11.2 Battery Systems — AGM and Lithium

Beginner Level: What Batteries Do

Myth: Batteries power the audio.
Reality: Alternator powers audio. Batteries buffer transient peaks that exceed instantaneous alternator output.

During 50ms bass transient at 300A, alternator can't respond fast enough. Battery supplies the burst.

More batteries help by: - Doubling capacity (2× runtime engine-off) - Halving internal resistance (less voltage sag) - NOT increasing alternator output (still limited by alternator)

Battery type comparison:

AGM (Absorbed Glass Mat): $200-400, sealed, maintenance-free, 300-500 cycle life. Standard choice.

LiFePO4 (Lithium): $400-800, 1/3 weight of AGM, flat 13.2V discharge curve, 2000+ cycles. Requires BMS. Competition use.

Installer Level: Dual Battery Installation

Isolator vs Parallel:

Parallel: Both batteries always connected. Audio can drain starting battery. Competition use only.

Isolator: Auxiliary disconnects when engine off. Starting battery protected. Daily driver use.

Automatic voltage-sensing relay ($50-80): Connects at 13.2V+ (alternator running), disconnects below 12.8V. Recommended.

Installation steps: 1. Mount auxiliary in trunk with secure bracket 2. 0 AWG from aux positive → isolator → starting battery positive 3. Inline fuse within 18" of aux terminal (200-250A) 4. Ground both batteries to same chassis point 5. Control wire from ignition-switched 12V to isolator coil

Engineer Level: LiFePO4 BMS Requirements

4S LiFePO4 (four 3.2V cells in series) = 12.8V nominal, compatible with 12V systems.

Absolute limits per cell: - Max charge: 3.65V (14.6V for 4S) - Min discharge: 2.5V (10.0V for 4S)

BMS mandatory functions: 1. Cell balancing (keep cells within 50mV) 2. Overcharge protection (disconnect at 3.65V/cell) 3. Overdischarge protection (disconnect at 2.5V/cell) 4. Overcurrent limiting 5. Temperature cutoff (no charging below 0°C)

Most automotive alternators at 14.4-14.8V are safe if BMS configured for 14.6V max.