How long does it take to charge an inverter battery?
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When outages come close together, the question isn’t just how long the battery lasts but how quickly it refills before the next one. Here is how to estimate it.
The basic sum
charging time (hours) ≈ energy to put back ÷ charging power
Or in amps, which is how most chargers are set:
charging time (hours) ≈ amp-hours to put back ÷ charging current (A)
Then allow extra for charging losses and, for lead-acid, a slow final stage.
Example: lead-acid bank on grid or generator
- Battery: two 200 Ah, 12 V batteries wired as a 24 V bank
- Used last night: half of it, so 100 Ah to put back
- Charger: set to 30 A
- Bulk charging: 100 ÷ 30 ≈ 3.3 hours
- Plus losses and the slow finishing stage (below): often a few more hours
Real total: roughly 5 to 7 hours to get properly full.
Why the last part of a lead-acid charge is slow
Lead-acid charging has stages:
- Bulk: the charger pushes its full current until the battery reaches its absorption voltage, usually somewhere around 80% full.
- Absorption: voltage is held steady while the current tapers off. The last 20% can take as long as the first 80%.
- Float: a gentle maintenance charge once full.
Skipping absorption, because the grid comes back only briefly or the generator is switched off early, leaves the battery part-charged. Repeated often, that shortens its life. See battery care.
Lithium charges faster
Lithium (LiFePO4) batteries accept close to their full charging current almost all the way to full, and lose less energy charging. The same amount of energy usually goes back in noticeably faster than with lead-acid, as long as the charger can supply the current.
The charger is often the limit
A big battery with a small charger charges slowly. Check:
- The inverter-charger’s maximum charging current, and what it is set to. Many are set conservatively out of the box.
- The battery maker’s recommended maximum charge current. Lead-acid batteries often have a recommended limit that is a fraction of their amp-hour capacity; lithium batteries usually accept more. Stay within it.
- The generator size, if charging from one. Charging power must fit within the generator’s output with headroom. See generator sizing.
Charging from solar
Solar charging depends on sunshine, not the clock:
energy solar can deliver in a day ≈ panel watts × peak sun hours × 0.75
For example, 1,800 W of panels in a place with 4 peak sun hours can deliver roughly 5.4 kWh on an average day. That’s enough to refill a 5 kWh battery used overnight, if the house isn’t using much during the day at the same time. On cloudy days it may not be.
Check your city’s peak sun hours, and size panels for your dullest month with the calculator.
Speeding it up
- Raise the charging current within the battery’s and inverter’s limits.
- Charge while the house uses less: whatever runs at the same time takes power away from the battery.
- Add solar, so the battery recovers during the day without relying on the grid or generator.
- Consider lithium if fast recovery between frequent outages matters most. See lithium vs lead-acid.