Battery Charge Time Calculator
Estimate how long a battery bank takes to charge from a given depth of discharge, allowing for charging losses.
Results
Why charging is slower than the sum suggests
Dividing amp-hours by charge current gives a floor, not an answer. Charging is not lossless: lead-acid banks return around 80 to 85 % of what goes in, lithium closer to 95 %.
The last 20 % is the slow part
Lead-acid charging tapers during the absorption phase. The bank reaches 80 % comparatively quickly and then spends hours creeping to full at a steadily falling current. Sizing an off-grid system on the fast part of that curve is how banks end up chronically undercharged and sulphated.
Common mistakes
Assuming the array can deliver its rated current all day. Charge current follows the sun, so the practical figure is closer to peak sun hours than to daylight hours.
Questions people ask
Can I charge faster with a bigger array? Up to the limit the battery accepts. Lead-acid banks take roughly 10 to 20 % of their capacity in amps; pushing beyond that heats the cells and shortens their life. Lithium tolerates far more.
Why does my bank never reach 100 %? Usually because the absorption phase is cut short — either the sun goes in, or the controller is set to a voltage the chemistry never quite reaches.