com (re-verified 2026-08-11: “Most APC batteries should last three to five years”).
6. Pull-A-Part, “FAQ”: the self-service pull-and-haul salvage-yard model; “our standard admission fee is $2,” “Pull-A-Part accepts cash and major credit cards,” and “you need to bring your own tools.” pullapart.com (re-verified 2026-08-11 at the current FAQ URL).
7. Fullriver Battery, Mobility applications (battery maker): mobility scooters and power chairs run on sealed, spill-proof deep-cycle AGM batteries, a 12 V product line built with thick lead plates that “can be recharged hundreds of times.” fullriverbattery.com (verified 2026-07-09).
8. Trojan Battery, Floor Machine Batteries (deep-cycle battery maker): a complete range of deep-cycle batteries for floor cleaning machines, from flooded lead-acid to AGM, in 6 V and 12 V sizes. trojanbattery.com (verified 2026-07-09).
9. David Linden and Thomas B. Reddy, eds., Handbook of Batteries, 3rd ed., McGraw-Hill, 2002 (Light Fighter Library, Field-Craft/Energy-Tech), chapters 22–24: lead-acid energy density “typically 30–40 Wh/kg”; the open-circuit cell voltage of 2.05 to 2.125 volts (12.3 to 12.75 volts for six cells, this module's multiplication); the VRLA family and the absorbed-glass-mat construction whose immobilized electrolyte “allows batteries to operate in different orientations without spillage”; SLI batteries built with thin plates, with “SLI batteries are not designed for deep-cycling service, and very short lives are generally obtained with such operation,” against deep-cycling designs built with “thick plates with high paste density”; self-discharge “more severe with batteries which use antimonial lead grid alloys in the positive plates,” minimized on calcium-lead nonantimonial grids; storing cells “in a discharged condition promotes sulfation and decreases capacity and life”; and “often batteries discarded as dead can be restored with a long, slow recharge (3 to 4 days at 2 to 3 A for SLI batteries).”
10. Battery University, BU-214 “Summary Table of Lead-based Batteries”: specific energy 30–50 Wh/kg for starter batteries and 20–30 Wh/kg, some higher, for deep-cycle batteries; nominal cell voltage 2.00 V. batteryuniversity.com (verified 2026-08-11).
11. Battery University, BU-201 “How does the Lead Acid Battery Work?”: deep-cycle cycle life by depth of discharge, 150–200 cycles at 100 percent, 400–500 at 50 percent, 1,000 and more at 30 percent. batteryuniversity.com (verified 2026-08-11).
12. Victron Energy, Gel and AGM Batteries datasheet and general technical information: self-discharge “less than 2% per month at 20°C,” doubling for every 10°C increase, on lead-calcium grids; AGM cycle design life 400 cycles at 80 percent discharge, 600 at 50 percent, 1,500 at 30 percent; the three-step bulk/absorption/float charge curve; and VRLA valves venting gas only on overcharging or cell failure. victronenergy.com (verified 2026-08-11).
13. Trojan Battery, Battery Maintenance (deep-cycle battery maker): “80% discharge is the maximum safe discharge,” “50% (or less) discharges are recommended,” and “shallow discharges will result in a longer battery life”; checking batteries once a month until the watering pattern is known; the 12.73 V full-charge open-circuit figure; and the storage rule that batteries “should be given a boost charge when they show a 70% charge or less” (maintenance figures re-verified 2026-08-11). trojanbattery.com (verified 2026-07-09).
14. Canadian Centre for Occupational Health and Safety (CCOHS), “Battery Safety — High-Voltage Batteries in Electric, Hybrid, or Plug-in Hybrid Vehicles”: “Electric, hybrid, and plug-in hybrid vehicles have both high-voltage (200-800 volt) and low-voltage (12-volt lead-acid batteries) battery systems”; stored energy remaining after the high-voltage battery is disconnected; and verifying no high-voltage energy is present with a CAT voltmeter while following the manufacturer's instructions. ccohs.ca (verified 2026-08-11).
15. Micah Toll, DIY Lithium Batteries: How to Build Your Own Battery Packs, 2017 (Light Fighter Library, Field-Craft/Energy-Tech): LiFePO4 rated for over two thousand charge cycles where an ordinary lithium cell is rated for roughly three hundred to a thousand, and the battery-management board's balance, low-voltage cutoff, and short/overload protection that a multi-cell bank never runs without.
16. ToolGuyd, “Comparing Power Tool Battery Specs: Watt-Hours vs Amp-Hours”: an intact salvaged lithium pack keeps its own matched cells and BMS, used within its rating through its own connector. toolguyd.com (re-verified 2026-08-11 at the current URL: 1 Wh = 1 V × 1 Ah, packs built from matched cells with a battery management system).
