Batteries

When the grid goes down, your battery bank determines how long you stay operational — and which systems you keep running. The gap between a 200 Wh phone charger and a 10 kWh home storage system is not just scale; it's a different class of decision involving chemistry, cycle life, wiring gauge, and charge management. Getting this right before you need it prevents expensive failures at the worst possible time.

Battery chemistry comparison

Three chemistries dominate preparedness and off-grid applications. Each involves real tradeoffs between upfront cost, usable capacity, service life, and safety profile.

Chemistry	Cycle life	Usable DoD	Cost per kWh (cells)
LiFePO4	3,000–6,000	80–95%	$120–$280
AGM lead-acid	300–600	50%	$150–$250
Flooded lead-acid	200–500	50%	$80–$150
NMC lithium-ion	500–1,500	80%	$100–$200

Chemistry	Weight (12V 100Ah)	Temp range (charge)	Self-discharge / month
LiFePO4	~31 lbs (14 kg)	32–113°F (0–45°C)	~2–3%
AGM lead-acid	~65 lbs (30 kg)	32–104°F (0–40°C)	~3–5%
Flooded lead-acid	~60 lbs (27 kg)	32–104°F (0–40°C)	~5%
NMC lithium-ion	~25 lbs (11 kg)	32–113°F (0–45°C)	~2%

The usable DoD column is where lead-acid chemistry loses ground quickly. A 100Ah AGM battery provides only about 50 Ah of usable capacity before you risk damaging the plates. A 100Ah LiFePO4 provides 80–95 Ah. To match the usable capacity of one LiFePO4 battery, you need two AGM batteries — which shifts the cost comparison significantly.

Sizing your battery bank

Accurate sizing starts with a load audit, not a battery catalog. Work through these steps in order.

Step 1 — List critical loads and estimate daily consumption.

Use this format for each device:

Device	Watts	Hours/day	Wh/day
LED lighting (4 fixtures)	40 W	6 h	240 Wh
Refrigerator (Energy Star)	60 W avg	24 h	1,440 Wh
Phone + radio charging	30 W	3 h	90 Wh
CPAP machine	30 W	8 h	240 Wh
Laptop	45 W	4 h	180 Wh
Daily total			2,190 Wh

Step 2 — Choose your autonomy window.

24 hours: minimum baseline, no solar or generator needed for one day
48 hours: covers most short-term outages and cloudy weather
72 hours: the preparedness standard; covers most regional emergencies

Step 3 — Account for depth of discharge.

Divide your raw capacity need by the usable DoD of your chosen chemistry. For LiFePO4 at 90% DoD:

2,190 Wh × 48 hours ÷ 24 = 4,380 Wh raw need 4,380 Wh ÷ 0.90 = 4,867 Wh battery capacity needed

Add a 15–20% buffer for inverter losses and aging capacity:

4,867 Wh × 1.18 = ~5,740 Wh total

Step 4 — Convert to amp-hours at your system voltage.

A 48V system is preferred for anything above 2 kWh — it reduces current, allows smaller cable gauges, and improves inverter efficiency.

5,740 Wh ÷ 48V = ~120 Ah at 48V

A pair of 24V 100Ah LiFePO4 batteries in series gives you a 48V 100Ah (4,800 Wh) bank — close enough for this load profile.

Scenario

A family of three running LED lights, a chest freezer (75 W average), and communication devices needs about 2,400 Wh per day. For 48-hour autonomy with a 15% buffer, they need roughly 6.2 kWh of LiFePO4 capacity — achievable with three 24V 100Ah batteries in a series-parallel configuration, or a single purpose-built 48V 200Ah unit. At current cell prices of around $120–$280 per kWh, this bank runs $750–$1,700 for cells alone, plus BMS, wiring, and housing.

LiFePO4 — the preparedness standard

Lithium iron phosphate is the dominant choice for serious preparedness systems for three compounding reasons: safety, cycle life, and lifetime cost.

On safety, LiFePO4 chemistry begins thermal runaway at approximately 270–300°C (518–572°F). NMC (nickel manganese cobalt) chemistry, used in many consumer electronics and some power walls, reaches that threshold at 150–210°C (302–410°F). NMC cells also release oxygen during failure, which feeds fires in adjacent cells. LiFePO4 does not. This distinction matters when batteries are in a confined space with limited ventilation.

On cycle life, real-world LiFePO4 banks routinely achieve 3,000–5,000 full cycles before dropping to 80% capacity. A system cycled daily lasts 8–14 years. The same daily cycling destroys a lead-acid bank in 2–4 years.

On lifetime cost: at $120–$280 per kWh for cells and a 10+ year lifespan, LiFePO4 amortizes to roughly $12–$30 per kWh-year of service. Lead-acid, despite lower entry cost, typically lands at $40–$70 per kWh-year when replacement cycles are included.

