How the calculation works
Charging time is the energy you need to put back divided by the rate you can put it back, adjusted for the losses that turn some of that energy into heat rather than stored charge.
Charging efficiency is real and chemistry-specific. Lithium returns roughly 95 cents of every dollar you put in; flooded lead-acid closer to 80, with the rest lost to heat and gassing. That difference alone stretches a four hour lithium charge into five hours or more for lead-acid.
The absorption tail
This calculator estimates the bulk phase, when the battery accepts everything the charger can supply. Lead-acid batteries then enter an absorption phase where current tapers steadily as the battery approaches full, and that final 20% can take as long as the first 80%. Lithium has a much shorter tail: it accepts near-full current until roughly 95% state of charge, which is why lithium banks recharge dramatically faster in real solar conditions where sun hours are limited.
Practical consequence for solar: a lead-acid bank that mathematically needs five hours of charging may not actually reach full on a five hour winter day, and chronic undercharging is the main killer of lead-acid banks.
Worked example
A 200 Ah LiFePO4 bank at 20% state of charge, charged at 40 A:
The same job on an AGM bank at 85% efficiency takes about 4.7 hours of bulk charging, then another one to two hours of absorption to genuinely finish, in a day that may only offer five useful sun hours.
Common mistakes
- Assuming solar delivers its rated current all day. A 40 A controller only produces 40 A near solar noon in good conditions. Real daily harvest is far closer to your peak sun hours figure than to controller rating times daylight hours.
- Charging lead-acid too slowly. Lead-acid wants a charge current around 10 to 20% of capacity. Much less than that and the bank may never reach a proper full charge, which causes sulfation.
- Charging lithium too fast for the BMS. Most LiFePO4 batteries accept 0.5C comfortably, some allow 1C, but every model has a limit and the BMS will disconnect if you exceed it. Check the datasheet before pairing a big charger with a small battery.
- Charging lithium below freezing. LiFePO4 must not be charged below 0 degrees Celsius without internal heating or low-temperature cutoff. Cold-climate installs need heated batteries or a charger that respects temperature.
Frequently asked questions
How long to charge a 100 Ah battery?
From empty at 20 A with lithium: roughly 5.3 hours. At 10 A: about 10.5 hours. Lead-acid takes longer both because of lower efficiency and because of the absorption tail, and it should not be routinely drained to empty in the first place.
What does a C-rate mean?
C-rate expresses current relative to capacity. Charging a 200 Ah battery at 40 A is 0.2C. Most LiFePO4 cells accept 0.5C routinely; lead-acid prefers 0.1 to 0.2C. The badge above shows your rate so you can compare it against your battery datasheet.
Can I charge from solar and shore power at once?
Yes, and the currents add. Two sources of 30 A and 20 A give 50 A of charging current, which shortens the time proportionally. Confirm the combined figure stays within the battery C-rate limit and that both chargers use the same voltage profile for your chemistry.