How long does it take to charge a car battery?
About 7.5 hours for a 50 Ah battery at half charge on a 5 amp charger, and twice that from flat. The rule behind it: amp-hours to put back, divided by charger amps, times 1.5.
Checked against sources on by the TrueAmps editorial team.
Charge time estimate
Where the 1.5 comes from
NOCO prints a charge-time table in its GENIUS manuals, measured from 50 percent charge. For the 1 amp GENIUS1 it lists 18 hours for a 24 Ah battery and 22.5 hours for 30 Ah (GENIUS1 user guide). For the 5 amp GENIUS5 it lists 3 hours for 20 Ah and 15 hours for 100 Ah (GENIUS5 user guide).
Put any of those rows through the arithmetic and the same ratio falls out. Half of 30 Ah is 15 Ah; at 1 amp that is 15 hours; NOCO says 22.5. Half of 100 Ah at 5 amps is 10 hours; NOCO says 15. The extra half accounts for charging losses and for the slow final stage, when a smart charger tapers the current to top the battery off.
One assumption is ours, not NOCO's: the tables are built on a half-charged battery, and the calculator applies the same 1.5 ratio when you pick flat, 25 or 75 percent. Treat those as estimates.
Charge times by battery size
Hours from half charged to full. Double them for a flat battery.
| Battery | 2 A charger | 5 A charger | 10 A charger |
|---|---|---|---|
| 40 Ah | 15 h | 6.0 h | 3.0 h |
| 50 Ah | 19 h | 7.5 h | 3.8 h |
| 60 Ah | 23 h | 9.0 h | 4.5 h |
| 70 Ah | 26 h | 11 h | 5.3 h |
| 100 Ah | 38 h | 15 h | 7.5 h |
When it takes longer than the estimate
NOCO's manual lists temperature and battery condition as the two things that move the real number. A battery that has sat flat for months may need a recovery or repair mode before normal charging even starts; on the GENIUS5 that is a separate mode that runs up to 16.5 volts, according to its manual.
The charger also has a ceiling. NOCO rates the GENIUS1 to recharge batteries up to 30 Ah and the GENIUS5 up to 120 Ah. Above that, NOCO rates these chargers to maintain the battery, not to recharge it. The calculator warns you when your battery is past the rating of the charger you picked.
Chargers in the calculator
| Charger | Amps | Recharges up to |
|---|---|---|
| NOCO GENIUS1 | 1 | 30 Ah |
| Battery Tender Junior Multi-Chemistry | 1 | not published |
| NOCO GENIUS2 | 2 | 40 Ah |
| NOCO GENIUS2D | 2 | 40 Ah |
| NOCO GENIUS5 | 5 | 120 Ah |
| NOCO GENIUS10 | 10 | 230 Ah |
Questions people ask
How long does it take to charge a car battery?
With a 5 amp charger, a 50 Ah battery at half charge takes about 7.5 hours and a fully flat one about 15 hours. Divide the amp-hours you need to put back by the charger current, then multiply by 1.5, the ratio in NOCO's own charge-time tables.
How long does a 10 amp charger take on a car battery?
About 3.75 hours for a 50 Ah battery at half charge and 7.5 hours from flat, using the same 1.5 ratio.
How long to charge a car battery with a 2 amp charger?
About 19 hours for a 50 Ah battery at half charge. A 2 amp charger is better suited to keeping a battery topped up than recharging a flat one.
Does driving charge the battery as fast as a charger?
We have not found a manufacturer source that gives a reliable figure, so we do not state one. A charger on a timer is the case we can calculate.