BMS requirement: Every LiFePO4 bank requires a Battery Management System (BMS) — either integrated into the cells (as with server rack batteries) or external. The BMS monitors cell voltages, temperature, and current, and disconnects the bank if any parameter goes out of safe range. A bank without a BMS can over-discharge, over-charge, or operate in a thermal runaway condition without any warning.

A 12V 100Ah LiFePO4 battery weighs approximately 28–33 lbs (13–15 kg), compared to 60–65 lbs (27–30 kg) for a similar-capacity AGM unit. At scale, this difference matters for transport and installation.

Field note

Server rack LiFePO4 batteries (48V 100Ah or 200Ah) from major manufacturers include the BMS, a built-in display, and rack mounting hardware in one unit. For a home preparedness system, they are often cheaper per usable kWh than assembling individual cells, and they simplify the wiring.

Lead-acid options

Lead-acid chemistry is not obsolete — it is appropriate in specific circumstances: lower cycling frequency, smaller budgets, and situations where local availability matters more than weight or longevity.

AGM (Absorbed Glass Mat) batteries are sealed, spill-proof, and require no water maintenance. They cost around $150–$250 per 100Ah at 12V from reputable brands. AGM works well for systems cycled less than once per day — seasonal cabins, backup supplies that sit most of the year, and vehicle auxiliary banks. The failure mode for AGM is sulfation from chronic partial discharge, which is largely irreversible in sealed batteries. Keep AGM banks above 50% state of charge whenever possible.

Flooded lead-acid is the cheapest entry point at $80–$150 per 100Ah and tolerates equalization charging that can reverse early-stage sulfation. The maintenance requirement is real: check electrolyte levels monthly, top up with distilled water, and equalize every 30–90 days. Store flooded batteries only in ventilated enclosures — they off-gas hydrogen during charging, and hydrogen accumulates to explosive concentrations in sealed spaces.

Sulfation is the primary lead-acid failure mode. When a lead-acid battery sits partially discharged, lead sulfate crystals form on the plates and harden. Early sulfation is reversible with equalization (a controlled overcharge at 15.5–16 V for flooded cells). Advanced sulfation is permanent — capacity loss becomes non-recoverable and the battery must be replaced. Symptoms include: charging voltage reaches absorption target quickly but runtime is far shorter than expected, and specific gravity readings that won't equalize across cells.

Hydrogen gas risk in confined spaces

Flooded lead-acid batteries release hydrogen gas during charging — particularly during equalization. Hydrogen is odorless, colorless, and ignites at concentrations as low as 4% in air. Never charge flooded batteries in a sealed room, closet, or enclosure without active ventilation to the outside. One spark from a nearby light switch or tool is sufficient to ignite accumulated gas. AGM batteries off-gas at much lower rates but still require adequate ventilation.

Charging and charge sources

Batteries only perform as well as their charging system. Three charge sources matter for preparedness systems:

Grid charger / AC charger: A quality smart charger matched to your chemistry handles the three-stage charge cycle automatically. Do not use a simple constant-voltage charger for multi-hundred amp-hour banks — it cannot manage the absorption phase properly and will either undercharge or overcharge cells.

Solar charge controller: Use an MPPT (Maximum Power Point Tracking) controller for any system above 200W of panels. MPPT extracts 20–30% more energy from panels than a PWM (Pulse Width Modulation) controller, particularly in cold weather when panel voltage is elevated above battery voltage. PWM is acceptable only for very small systems (under 100W) where the cost savings outweigh the efficiency loss.

Generator charging: Most inverter-chargers can accept generator power and charge at 50–100A while simultaneously powering loads. Size your generator to handle both the inverter-charger's charging draw and essential loads simultaneously — not just one at a time.

Three-stage charge cycle — why it matters:

Bulk — full current flows into the battery; voltage rises from resting to the absorption target (14.4–14.6V for 12V LiFePO4, 57.6V for 48V)
Absorption — voltage holds at the absorption setpoint; current tapers as cells approach full charge; this stage completes cell balancing and prevents chronic undercharge
Float — voltage drops to 13.6V (12V systems) to compensate for self-discharge without overcharging

Skipping absorption — which happens when you disconnect the charger too early or use an undersized solar array that can't maintain absorption voltage — leads to chronic partial state of charge. In lead-acid batteries, this causes sulfation within weeks. In LiFePO4, cell imbalance gradually develops and capacity degrades faster than the chemistry would otherwise allow.

Temperature compensation: At temperatures below 50°F (10°C), lead-acid batteries require a higher charge voltage to reach full charge — approximately 3mV per cell per degree Celsius below 25°C. Most quality MPPT controllers include automatic temperature compensation via a sensor at the battery terminal.

