Battery system

LiFePO4 vs AGM

LiFePO4 is lighter, usually offers far more usable energy per rated Ah, holds voltage well and cycles deeply. AGM is cheaper upfront, works in cold charging conditions better, and is familiar, but is heavy and suffers when repeatedly discharged deeply. Follow the exact battery charge profile and limits.

Capacity and sizing

Energy = nominal voltage x Ah. A 12.8 V, 300 Ah LFP bank is 3.84 kWh nominal. If your planning window uses 90%, you have about 3.46 kWh before conversion losses. For 2.0 kWh/day and two days, 4.0 kWh usable is the starting target, then add reserve.

BMS and cold

A battery management system watches cells, temperature and current and can disconnect to prevent damage. That disconnect is a last line of defense, not normal control. Many LFP batteries must not accept charge around or below freezing unless their design provides safe low-temperature control or heating. Use the maker's exact limits.

Parallel banks

Use identical approved batteries, same model/age where required, equal-resistance cabling and balanced takeoff. Prefer positive from one end and negative from the opposite end for small parallel banks, or equal cables to busbars. Never assume the internal BMS ratings add perfectly for surge loads.

Protection, monitoring and mounting

Install a main fuse with adequate interrupt rating close to the positive source, then a rated disconnect. Place the shunt in the single path from battery negative to the rest of the system. Secure batteries for crash loads, protect terminals, keep away from heat/water, and ventilate as the manufacturer requires.

Rule of thumb

If you cannot state the expected current, full conductor length, protection rating and manufacturer limit for a circuit, the circuit is not ready to build.

CHAPTER CHECK

• I can explain the purpose of every component covered here. • I can identify the exact manufacturer limit that controls my design. • I have added the decision, rating or measurement to my schematic/worksheet.