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Battery charging time calculator
How long will it take to charge your battery? Estimates bulk and absorption time for LiFePO4, AGM, gel and flooded batteries from charger amps, plus wall power.
Results
| Estimated charging time | 4 h 20 min |
|---|---|
| Bulk phase (full current) | 3 h 50 min |
| Absorption phase (tapering) | 0 h 30 min |
| Charge put into the battery | 81.6 Ah |
| Charge rate (share of capacity per hour) | 20 % |
| Peak charger output | 288 W |
| Peak power from the wall or generator | 327 W |
| Energy from the wall (approx.) | 1.26 kWh |
- LiFePO4 must not be charged below 0 °C (32 °F) unless the battery has built-in heating or low-temperature protection.
About this calculator
"Amp-hours divided by charger amps" is the rule of thumb everyone starts with, and it's only half right. Batteries charge in stages. In the bulk stage the charger pushes its full current while the voltage climbs. Once the battery reaches its absorption voltage, the charger holds that voltage and the current tapers off as the battery fills. That absorption stage is short for LiFePO4 but can take hours for lead-acid, which is why a lead-acid battery that's "80% full in two hours" can take another two to four hours to reach 100%.
This calculator models both stages for LiFePO4, AGM, gel and flooded batteries. It also tells you how much power the charger pulls from the wall or a generator, and warns you if the charger is too big or too small for the battery.
How to use it
- Enter the battery's amp-hour rating and voltage. For a bank, use the total Ah of the bank at its voltage.
- Choose the chemistry. It sets where the bulk stage ends (about 95% for LiFePO4 and 80% for lead-acid) and the typical absorption time.
- Enter the current state of charge (the battery voltage chart can help) and your target.
- Enter the charger's DC output amps from its label. For a DC-DC charger or solar controller, use the amps it actually delivers.
- Optionally enter the absorption time your charger is set to, and its efficiency, to estimate wall or generator power.
The formula
Bulk time = Ah from the starting charge up to the bulk limit ÷ (charger amps × charge efficiency). The bulk limit is 95% for LiFePO4 and 80% for lead-acid. Charge efficiency is 98% for LiFePO4 and 95% for lead-acid, the values Victron uses for its battery monitors.
Absorption time = typical absorption time × the share of the absorption range you need to cover. The calculator never allows less time than the remaining Ah ÷ (amps × efficiency). Typical absorption times are 0.5 h for LiFePO4 (charge to 3.65 V per cell, then hold until current falls to 5% of capacity, per the EVE cell specification) and Trojan's maximum absorption times of 2 h for AGM and gel and 4 h for flooded.
Peak charger output = amps × absorption voltage (3.6 V per LiFePO4 cell, 2.40-2.45 V per lead-acid cell). Wall power = output ÷ charger efficiency. Wall energy ≈ Ah in × average of nominal and absorption voltage ÷ charger efficiency.
Recommended charge current
| Chemistry | Typical max charge current | Absorption voltage (12 V) | Source |
|---|---|---|---|
| LiFePO4 | 0.5C (50 A for 100 Ah) | 14.4-14.6 V | EVE cell specification (standard charge) |
| AGM | 20% of C20 (20 A for 100 Ah) | 14.4 V | Trojan User's Guide, Table 6 |
| Gel | 10-13% of C20 | 14.4 V | Trojan User's Guide, Table 6 |
| Flooded | 10-13% of C20 | 14.7 V | Trojan User's Guide, Table 5 |
Sources: Trojan Battery User's Guide, EVE LF80 LiFePO4 cell specification, Victron SmartShunt quick install guide (charge efficiency). Always follow your own battery's datasheet where it differs.
Worked example (default values)
Inputs
| Battery capacity | 100 Ah |
|---|---|
| Battery voltage | 12 V |
| Chemistry | LiFePO4 |
| Starting state of charge | 20 % |
| Target state of charge | 100 % |
| Charger output current | 20 A |
| Absorption time at full charge (0 = typical for chemistry) | 0 h |
| Charger efficiency (wall to battery) | 88 % |
Results
| Estimated charging time | 4 h 20 min |
|---|---|
| Bulk phase (full current) | 3 h 50 min |
| Absorption phase (tapering) | 0 h 30 min |
| Charge put into the battery | 81.6 Ah |
| Charge rate (share of capacity per hour) | 20 % |
| Peak charger output | 288 W |
| Peak power from the wall or generator | 327 W |
| Energy from the wall (approx.) | 1.26 kWh |
Tips and common mistakes
Loads steal charging current. If a fridge or inverter is running while you charge, subtract its current from the charger's output. A 20 A charger feeding a 5 A fridge only puts 15 A into the battery.
Alternator and DC-DC charging follows the same math. Enter the DC-DC charger's rated output (often 20-50 A), and remember it only charges while the engine runs.
Generator charging is most fuel-efficient in bulk. With lead-acid, running a generator through the long absorption tail burns fuel for very little energy. A common approach is to charge to about 80-90% with the generator and let solar finish the absorption stage.
Bigger isn't always faster for lead-acid. Doubling the charger roughly halves the bulk stage, but absorption is limited by the battery's chemistry, not the charger. For LiFePO4 nearly the whole charge is bulk, so charger size matters almost linearly.
Frequently asked questions
How long does it take to charge a 100Ah lithium battery?
With a 20 A charger from 20% to 100%, about 4 hours 20 minutes: about 3.8 hours of full-current bulk charging plus roughly 30 minutes of absorption. A 10 A charger takes about twice as long, and a 50 A charger about 2 hours.
How long does it take to charge a 100Ah AGM battery?
From 50% with a 20 A charger, roughly 3.5-4 hours: about 1.6 hours of bulk charging to around 80%, then up to about 2 hours of absorption (Trojan's maximum for AGM) as the current tapers. A small charger takes much longer: with 5 A it is more like 10 hours.
Can I use any charger for a LiFePO4 battery?
Use a charger with a LiFePO4 profile, or one that lets you set about 14.2-14.6 V absorption and either no float or a low float around 13.5 V. Avoid lead-acid equalization and desulfation modes, which can push the voltage too high.
Why does my battery charge slower at the end?
Once the battery reaches its absorption voltage the charger holds the voltage steady and the current falls as the battery fills. It's normal, and it protects the battery.
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Estimates for planning only, based on the assumptions shown. Real-world performance varies with equipment, temperature and installation. Updated October 19, 2026. See our methodology.