About the Battery Charge Time Calculator
This calculator answers two practical battery questions. The first is how long a charger needs to bring a battery from one level to another, given the capacity in Ah or mAh, the charger current and the efficiency of charging. The second is how big a battery bank has to be to run a load for a number of hours, taking account of how deeply you are willing to discharge the batteries and what the inverter wastes. It then counts how many batteries to wire in series and in parallel. All the maths is done on your device, using the standard charge and energy formulas.
Updated September 2026
How to use the Battery Charge Time Calculator
- 1Choose Charge time or Size a battery bank.
- 2Pick the battery type. It fills in a typical efficiency and usable capacity that you can change.
- 3For charge time, enter the capacity, the start and end charge level and the charger current.
- 4For a battery bank, enter the load in watts, the hours you need, the system voltage and the size of one battery.
- 5Read the result and the notes underneath it.
How charge time is worked out
The basic rule is time equals the charge to be added divided by the charging current. Adding 80 Ah to a 100 Ah battery with a 10 A charger would take 8 hours if nothing were lost. Batteries do not store every amp-hour they receive, so the calculator divides by the charging efficiency first. At 85% the same job takes about 9 hours 25 minutes.
Typical efficiencies are around 85% for lead-acid and 90 to 95% for lithium types. They are planning figures, not guarantees, so you can overwrite the value when you know your own battery.
The answer assumes the charger keeps its current steady. Most chargers taper the current toward the end, so the final part takes longer than the maths says. Read the result as the shortest time you can expect.
Sizing a battery bank
Start with the energy the load needs: watts multiplied by hours. Divide by the share of the battery you are happy to use, and by the inverter efficiency if the load is fed through one. A 200 W load for 5 hours on lead-acid at 50% usable and 90% inverter efficiency needs about 2,222 Wh, or 185 Ah at 12 V.
The tool then turns that into batteries. With 100 Ah, 12 V units it says two in parallel. For a 48 V system built from 12 V batteries, four go in series, and extra strings go in parallel to add capacity.
- Lead-acid lasts much longer if you stay near 50% usable capacity.
- LiFePO4 can usually be discharged to 80 to 90% each cycle.
- Use 100% inverter efficiency only if the load runs on the battery voltage directly.
- Add a margin for cold weather, battery age and start-up surges.
Picking a charger
A common guide is to keep lead-acid charging near 10 to 20% of the capacity per hour, so 10 A to 20 A for a 100 Ah battery. Lithium batteries often accept more, but follow the maker's limits. Try a few currents in the tool to see what a bigger charger saves in time.
For small devices, use mAh and mA. A 3,000 mAh phone battery with a 1 A charger and 90% efficiency takes about 3 hours 20 minutes from empty, though fast-charging protocols change the current during the charge. Charging time depends on the size and the supply. Convert the capacity first with the mAh to Wh Calculator, check the draw with the Watts, Amps and Volts Calculator, see what a full charge costs with the Electricity Cost Calculator, and use the Power Factor Calculator for AC chargers.
Features
- Charge time between any two charge levels
- Works in Ah or mAh and in A or mA
- Presets for lead-acid, LiFePO4 and lithium-ion
- Battery bank size for a load and run time
- Counts batteries in series and parallel
- Shows the real run time of the bank you would build
Frequently asked questions
How do I calculate battery charging time?
Divide the amp-hours to add by the charger current and by the charging efficiency. A 100 Ah battery going from 20% to 100% with a 10 A charger at 85% takes about 9 hours 25 minutes.
Why does my battery take longer than the calculator says?
Chargers reduce their current as the battery fills, especially past 80%. The tool assumes a steady current, so it gives the shortest realistic time.
What efficiency should I use when charging?
Use about 85% for lead-acid and 90 to 95% for lithium batteries, or the figure from the battery's datasheet. Choosing the battery type fills these in, and you can change them.
How many batteries do I need for an inverter?
Switch to Size a battery bank, enter the load in watts, the hours you want and the size of one battery. The result shows the count, and how many go in series and in parallel.
What is depth of discharge?
It is the share of the capacity you use before recharging. Lead-acid batteries last longer at about 50%, while LiFePO4 batteries can use 80 to 90% each cycle.
Can I use it for a phone or laptop battery?
Yes. Choose mAh and mA for small batteries. Fast chargers vary the current during the charge, so the result is a rough guide for those devices.
Does it work out the watts of the charger?
No. It works with current in amps. To use a charger rating in watts, divide the watts by the battery voltage to get amps, then enter that number.