--- title: "Battery Charge Time Calculator: How Long to Charge?" description: "How long does it take to charge a battery? Enter Ah, charger amps and state of charge to get the charging time for lithium, AGM and lead-acid batteries." canonical: https://batteryruntimecalculator.com/battery-charge-time-calculator language: en updated: 2026-11-10 source_revision: 59f9d32 maintainer: Battery Runtime Calculator (hello@batteryruntimecalculator.com) --- # Battery Charge Time Calculator Work out how long it takes to charge a battery from any state of charge, using the charger’s amps and the battery type. Updated November 10, 2026 Quick answer Charging time ≈ Ah to put back ÷ (charger amps × efficiency), plus extra time for the slow final stage. A 100 Ah LiFePO4 battery from 20% to full on a 20 A charger takes about **4 h 28 min**; a 100 Ah AGM battery from 50% on a 10 A charger takes about **8 h 14 min**. Estimated charging time 4 h 28 min 20% → 100%, ≈ 4.5 hours Full-current (bulk) 3 h 57 min Tapering (absorption) 32 min Charge rate 0.2C Charge efficiency 95% 80 Ah ÷ 95% = 84.2 Ah from a 20 A charger, tapering above 95% Charging lithium to 80–90% for daily use, and to 100% only when you need the full range, helps it last longer. ## Battery charging time formula ``` Ah to add = capacity (Ah) × (target % − current %) Charge time (h) = Ah to add ÷ (charger A × efficiency) ``` That simple formula is right for the part of the charge at full current. The calculator adds two corrections: - **Charge efficiency.** Some energy is lost as heat, so more amp-hours go in than the battery stores: about 95% efficiency for lithium and 85% for lead-acid. - **The taper.** Above about 95% for lithium and 80% for lead-acid, the charger holds the voltage and the current drops. We count that stage at 50% of the charger’s current on average. Worked example for the default: a 100 Ah LiFePO4 battery from 20% on a 20 A charger. The bulk stage, 20% to 95%, takes 3.95 h; the final 5% takes 0.53 h at the lower current; 4.47 hours in total. ## How long does it take to charge a 100Ah battery? Charging time for a 100 Ah battery by charger size. LiFePO4 is charged from empty; AGM from 50%, the deepest you should normally discharge it. | Charger | LiFePO4, 0–100% | LiFePO4, 20–80% | AGM, 50–100% | | --- | --- | --- | --- | | 5 A | 22 h 6 min | 12 h 38 min | 16 h 28 min | | 10 A | 11 h 3 min | 6 h 19 min | 8 h 14 min | | 20 A | 5 h 32 min | 3 h 9 min | 4 h 7 min | | 30 A | 3 h 41 min | 2 h 6 min | 2 h 45 min | | 50 A | 2 h 13 min | 1 h 16 min | 1 h 39 min | Doubling a lead-acid charger’s current does not halve the time, because only the bulk stage speeds up. For lithium, the time falls almost in proportion, as long as the charger stays within the battery’s maximum charge current. ## Charge time chart by battery size LiFePO4 batteries charged from 20% to 100%. | Battery | 10 A charger | 20 A charger | 40 A charger | | --- | --- | --- | --- | | 50 Ah | 4 h 28 min | 2 h 14 min | 1 h 7 min | | 100 Ah | 8 h 57 min | 4 h 28 min | 2 h 14 min | | 200 Ah | 17 h 54 min | 8 h 57 min | 4 h 28 min | | 300 Ah | 26 h 51 min | 13 h 25 min | 6 h 43 min | ## Charging from solar panels or a car alternator Solar charge controllers and DC-DC chargers are rated in amps, so enter that figure as the charger current. If you only know the panel watts, an MPPT controller delivers roughly panel watts × 0.8 ÷ battery voltage in good sun: a 200 W panel charging a 12.8 V battery gives about 12.5 A. Solar output varies through the day, so treat the result as hours of good sun, not clock hours. To size a solar battery bank, use the [solar battery calculator](https://batteryruntimecalculator.com/solar-battery-calculator). Charge time is the other