17. Lifeline Batteries, “How to Program BSE Charge Controllers for LiFePO4 Batteries” (battery maker): a charge controller charging a lithium battery must be set to the lithium profile, not lead-acid, with equalization disabled and temperature compensation off; LiFePO4 charging at about 14.4 V absorption and 13.6 V float for a 4S battery; and “ask the manufacturer for the recommended charge profile” when in doubt. lifelinebatteries.com (verified 2026-07-09).
18. National Electrical Code (NFPA 70), §240.4 (protection of conductors in accordance with their ampacities), 2005 ed. The same overcurrent principle Module 07 cites for the tap.
19. Littelfuse, Fuseology Design Guide (fuse maker): the designer “generally loads the fuse not more than 75% of the nominal rating listed by the manufacturer,” and “fuses at room temperature should last indefinitely if operated at no more than 75% of nominal current rating.” littelfuse.com (re-verified 2026-08-11 from the current design guide; the 2026-07 URL is dead).
20. PowerStream, wire gauge and ampacity chart: the current each wire gauge can carry; the gauge-to-amperage pairings here are a conservative reading of that chart for short 12 V branch runs. powerstream.com (verified 2026-07-08).
21. Victron Energy, “Which solar charge controller: PWM or MPPT?” (technical note): “The PWM controller is in essence a switch that connects a solar array to the battery,” pulling the array's voltage down to near the battery's; the MPPT controller “will adjust its input voltage to harvest the maximum power from the solar array”; the worked example where the same 100 W panel yields 81 W through PWM against 100 W through MPPT at 25°C; and “it is generally accepted that MPPT will outperform PWM in a cold to temperate climate, while both controllers will show approximately the same performance in a subtropical to tropical climate.” victronenergy.com (verified 2026-07-09).
22. NREL PVWatts Calculator: NREL's public tool returning a location's peak sun hours (annual-average daily solar resource) for sizing a panel array to a daily load; enter your own location for the figure that applies to you. pvwatts.nrel.gov.
23. Guerrilla-Tech salvage and component working notes (SOLAR-SIZING), Light Fighter working library: sizing a charge controller's current rating above the panel's short-circuit current times 1.25. Stated here as the working-notes rule; the underlying code provision (NEC 690.8) was not re-verified against the code text for this module.
24. Canadian Centre for Occupational Health and Safety (CCOHS), “Battery Charging — Industrial Lead-Acid Batteries”: when lead-acid batteries are being recharged they generate hydrogen gas, “explosive limits are 4.1 to 72 percent hydrogen in air,” and ventilation guidance limiting accumulation to 25 percent of the lower explosive limit, about 1 percent of the volume. ccohs.ca (verified 2026-08-11).
25. U.S. Department of Energy, DOE Fundamentals Handbook: Electrical Science (Basic Battery Principles), DOE-HDBK-1011/2-92 (Light Fighter Library, Field-Craft/Energy-Tech): “since hydrogen is highly explosive, it is necessary to provide adequate ventilation to the battery whenever charging is in progress” and “no smoking, electric sparks, or open flames are allowed near a charging battery”; and low temperatures lowering battery capacity.
26. National Electrical Code (NFPA 70), §702.6 “Transfer Equipment,” 2005 ed.: transfer equipment “designed and installed so as to prevent the inadvertent interconnection of normal and alternate sources of supply.” The same never-back-feed principle Module 07 cites for the tap.
27. EcoFlow, DELTA 3 Plus portable power station specifications: “Battery Chemistry LFP (LiFePO4 battery),” “Cycle Life 4000 cycles to 80+% capacity,” battery pack IP65 with BMS. ecoflow.com (verified 2026-08-11).
28. Battle Born Batteries, 100Ah 12V LiFePO4 Deep Cycle Battery product page: a built-in BMS with “integrated protections,” marketed as “a straightforward upgrade from traditional lead-acid batteries” fitting common Group 27/31 compartments; charge 14.2–14.6 V, float 13.4–13.8 V; listed at $799.00 (regular $949.00). battlebornbatteries.com (verified 2026-08-11).
29. Battery University, BU-903 “How to Measure State-of-charge”: the BCI open-circuit-voltage table for a 12-volt starter battery (12.65 V at 100 percent, 12.45 at 75, 12.24 at 50, 12.06 at 25, 11.89 at 0, “taken at 26°C (78°F) after a 24h rest”), and “the battery needs to rest in the open circuit state for at least four hours; battery manufacturers recommend 24 hours for lead acid.” batteryuniversity.com (verified 2026-08-11).