How long does it take to charge a car battery?
A typical car battery holds 40 to 80 Ah. A 60 Ah battery that is half flat needs 30 Ah, or about 33 Ah at lead-acid's 90% efficiency. On a 10 A charger that takes 3 hr 20 min. On a 2 A trickle charger it takes 16 hr 40 min.
Real chargers taper off near full, so the last 20 to 30% takes longer than a straight division suggests. Battery University puts a full lead-acid charge at 12 to 16 hours. The car battery charging guide walks through it.
hours = Ah x share to fill / (charger amps x 0.9)
12V battery charging time chart
From 50% to full, lead-acid at 90% efficiency.
| Battery | 2 A | 4 A | 10 A | 15 A | 25 A |
|---|---|---|---|---|---|
| 40 Ah | 11 hr 7 min | 5 hr 33 min | 2 hr 13 min | 1 hr 29 min | 53 min |
| 50 Ah | 13 hr 53 min | 6 hr 57 min | 2 hr 47 min | 1 hr 51 min | 1 hr 7 min |
| 60 Ah | 16 hr 40 min | 8 hr 20 min | 3 hr 20 min | 2 hr 13 min | 1 hr 20 min |
| 70 Ah | 19 hr 27 min | 9 hr 43 min | 3 hr 53 min | 2 hr 36 min | 1 hr 33 min |
| 80 Ah | 22 hr 13 min | 11 hr 7 min | 4 hr 27 min | 2 hr 58 min | 1 hr 47 min |
| 100 Ah | 27 hr 47 min | 13 hr 53 min | 5 hr 33 min | 3 hr 42 min | 2 hr 13 min |
What about charging by driving?
An alternator can refill a battery after a start, but a deeply flat battery is better recharged on a plug-in charger, which holds a steady current and handles the slow top-off. The battery charging current calculator shows the amps a battery can take.