With an 8-amp charger, a typical car battery that is about half discharged may take roughly 4 to 5 hours, and a very flat one may take around 8 to 10 hours or more. These are estimates: the real time depends on the battery’s size, condition and temperature.
Short answer for 8 amps
For a common 60 Ah (amp-hour) car battery:
- About 50% discharged: roughly 4 to 5 hours
- About 80% discharged: roughly 7 to 8 hours
- Fully flat: often 8 to 10 hours or longer
Your battery’s amp-hour rating may differ, so check the label or manual.
The basic math
A rough formula:
Hours = (amp-hours to replace ÷ charger amps) × about 1.2
The extra 20% or so accounts for charging losses and the slower final stage. For example, a 60 Ah battery that is 50% discharged needs about 30 Ah back. At 8 amps, that is 30 ÷ 8 = 3.75 hours, or around 4.5 hours with the allowance. Smart chargers taper their current near the end, so they can take longer than the simple figure.
Estimated times table (60 Ah example)
| Charger output | 50% discharged | Fully flat |
|---|---|---|
| 2 amps | about 15 to 18 hours | about 30 to 36 hours |
| 4 amps | about 8 to 9 hours | about 15 to 18 hours |
| 8 amps | about 4 to 5 hours | about 8 to 10 hours |
| 10 amps | about 3 to 4 hours | about 6 to 8 hours |
Treat these as ballpark figures only. Your charger’s manual may give its own guidance.
Is 8 amps safe?
For many standard 12 V car batteries, a rate of about 8 to 10 amps is generally within normal use, and a smart charger will manage the current for you. A common rule of thumb is to keep the charge rate to roughly 10 to 20% of the battery’s amp-hour rating, which puts 8 amps in range for a 60 Ah battery. Smaller batteries (motorcycle, lawn tractor) may need a much lower rate, so check the battery’s label or manual. If the battery becomes hot, swells or smells strongly, disconnect and stop.
Slower rates: 2 amps and maintainers
A 2-amp setting is gentle and suits topping up or maintaining a battery, but a flat car battery may need a day or more. With a smart charger or maintainer, it can usually stay connected for long periods, since it reduces output when full. A basic charger without automatic shutoff should not be left on for days. See how to choose a car battery charger for the differences.
Faster rates: 10 amps and above
At 10 amps a dead battery may take several hours. Higher “boost” or “engine start” settings (often 40 amps or more) are meant for short use and are not for routine full charging, since high current can heat the battery. If your charger has such a mode, read its instructions first.
What changes the time
- Battery size: Larger amp-hour ratings take longer.
- Depth of discharge: The flatter the battery, the longer it takes.
- Battery age and health: Old or sulfated batteries may charge slowly or never fully.
- Temperature: Cold batteries accept charge more slowly.
- Charger type: Smart chargers taper current at the end.
- Battery type: AGM, gel and lithium batteries may need specific charger settings.
Charging by driving
Driving can recharge a battery, but alternators are designed mainly to keep a healthy battery topped up, not to fully recharge a flat one. A long drive may help, but a short trip often will not. If the battery is badly drained, a charger is usually the better option. For the connection steps, see how to use a car battery charger.
FAQ
How long does an 8-amp charger take? Around 4 to 5 hours for a half-discharged 60 Ah battery, longer if it is flatter.
How long can I leave a battery on 2 amps? With a smart charger or maintainer, usually long periods; with a basic charger, follow the manual and avoid leaving it unattended for days.
How long with 10 amps for a dead battery? Often around 6 to 8 hours for a 60 Ah battery, as an estimate.
How do I know it is done? Use the charger’s full indicator, or check resting voltage afterward, which is typically around 12.6 V or higher for a healthy, full lead-acid battery.