Car Battery Runtime Calculator

Car batteries (SLI — starting, lighting, ignition) pack many thin lead plates close together. That design maximises the surface area behind the burst of current that turns the starter, the job measured by the cold cranking amps a starter battery delivers, and it leaves the same plates fragile under a long, slow discharge.

Discover Battery prints the design limits as plain numbers. Regular flooded starting batteries "are designed for discharges of less than 3%" of capacity. Enhanced flooded start-stop batteries are designed for up to 17.5%. High-cycle anti-idle batteries, the ones built for running house loads with the engine off, are designed for up to 30%. A plain car battery is the first of those three, so running accessories from it is a one-off rather than a habit. Our depth of discharge guide explains why the same percentage costs SLI, AGM and LiFePO4 chemistries such different amounts of life.

How deep is too deep? EnerSys prints the line for its ODYSSEY batteries: at 12.0 volts, which its manual calls 35% state of charge, the battery "should be recharged immediately to avoid permanent battery damage". That is 65% depth of discharge. The calculator defaults to 30%, flags every setting past 3%, and marks anything past 65% as damage territory. If key-off power is part of how you use the vehicle, an AGM battery sized for cycling on its own isolator is the fix, not a deeper draw from the starter.

The wattages are the figures worked, not measurements of your car: read yours off the nameplate or measure it. The runtimes are this calculator's own output at the page defaults, a 60Ah flooded battery at 30% depth of discharge with no inverter in the path. The dash cam rows draw less than 8W, which is slower than the slowest discharge Trojan prints for a flooded battery, so those two are extrapolations. A lead-acid battery also loses charge sitting idle, so a long runtime at a tiny load is not a shelf-life figure.

The most common way people flatten a car battery is leaving the headlights on. At 120W, halogen headlights take a 60Ah battery to 30% depth in about 1 hour 25 minutes. By the time you come back, the starter may not turn.

How the calculator gets there: the energy a lead-acid battery gives up falls as you draw it faster, so instead of assuming the label amp-hours are available at any current it applies a discharge-rate correction fitted to the tables Trojan, Lifeline and Victron print for their own batteries. Those are deep-cycle products, which is worth saying plainly. The fit lands within about 10% of Trojan's printed flooded points and about 6% of Lifeline's AGM points, and checked against the constant-power table EnerSys prints for a 12V automotive AGM it sits within about 5% from the 20-hour rate down to the 1-hour rate. Faster than that it turns optimistic, which is why a heavy load gets a warning instead of a confident number.