LiFePO4 Batteries: How to Choose, Size, and Use Them

LiFePO4 (Lithium Iron Phosphate) batteries offer long cycle life, stable performance, and excellent safety for deep-cycle use. They are a strong upgrade from lead-acid in solar storage, RVs, marine systems, and backup power. This guide focuses on practical selection, sizing, and compatibility so you can deploy LiFePO4 with confidence.

When LiFePO4 Makes Sense

• Use if you need high cycle life (≈2,000–6,000 cycles at 80% DoD), stable voltage, and low maintenance.
• Use for deep-cycle loads: inverters, fridges, pumps, trolling motors, telecom, and off-grid storage.
• Avoid if the only requirement is lowest upfront price and infrequent cycling (basic standby with rare discharges may suit lead-acid).

Quick Specs You Can Expect

• Nominal cell voltage: 3.2 V (4 cells in series for ~12.8 V)
• Cycle life: ~2,000–6,000 cycles at 80% DoD (varies by brand, temperature, and charge profile)
• Usable DoD: up to 80–100% (80–90% recommended for longevity)
• Self-discharge: ~2–3% per month at 77°F (25°C)
• Operating temp (typical): Discharge −4 to 140°F (−20 to 60°C); Charge 32 to 113°F (0 to 45°C)

How to Size Your Battery Bank

1. Choose system voltage:
   • 12 V: small systems and mobile (≤1,500 W inverter typical)
   • 24 V: mid-size (1,500–3,000 W)
   • 48 V: larger systems (>3,000 W), lower current and smaller cables
2. Estimate daily energy use (Wh): Sum each device’s watts × hours per day, then add 15–25% for losses.
3. Convert to required capacity: Required Wh ÷ (Nominal system V × planned DoD). Example: A 12 V system needs 1,000 Wh/day. With 90% DoD: 1,000 ÷ (12.8 × 0.9) ≈ 87 Ah. Choose a 100 Ah battery for headroom.
4. Check runtime: Runtime (hours) ≈ Usable Wh ÷ Load W. Example: A 12 V 100 Ah LiFePO4 (~12.8 V × 100 Ah × 0.9 ≈ 1,152 Wh usable) powering a 100 W load ≈ 11–12 hours.
5. Match current ratings to your inverter and loads:
   • Typical 12 V 100 Ah pack: 100 A continuous, ~200 A surge (few seconds).
   • A 2,000 W inverter at 12 V can draw >160 A; use multiple batteries in parallel or move to 24/48 V.

Charging Requirements

Use a charger or solar controller with a LiFePO4 profile. Avoid equalization. Float is optional and usually lower than lead-acid.

System	Bulk/Absorb	Float (optional)	Recommended Charge Rate
12 V	14.2–14.6 V	13.4–13.6 V	0.2–0.5C (max up to 1C if manufacturer allows)
24 V	28.4–29.2 V	26.8–27.2 V	As above
48 V	56.8–58.4 V	53.6–54.4 V	As above

• Do not charge below 32°F (0°C) unless the battery has a low-temp cut-off and/or internal heater.
• For alternator charging (vehicle/boat), use a DC-DC charger to current-limit and protect the alternator.

Compatibility Checklist

• Inverters/Chargers: Must support LiFePO4 or user-set voltage. Verify surge current vs. BMS limits.
• Solar Charge Controllers: MPPT/PWM with adjustable setpoints; disable equalize.
• Series/Parallel: Only combine identical models/age. Follow manufacturer limits. Balance to the same state of charge before connecting.
• Communication: Some rack batteries support CAN/RS485 for closed-loop control with inverters—confirm protocol compatibility.

What to Look For When Buying

• Cycle life rating (state the DoD and temperature used for the claim).
• BMS specs: continuous/peak discharge, max charge current, low-temp cut-off, short-circuit/over-voltage/under-voltage protection.
• Features: Bluetooth monitoring, internal heater for cold climates, serviceable design.
• Certifications: UN38.3 (transport), UL1973/IEC 62133 (safety), UL9540A data for fire behavior (where applicable).
• Warranty: Clear terms; many reputable brands offer 5–10 years.
• Form factor: 12 V “drop-in” (Group 24/27/31) vs. 24/48 V rack modules for larger systems.

Pros and Cons

• Pros: Long life, high usable capacity per cycle, fast charging, low maintenance, stable thermal behavior, lightweight (e.g., many 12 V 100 Ah units are ~24–28 lb).
• Cons: Higher upfront cost, lower energy density than cobalt-based lithium, requires correct charging profile.

LiFePO4 vs. Other Chemistries

Chemistry	Cycle Life (typ.)	Safety/Thermal Stability	Maintenance	Cost Over Life
LiFePO4	2,000–6,000 @ 80% DoD	Excellent	Low	Low per kWh delivered
NMC/NCA (Lithium-ion)	1,000–2,000	Moderate (higher energy density)	Low	Moderate
AGM/Gel Lead-Acid	300–800 @ 50% DoD	Good	Low–Moderate	Higher per kWh delivered
Flooded Lead-Acid	300–700 @ 50% DoD	Good	High (watering, equalize)	Higher per kWh delivered

Installation Tips

• Mount securely; orientation is generally flexible. Allow airflow to avoid heat buildup.
• Use appropriately sized cables and fusing near the battery. High-current 12 V systems often need 2 AWG to 2/0 AWG cabling.
• Place a main disconnect and proper overcurrent protection for each series/parallel string.
• For storage >1 month: store around 40–60% state of charge at 50–77°F (10–25°C); top up every 6–12 months.

Common Use Cases

• Solar/off-grid: Daily cycling with MPPT controller; pair with 24 V or 48 V for >2 kW inverters.
• RVs/Vanlife: House banks for fridges, inverters, and shore/alternator charging via DC-DC chargers.
• Marine: Trolling motors, house loads; ensure waterproof enclosures and corrosion-resistant hardware.
• Backup power/UPS: Quiet, low-maintenance alternative to generators for short outages.

FAQ

Can I use my old lead-acid charger?

Yes if it is adjustable or has a LiFePO4 mode. Set bulk/absorb to ~14.2–14.6 V (12 V systems) and disable equalize. Float is optional at ~13.4–13.6 V.

Can I charge below freezing?

Do not charge below 32°F (0°C) unless the battery has a low-temperature cut-off and/or internal heaters. Discharging below freezing is usually allowed (check the spec sheet).

How many batteries can I connect?

Follow the manufacturer’s series/parallel limits. Only connect identical batteries of the same age and state of charge, and fuse each parallel string.

How does LiFePO4 compare in weight?

A typical 12 V 100 Ah LiFePO4 weighs ~24–28 lb versus ~60–70 lb for AGM of similar capacity.

Do I need to float charge?

No. LiFePO4 doesn’t require float. If your charger cannot disable float, set a low float voltage or use a timer to terminate charging after absorption.