🔄 Battery Charge Time Calculator
Enter the battery capacity, the charge current and a losses allowance to estimate the charge time and the C-rate.
Charge time (empty → full)
5 h 45 m
Charge rate
0.20C
gentle — fine for any battery including lead-acid
Time ≈ capacity ÷ charge current, plus ~10–20% for charging losses. The C-rate is the current relative to capacity (a 100 Ah battery at 20 A = 0.2C). Keep it gentle for lead-acid (~0.1–0.3C); lithium (LiFePO4) accepts faster charging (0.5C and up). For solar, the "charge current" is what your panels/controller actually deliver, which varies through the day — so real charging takes longer than this best-case figure. 🔒 In your browser.
How the battery charge time calculator works
Charge time is roughly the battery's capacity divided by the current going into it, plus a bit extra because charging isn't perfectly efficient. The tool computes that time and the C-rate — the charge current relative to the capacity — which tells you whether you're charging gently or fast, and whether that's safe for your battery chemistry.
The C-rate is the safety check: lead-acid batteries want a gentle charge (about 0.1–0.3C), while lithium (LiFePO4) accepts much faster charging (0.5C and beyond). For solar charging, the "charge current" is what your panels and controller actually deliver, which rises and falls through the day and with cloud cover — so real charging takes longer than this best-case figure suggests.
Frequently asked questions
How long does it take to charge a battery?
Roughly capacity (Ah) ÷ charge current (A), plus 10–20% for charging losses. A 100 Ah battery at 20 A takes about 100 ÷ 20 × 1.15 ≈ 5.75 hours from empty to full.
What is C-rate?
The charge (or discharge) current relative to capacity: a 100 Ah battery charged at 20 A is at 0.2C, at 50 A is 0.5C. It tells you how gentle or aggressive the charge is — important because each battery chemistry has a safe maximum.
What is a safe charge rate for my battery?
Lead-acid prefers a gentle 0.1–0.3C (10–30 A per 100 Ah); lithium (LiFePO4) commonly accepts 0.5C and many cells up to 1C. Charging faster than the recommended C-rate shortens life or trips protection — check your battery's spec.
Why does solar charging take longer than this?
Because panels don't deliver full current all day — output ramps up and down with the sun and drops under cloud. The calculator assumes a steady charge current, so treat its time as a best case and expect real solar charging to take longer.
Why add a losses allowance?
Charging isn't 100% efficient — some energy becomes heat, especially near full charge for lead-acid. Adding roughly 10–20% to the ideal time gives a more realistic figure for a full charge.