The formula
days = (Ah × depth of discharge) ÷ (parasitic drain × 24)
Where it comes from
Amp hours and cold cranking amps measure different things and neither follows from the other: the first says how long the battery can supply current, the second how hard it pushes with a freezing engine. On top of that, only part of the nominal amp hours can actually be used: a lead-acid battery taken below half charge sulphates and loses life, so the number that matters is the usable capacity, not the one on the label.
How to work it out by hand
- Multiply the capacity by the discharge it tolerates: that is the usable capacity
- Divide by the parasitic drain times twenty-four for the days parked
- For recharging, work out the missing amp hours and add 20 % for losses
- Divide by the charger current
What is worth knowing
A modern car draws about 20 to 30 milliamps at rest: alarm, memories and control units that never fully sleep. At 30 mA, a 70 Ah battery with fifty per cent usable depth lasts around forty-eight days; if a badly fitted accessory pulls 100 mA, that becomes fourteen. Reserve capacity is the number of minutes the battery can supply 25 amps, which is the standard used to compare two batteries: it is what you would have if the alternator failed while driving.