How to use the car battery charge time calculator
- Enter the battery capacity in amp-hours from its label.
- Enter how discharged it is, as the percent of the capacity that has been used.
- Enter the charger current in amps and the charging efficiency.
- Read the amp-hours to replace and the estimated charge time.
Formula
Amp-hours to replace = capacity x share of charge used
Effective charge current = charger amps x efficiency
Hours = amp-hours to replace / effective charge current
The last part of a charge goes in more slowly than the first, so the real time is longer than this simple division, especially from a deep discharge.
A half-discharged 60 Ah battery
A 60 amp-hour battery is 50% discharged, so 30 amp-hours must go back in. The charger supplies 10 amps at 85% efficiency, which is 8.5 amps into the battery.
30 / 8.5 = 3.53 hours, or 3 hours 32 minutes. The charger current is about 1/6 of the capacity, shown as C/6.0.
Why a rough estimate is useful
Knowing roughly how long a charge will take helps you plan: whether it will be ready by morning, whether a small trickle charger is enough or whether a bigger one is worth buying. The division gives the time at a steady current, which is a fair guide for the bulk part of a charge.
Charging efficiency is a rough allowance for losses as heat. Lead-acid batteries often need more energy in than they give back, so a value between 80% and 90% is a reasonable starting point to test, but your charger and battery type decide.
What the estimate leaves out
Most chargers hold a steady current until the battery reaches a set voltage, then lower the current to finish without overheating. That taper can double the time for the last 20% of the charge. Cold batteries accept charge more slowly, and an old or damaged battery may never reach its label capacity.
Always follow the charger and battery instructions. Charge in a ventilated place, keep sparks and flames away, and do not charge a frozen, swollen or leaking battery. This page is a planning aid and gives no guidance on safe charging.
Assumptions and limits
- The charger holds its stated current for the whole time, and there is no taper.
- The capacity on the label is available. A worn battery has less.
- The efficiency is your estimate and is applied to the charger current.
- The figures are examples. Nothing is uploaded.
Frequently asked questions
How do I know how discharged the battery is?
A resting voltage reading gives a rough idea, and the table from the battery maker shows the charge for each voltage. Use the percent that you estimate.
Why does the charge take longer than the estimate?
Chargers slow down near full to protect the battery, and cold or aged batteries accept charge slowly.
What is C/6?
A charge rate of one sixth of the capacity per hour. A 60 Ah battery at 10 amps is charged at about C/6.
Can I use this for lithium batteries?
The division is the same, but the charger profile, safe current and efficiency differ. Follow the maker instructions for the battery.