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. The battery bank is only one component — it needs to be sized in concert with your solar array and charge controller for recharging and an inverter matched to your AC load profile.

Educational use only

This page is for educational purposes only. Hands-on skills should be learned and practiced under qualified supervision before relying on them in emergencies. Use this information at your own risk.

Before you start

Skills: Understanding of LFP, AGM, and flooded lead-acid chemistry differences; familiarity with solar basics concepts (Voc, Vmp, charge stages); comfort with voltage, amp-hour, and watt-hour math.

Materials: Battery monitor with shunt (Victron BMV-712 or equivalent); battery temperature sensor; DC cables sized for at least 125% of maximum continuous current per NEC 706.30 (Energy Storage Systems — Circuit Sizing and Current) with ampacity from NEC Table 310.16, and (for battery-side and marine-style installations) per American Boat and Yacht Council (ABYC) E-11; Class T fuse and holder for the main bank disconnect (20,000 A AIC at 125 VDC — required at the battery terminal on any lithium bank because NEC 110.9 mandates the overcurrent protection device (OCPD) interrupting rating meet or exceed the available fault current, and LFP prospective short-circuit current commonly reaches several thousand amps; ABYC E-11 codifies the same practice for marine and adopted off-grid installations; ANL fuses are acceptable only for downstream DC branch circuits with lower fault current); battery box or enclosure rated for your chemistry.

Conditions: Confirm site temperature range against chemistry limits — LFP: operating range −4°F to 140°F (−20°C to 60°C), charge only above 32°F (0°C); AGM/flooded lead-acid: operating range 32°F to 104°F (0°C to 40°C). Flooded lead-acid requires active ventilation to the outside due to hydrogen evolution during charging per NEC 480.10.

Time: 30–60 min for chemistry selection and sizing research; 2–4 hours for physical installation depending on bank size.

Action block

Do this first: List every critical device's wattage and daily hours of use, then multiply each to get Wh/day and sum the column (15 min) Time required: Active: 30–60 min for chemistry selection and load sizing; 2–4 hours for physical installation Cost range: Affordable for a single 100Ah AGM baseline bank; significant investment for a 10+ kWh LiFePO4 home storage system Skill level: Beginner for AGM maintenance; advanced for lithium system design; needs licensed electrician for permitted whole-home install Tools and supplies: Tools: multimeter, torque wrench, hydrometer (flooded lead-acid only), cable cutter/crimper, wire stripper. Supplies: DC-rated cable (sized per load), ring lugs, heat-shrink, anti-corrosion terminal spray, distilled water (flooded lead-acid). Infrastructure: Class T fuse and holder, battery monitor with shunt, ventilated enclosure. Safety warnings: See Never charge LiFePO4 below freezing below — irreversible lithium plating if charged below 32°F (0°C); See Hydrogen gas risk in confined spaces below — flooded lead-acid only

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 (depth of discharge) 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 (nickel manganese cobalt) 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. For a complete worked example that integrates battery sizing with solar panel sizing and generator backup, see whole-home off-grid system design.

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 less expensive per usable kWh than assembling individual cells, and they simplify the wiring.

Lead-acid options

Field note

Flooded lead-acid batteries that have lost capacity to sulfation often show it clearly: the charger hits absorption voltage in minutes instead of an hour or more, but the battery won't hold a load for anywhere near its rated time. If you see that pattern, run a full equalization cycle at 15.5–16V before writing the battery off — early sulfation is reversible and equalization has saved plenty of batteries that looked dead.

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 are an affordable to moderate-investment option 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 inexpensive entry point 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:

  1. 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)
  2. Absorption — voltage holds at the absorption setpoint; current tapers as cells approach full charge; this stage completes cell balancing and prevents chronic undercharge
  3. 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

Failure modes

LFP charging below freezing — Recognition: LFP bank shows full nameplate open-circuit voltage but capacity collapses 50% or more in winter; the BMS triggers a low-temperature cutoff; cells develop an internal short that is not visible externally and manifests as progressive capacity loss. Remedy: never charge LFP below 32°F (0°C) — install a battery heating pad or blanket triggered at 35°F (1.7°C) per most LFP manufacturer specifications; or relocate the bank to an insulated, temperature-controlled space. The BMS low-temperature cutoff is the last line of defense, not the primary protection strategy.

