Battery Charge Time Calculator
How long until the batteries are back to full after an outage?
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Battery Charge Time Calculator by TheCalcTools
How the Battery Charge Time Calculator works
Enter the bank capacity and how deeply it was discharged.
Enter the charge current the rectifier can spare after supplying the load.
Time = Ah to return ÷ (current × efficiency). The last 10–20% takes longer in practice due to the absorption stage.
The formula
Worked example
Using the default values shown in the calculator:
| Bank capacity (Ah) | 400 |
|---|---|
| Discharged (% of capacity) | 60 % |
| Charge current (A) | 40 |
| Charge efficiency (%) | 90 % |
- Ah to return = 400 × 60% = 240 Ah.
- At 40 A and 90% efficiency, charging takes about 6.7 hours (C/10.0).
Result:
| Ah to return | 240 |
|---|---|
| Charge time (hours) | 6.7 |
| Charge rate (C/n) | C/10.0 |
| Charge current check | Within common limits (0.05C–0.2C) |
Frequently asked questions
What is a safe charge current?
VRLA batteries are commonly charged at 0.1–0.2C (10–20% of Ah). Check the manufacturer’s limit and the temperature compensation.
Why does charging take longer than calculated?
As voltage reaches the absorption level, the current tapers. Allow extra time, especially to reach 100%.
Sources and further reading
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This calculator gives estimates for planning only. Electrical installations must be designed, checked and carried out by qualified personnel following local codes and the equipment manufacturer's data. Never rely on this result alone for safety-critical decisions. See our disclaimer.