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How long to charge by driving
From 20% to 100%, with a DC-DC charger at 90% efficiency. Each cell is hours of engine running at that charging current.
| Battery | 20 A | 40 A | 60 A |
|---|---|---|---|
| 100 Ah lithium | 4 h 13 min | 2 h 6 min | 1 h 24 min |
| 200 Ah lithium | 8 h 25 min | 4 h 13 min | 2 h 48 min |
| 300 Ah lithium | 12 h 38 min | 6 h 19 min | 4 h 13 min |
The charging current matters more than the battery size. A big battery on a small charger takes many hours, and a 20 amp charger can need a long drive to fill a large bank.
How it is calculated
- Energy to put in (Wh) = capacity × (target % − start %) ÷ battery charging efficiency (95% lithium, 80% lead-acid).
- Charging power (W) = charging current × battery voltage.
- Drive time (h) = energy to put in ÷ charging power.
- Load on the alternator (A) = charging power ÷ charger efficiency ÷ 14 V, the typical vehicle charging voltage.
- Battery gained per hour = charging power × battery efficiency ÷ battery capacity.
It assumes the current stays constant. In practice the alternator output, the cable length and the battery’s charge stage all change it.
Using it anywhere
- Amps, volts and Wh are the same everywhere, so nothing needs converting.
- Enter your own system voltage: 12 V is common in vans and trailers, and 24 V in many trucks and boats.
- Numbers follow your region’s format.
What limits charging from an alternator
- Direct connection. A plain cable or relay lets the battery pull whatever the alternator gives, and wiring can overheat. A DC-DC charger limits the current and charges in stages.
- Alternator rating. The extra load should stay well under what the alternator can supply, especially at low engine speed.
- Cable size. Long runs lose voltage. Use the wire gauge and fuse calculator for the cable and fuse.
- Battery acceptance. Lead-acid batteries accept less current as they fill, so the last part takes longer.
Common questions
How long does it take to charge a 100 Ah battery by driving?
With a 40 amp DC-DC charger, a 100 Ah lithium battery goes from 20% to 100% in about 2 h 6 min. With a 20 amp charger it takes about 4 h 13 min, and with a 60 amp charger about 1 h 24 min.
Can my alternator charge a second battery?
Yes, usually through a DC-DC charger or a split-charge relay. Check that your alternator has spare capacity, because it also runs the vehicle. A 40 amp charger adds roughly 40 amps of load at the alternator.
Do I need a DC-DC charger?
A DC-DC charger is strongly recommended for lithium batteries and for modern vehicles with smart alternators. It limits the current, boosts the voltage for proper charging, and protects both batteries. A simple relay can work for lead-acid on older vehicles.
Does idling charge the battery as fast as driving?
Not always. Alternators produce less current at idle, so charging can be slower or limited. Driving at normal speeds usually gives the alternator more to spare.
How many amps can I safely draw from my alternator?
It depends on your alternator’s rating and the vehicle’s own load. Many installers keep the extra continuous load well under the rated output, often around half. Check the alternator’s output at the engine speed you will use.
Why does the last 20% take longer?
Batteries accept less current as they fill. Lead-acid batteries have an absorption stage that can add hours, and lithium batteries taper at the end. The calculator assumes a steady current, so real charging near full is slower.
Related calculators
Size the cable and fuse for the charger with the wire gauge and fuse calculator. Plan the battery with the van and RV solar and battery calculator, and see how long solar takes with the solar charge time calculator.