Wiring and safety

Poor wiring is the leading cause of field failures, fires, and capacity loss in battery systems. The physics are unforgiving: resistance in a high-current DC circuit generates heat proportional to the square of the current.

Cable sizing by system voltage and current draw:

Current (continuous)	12V system	24V system	48V system
50A	4 AWG (21 mm²)	8 AWG (8 mm²)	10 AWG (5 mm²)
100A	2 AWG (35 mm²)	4 AWG (21 mm²)	8 AWG (8 mm²)
200A	2/0 AWG (67 mm²)	2 AWG (35 mm²)	4 AWG (21 mm²)

Use flexible stranded welding cable or battery cable — not solid copper wire. Keep cable runs as short as possible. Size cables for at least 125% of continuous current rating.

Fusing: Install a fuse or breaker within 18 inches (46 cm) of the positive battery terminal on every cable leaving the bank. This protects against short-circuit faults. For LiFePO4 banks, the prospective short-circuit current can reach several thousand amps within milliseconds — use fuses rated for DC interrupt ratings, not AC fuses.

Terminal torque: Loose terminals cause resistance, heat, and arcing. Tighten lugged connections to the torque spec listed on the battery terminal (typically 70–100 in-lb / 8–11 N·m for M8 terminals). Check and re-torque at every maintenance interval.

LiFePO4 cold charging: Do not charge LiFePO4 batteries below 32°F (0°C). Below freezing, lithium ions cannot embed normally into the anode graphite and instead deposit as metallic lithium dendrites — thin crystalline structures that can pierce the separator between anode and cathode and cause internal short circuits. Quality batteries include low-temperature cutoff in the BMS; verify this feature before purchasing.

Never charge LiFePO4 below freezing

Charging a LiFePO4 battery below 32°F (0°C) causes lithium plating on the anode — metallic lithium deposits that degrade capacity permanently and can puncture the cell separator. The damage is cumulative and not always immediately visible. In cold environments, install batteries in insulated, temperature-controlled enclosures, or use batteries with integrated heating pads. The BMS low-temperature cutoff is your last line of defense, not your primary protection strategy.

Maintenance schedule

Monthly:

Check all terminal connections for corrosion, looseness, or heat discoloration
Verify BMS display readings: state of charge, cell voltage balance, temperature
Confirm charger is cycling correctly (not stuck in bulk or float)
Check flooded battery electrolyte levels; top off with distilled water only

Quarterly:

Test actual runtime against a known load and compare to calculated baseline
Check cell voltage balance at full charge (LiFePO4: all cells should be within 50 mV of each other)
Run an equalization cycle on flooded lead-acid batteries
Inspect cable insulation for cracking, chafing, or heat damage
Record battery bank capacity and compare to previous quarter

Annually:

Perform a full load test: discharge to minimum SoC under a known load, record Ah delivered
Compare actual capacity to nameplate; below 80% capacity signals end-of-life planning
Inspect battery enclosure for moisture, pests, and structural integrity
Update system documentation and battery age records

Common failure modes

Failure	Chemistry	Cause	Symptom	Prevention
Sulfation	Lead-acid	Chronic partial discharge or sitting discharged	Short runtime; won't fully charge; low specific gravity	Keep above 50% SoC; equalize flooded cells every 30–90 days
Cell imbalance	LiFePO4	Missed absorption; cell aging	Capacity loss; BMS trips on one weak cell	Complete absorption stage every cycle; monitor cell voltages quarterly
Thermal damage	All	Overcharge, high ambient temperature	Swelling (lithium), cracking, electrolyte loss	BMS protection; temperature monitoring; ventilation
Over-discharge	All	Running below minimum SoC	Lead-acid: sulfation; lithium: BMS lockout requiring reconditioning	Set inverter low-voltage disconnect above minimum; program charger cutoffs
Poor connections	All	Under-torqued or corroded terminals	Heat at terminals; capacity loss; voltage drop under load	Re-torque every maintenance interval; use dielectric grease on terminals
Lithium plating	LiFePO4	Charging below 32°F (0°C)	Capacity loss; internal short; BMS fault	Temperature-controlled enclosure; BMS low-temp cutoff

Practical checklist

A well-designed battery bank is the foundation that makes everything else reliable. Pair it with the right inverter sized for your load profile, a generator for fast recharging and surge capacity, and disciplined fuel storage to support generator runtime during extended cloudy periods. For smaller or mobile installations, portable power stations offer an all-in-one alternative that eliminates most of the wiring complexity at the cost of lower capacity. The energy foundation index maps how these components fit together as a complete resilience system.