half of runtime: the [battery runtime calculator](https://batteryruntimecalculator.com/) tells you how long a full battery lasts, and the [battery capacity calculator](https://batteryruntimecalculator.com/battery-size-calculator) how big it needs to be. The method behind all the calculators is on [how battery runtime is calculated](https://batteryruntimecalculator.com/how-battery-runtime-is-calculated). ## Frequently asked questions ### How long does it take to charge a 100Ah battery? With a 20 A charger, a 100 Ah LiFePO4 battery takes about 4 h 28 min from 20% to full, and about 5 h 32 min from empty. A 100 Ah AGM battery charged from 50% takes about 4 h 7 min on 20 A and 8 h 14 min on 10 A, most of it in the slow absorption stage. ### How long does it take to charge a car battery at 10 amps? A typical 60 Ah car battery that is half flat takes about 4 h 56 min to charge fully at 10 A. Getting it to 80%, enough to start the car, takes about 2 h 7 min. A completely flat car battery takes longer and may be damaged; have it tested. ### What size charger do I need for a 100Ah battery? For lead-acid (AGM, gel or flooded), a charger of 10–30 A, which is 10–30% of the Ah rating, is the usual recommendation. LiFePO4 batteries accept more: 20–50 A for a 100 Ah battery is common, up to the maximum charge current in the battery’s datasheet. A bigger charger shortens the bulk stage but not the absorption stage of lead-acid. ### Why does the last 20% take so long to charge? Chargers switch from constant current to constant voltage as the battery nears full, and the current then falls away. Lead-acid enters this absorption stage at about 80% charge, so the last fifth can take as long as everything before it. Lithium batteries stay at full current until about 95%, which is why they charge much faster. ### Can I charge a LiFePO4 battery with a lead-acid charger? Only if the charger has a lithium or LiFePO4 mode. Lead-acid chargers may use a desulfation or equalisation stage at high voltage, or a float voltage that keeps the lithium battery at 100% all the time, which shortens its life. A charger with a LiFePO4 profile (about 14.2–14.6 V for a 12 V battery) is the safe choice. ## More battery calculators - [Battery runtimeRuntime for any battery and load, in Ah or Wh, AC or DC.](https://batteryruntimecalculator.com/) - [Battery life (mAh)mAh and mA to hours, with sleep current and duty cycle.](https://batteryruntimecalculator.com/battery-life-calculator) - [Battery sizeHours you need in, battery Ah, Wh and count out.](https://batteryruntimecalculator.com/battery-size-calculator) - [Solar battery bankDaily kWh and days of autonomy to a battery bank size.](https://batteryruntimecalculator.com/solar-battery-calculator) - [UPS runtimeBackup minutes for a computer, router or NAS on a UPS.](https://batteryruntimecalculator.com/ups-runtime-calculator) - [Inverter backupHome inverter backup hours for 150 Ah to 220 Ah batteries.](https://batteryruntimecalculator.com/inverter-battery-backup-calculator) - [Power station runtimeWh rating to runtime for portable power stations.](https://batteryruntimecalculator.com/power-station-runtime-calculator) - [12V battery runtime12V deep-cycle runtime for RVs, vans and boats.](https://batteryruntimecalculator.com/12v-battery-runtime-calculator) - [Ah to Wh converterAmp hours, watt hours and kWh at any voltage.](https://batteryruntimecalculator.com/ah-to-wh-calculator) - [mAh to Wh convertermAh to Wh for power banks, with the 100 Wh flight limit.](https://batteryruntimecalculator.com/mah-to-wh-calculator) - [How runtime is calculatedThe formulas behind every calculator, with worked examples.](https://batteryruntimecalculator.com/how-battery-runtime-is-calculated)