Lead-acid sulfation from chronic undercharging — Recognition: bank no longer reaches 100% state of charge; specific gravity stays below 1.225 after extended charging; bank capacity drops measurably year over year. Remedy: run weekly absorption and equalization charges per the battery manufacturer's schedule; verify the charge controller absorption voltage setpoint (14.4–14.8 V for 12 V nominal lead-acid); audit daily Ah-in vs. Ah-out using a shunt-based battery monitor and identify any days where the charge controller is not completing the absorption stage.

Battery bank thermal runaway risk in confined spaces — Recognition: bank temperature rises more than 20°F (11°C) above ambient during discharge; LFP cells show more than 5°F (3°C) inter-cell temperature variation; flooded lead-acid bank shows excessive electrolyte water loss between monthly checks. Remedy: ensure at least 6 inches (15 cm) of clearance on each cell side for heat dissipation; provide ventilation per NEC 480.10(C); install thermal sensors with BMS-triggered shutoff at chemistry-specific cutoffs — LFP: 140°F (60°C) charge cutoff, 158°F (70°C) discharge cutoff per typical datasheets.

Parallel battery bank imbalance — Recognition: voltage divergence between paralleled banks exceeds 0.2 V at rest; one bank discharges faster than the others under identical loads; total measured bank capacity is meaningfully below the sum of individual bank capacities. Remedy: use a bus bar with equal-length, equal-gauge cables to each bank in parallel; install per-string fuses sized to manufacturer specification (Class T or ANL depending on position in the circuit); verify all banks are within 0.05 V per cell before paralleling new or replacement units into an existing bank.

Class T vs. ANL fuse fault-current mismatch — Recognition: ANL fuse installed at the main battery-bank disconnect on an LFP system; AHJ inspection flagged this as non-compliant; insurance audit identified the issue. Remedy: a Class T fuse is required at the LFP main bank disconnect (20,000 A AIC at 125 VDC) because NEC 110.9 (Interrupting Rating) requires the OCPD's interrupting rating to meet or exceed the available fault current, and LFP banks routinely produce prospective short-circuit currents that exceed ANL fuse AIC; ABYC E-11 codifies the same practice for marine and adopted off-grid installations. ANL fuses are acceptable only downstream of the Class T fuse on branch circuits where the upstream Class T limits prospective fault current to within ANL interrupt ratings.

Practical checklist

  • Audit critical loads; calculate daily Wh total before selecting any battery
  • Size bank for 48-hour autonomy minimum, with 15–20% buffer for losses and aging
  • Choose chemistry based on cycling frequency, budget, and weight constraints
  • For LiFePO4: confirm BMS includes over-voltage, under-voltage, over-current, and low-temperature cutoff
  • Install fusing within 18 inches (46 cm) of positive terminal on every output cable
  • Use cable sized for 125% of expected continuous current; run cables as short as possible
  • Use MPPT charge controller for solar systems above 200W of panels
  • Configure charger profiles to match your battery chemistry — do not use lead-acid profiles on lithium banks
  • Test actual runtime under load quarterly; log results to track capacity aging
  • Store batteries above 32°F (0°C); never charge LiFePO4 below freezing

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. For a complete off-grid installation that combines solar panels, charge controller, and battery bank, see off-grid solar system design. The energy foundation index maps how these components fit together as a complete resilience system.

Sources and next steps

Last reviewed: 2026-05-16

Source hierarchy:

  1. NEC Article 706.30 + 706.31 — Energy Storage Systems: Circuit Sizing and Overcurrent Protection (Tier 1, federal standard — governs battery-bank DC conductor ampacity at 125% of maximum continuous current, with OCPD per Article 240 and ampacity per Article 310)
  2. NEC 110.9 — Interrupting Rating (Tier 1, federal standard — requires OCPD interrupting rating ≥ available fault current; basis for Class T fuse selection on LFP banks where prospective fault current reaches several thousand amps)
  3. ABYC E-11 — DC Electrical Systems on Boats / Class T Fuse Requirements (Tier 1, standards body — governs Class T fuse AIC rating at battery disconnect in marine and adopted off-grid practice)

Legal/regional caveats: Battery-based Energy Storage Systems (ESS) above certain thresholds require a permit and AHJ inspection under NEC Article 706 and, where NFPA 855 is adopted, additional fire-separation and venting requirements. California, New York, and Texas have adopted NFPA 855 with state amendments. Check with your local building department before installing any bank larger than a single 100Ah 12V unit inside a living space.

Safety stakes: high-criticality topic — recommended to verify thresholds before acting.

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