How long does a battery take to charge?
Divide the charge the battery needs by the charger's current, then allow for charging losses. A 100 Ah lead-acid battery on a 10 A charger needs 100 Ah, or about 111 Ah at 90% efficiency, so it takes about 11 hr 7 min from empty. From half full it takes 5 hr 33 min.
The calculator does the same math for any size. Pick the battery type and it sets a typical charge efficiency: about 90% for lead-acid, over 99% for lithium and 70% to 90% for NiMH, the figures in Battery University's efficiency notes.
charging time (hours) = battery Ah x share to fill / (charger amps x efficiency)
Battery charging time chart
Hours from empty to full for common battery sizes, at lead-acid's 90% efficiency. Lithium charges about 10% faster on the same charger.
| Battery | 2 A | 5 A | 10 A | 20 A | 40 A |
|---|---|---|---|---|---|
| 20 Ah | 11 hr 7 min | 4 hr 27 min | 2 hr 13 min | 1 hr 7 min | 33 min |
| 50 Ah | 27 hr 47 min | 11 hr 7 min | 5 hr 33 min | 2 hr 47 min | 1 hr 23 min |
| 100 Ah | 2.3 days | 22 hr 13 min | 11 hr 7 min | 5 hr 33 min | 2 hr 47 min |
| 200 Ah | 4.6 days | 44 hr 27 min | 22 hr 13 min | 11 hr 7 min | 5 hr 33 min |
| 300 Ah | 6.9 days | 2.8 days | 33 hr 20 min | 16 hr 40 min | 8 hr 20 min |
Charging a phone or power bank
Small batteries are rated in milliamp-hours (mAh) and chargers in watts, so the mAh tab converts the battery to watt-hours first: mAh x 3.7 V / 1000. A 5,000 mAh battery holds 18.5 Wh. On a 20 W charger at 85% efficiency that is about 65 minutes of charging, before the slow top-off that phones use past about 80%.
Why the last part takes longer
Chargers slow down near full to protect the battery. For lead-acid, Battery University puts the fast stage at about 70% in 5 to 8 hours, then a topping charge that takes another 7 to 10 hours. Lithium reaches about 85% at the end of its fast stage and tops off from there (BU-409). The calculator shows the steady-current time; read it as the minimum for the last stretch. The lithium vs lead-acid charging guide covers the stages.