Battery calculator
Charging is fast while current is constant and slower near full. This estimate separates those stages, accounts for loads using charger current, and shows a range when lead-acid absorption time is genuinely variable.
Calculation breakdown
| Capacity to replace20% to 100% | 80 Ah |
|---|---|
| Net current into charging20 A charger minus active DC load | 20 A |
| Constant-current stage75 Ah below 95% SOC | 3 h 45 min |
| Finishing stageLithium profile | 30 min |
| Charger rate | 0.2 C |
Total time = bulk charge time + finishing-stage time = (75 Ah ÷ (20 A × 100%)) + 30 min = 4 h 15 min
Before you rely on this
Use a charger profile approved for your battery chemistry and stay within the battery and BMS voltage, current and temperature limits. A time estimate cannot make an incompatible charger safe.
Method
A charger's current rating only predicts the constant-current part of a charge. Divide the amp-hours that must be replaced before the finishing stage by the current that actually reaches the battery, after any active DC load and charge-efficiency loss.
Near full charge, voltage is held constant and current tapers. Victron's charger guidance places the start of this stage around 80% SOC for lead-acid and above 95% for lithium. Lithium typically finishes in about 30 minutes; adaptive lead-acid absorption can run from 30 minutes to 8 hours after a deep discharge, so this calculator reports a range instead of inventing a precise midpoint.
Net charge current (A) = Charger current − Active DC load Bulk Ah = Capacity × (bulk-end SOC − starting SOC) Bulk time (h) = Bulk Ah ÷ (net current × charge efficiency) Total time = Bulk time + finishing-stage time
Symbols
Worked examples
Example
A 20 A lithium-compatible charger charges a 100 Ah LiFePO4 battery from 20% to 100%, with no DC loads running.
Bulk charge = 100 Ah × (95% − 20%) = 75 Ah Bulk time = 75 Ah ÷ 20 A = 3.75 h Finishing stage ≈ 0.5 h Total = 3.75 h + 0.5 h = 4.25 h
Result: About 4 h 15 min.
Example
A 10 A charger refills a fully discharged 100 Ah lead-acid battery. Efficiency is set to 100% here to reproduce the staged example in the cited charger manual.
Bulk charge = 100 Ah × 80% = 80 Ah Bulk time = 80 Ah ÷ 10 A = 8 h Adaptive absorption = 0.5–8 h Total = 8.5–16 h
Result: About 8 h 30 min–16 h. The upper end matches the manual's deeply discharged example; a shallower or adaptively managed charge can finish sooner.
Detail
The defaults deliberately separate lithium from lead-acid. Open Advanced assumptions to replace them with the values from your battery and charger manuals.
| Battery type | Bulk stage ends | Finishing-stage window | Charge efficiency |
|---|---|---|---|
| LiFePO4 / lithium-ion | About 95% SOC | About 30 min | 100% starting assumption |
| AGM / gel / flooded lead-acid | About 80% SOC | 30 min to 8 h | 95% starting assumption |
These are editable planning defaults from the cited Victron manuals. Always use the charge profile and current limit specified for your actual battery.
Detail
Limits of the model
Assumptions
Charger output and active DC load stay constant during the bulk stage.
The selected bulk-end SOC and finishing-time window represent the actual battery and charger profile.
If a target crosses only part of the finishing band, its finishing time is scaled in proportion to the SOC span. This is a planning approximation, not a charger-control algorithm.
Charge efficiency is applied to the constant-current stage; taper and late-stage losses are represented by the finishing-time window.
Not covered
The battery or BMS can reduce current because of temperature, voltage, cell balance or its own current limit, making charging slower.
State-of-charge readings are estimates and can drift, particularly on batteries without a calibrated shunt monitor.
Battery age, sulfation and cell imbalance can extend finishing time.
This model assumes a steady charger. Solar output, alternator duty cycles and generator interruptions require source-specific modeling.
It does not choose a safe charging voltage or current. Those must come from the battery manufacturer.
Sources
Next steps
Enter your battery and your load to estimate runtime. The calculator applies depth of discharge, battery health, conversion losses and — for lead-acid only — the Peukert effect, and shows each step of the energy budget.
Turn daily energy use into the nominal kWh and amp-hours your bank needs. The model accounts for days without charging, conversion loss, battery health, usable depth of discharge and whole-battery rounding.
Choose a charging window and SOC target to estimate the DC amps a charger must supply. The model reserves time for current taper near full charge and adds any loads running while charging.
C-rate compares current with battery capacity. A 100 Ah battery at 20 A is running at 0.2 C; the same 20 A is 2 C for a 10 Ah battery.
Amp-hours describe charge, not energy. Multiply by the pack's nominal voltage to get watt-hours, the figure you need for runtime, shipping limits and solar sizing.
Energy targets are quoted in watt-hours, but batteries are sold in amp-hours. Divide by your system voltage to find the capacity you actually need to buy.
Last meaningful update: 2026-08-19. This date changes only when the model, the sources or the guidance change.