Battery & Alternator Sizing
Add up everything that runs at once, then size the alternator for running load and the battery for reserve — with honest idle-output derating instead of catalogue optimism.
What’s running at the same time?
Total draw: 66 A
Edge cases to remember: a fan and pump both spiking at once, headlights + light bar + wipers in a storm, and the starter pulling 150–250 A for a couple of seconds (that’s the battery’s job, not the alternator’s).
Sizing
Continuous load
66 A
Alternator @ 1.25×
83 A
If only idle output counts
120 A
~55% at idle
Battery needed
66 Ah
0.5 h reserve @ 50% usable
Pick the alternator ABOVE the idle number if you sit in traffic with everything on. For racing, a small LiFePO4 pack plus a modest alternator beats a huge battery — weigh it, you’ll agree. Undersizing shows up as dim lights at idle, a reset clock and a flat battery after a parade lap.
Good to know
- Alternators make only ~50–60% of rated output at idle: size for the idle number if you sit in traffic with fans, lights and pumps all running.
- Battery reserve assumes you won’t run it flat: lead-acid is good for ~50% depth of discharge, LiFePO4 more like 80% — but deep cycling kills ordinary starters.
- Starter current (150–250 A) is the battery’s job, not the alternator’s. Cranking requirements are about CCA, not amp-hours.
- Race car rule: lightweight LiFePO4 + modest alternator beats a huge lead battery that costs you lap time every lap.