Battery Charge Time Calculator: How long does it take to charge the house battery?

A 100 Ah LiFePO4 battery is full from 20% in about three hours with a 30 A charger. An AGM battery of the same size takes considerably longer with the same charger, because the last 20% is charged with a decreasing current in the absorption phase. This calculator estimates the charge time for a shore power charger or DC-DC charger.

Estimated charge time

2 h 50 min

from 20% to 100% at 30 A

Charge to replace: 80 Ah; ≙ 1,024 Wh
Constant-current phase: 2 h 35 min
Absorption/CV phase: 15 min
Charging current relative to capacity: 0.3C

LiFePO4 usually must not be charged below 32 °F (0 °C) – in the cold the BMS disables charging or the cells get damaged.

Show calculation

1To recharge: 100 Ah × (100% − 20%) = 80 Ah
2Effective charging current: 30 A × 97% = 29.1 A
3Constant-current phase to 95%: 75 Ah ÷ 29.1 A = 2.58 h
4Absorption phase from 95% (factor 1.5): 5 Ah ÷ 29.1 A × 1.5 = 0.26 h
5Total: 2.84 h (2 h 50 min)

How it's calculated

The charge to be replaced follows from the capacity and the difference between target and starting state of charge. Divided by the effective charging current, this gives the charge time of the constant-current phase.

Charge [Ah] = capacity [Ah] × (target SoC − start SoC)
Time [h] = charge [Ah] ÷ (charging current [A] × charge efficiency)

Example: 100 Ah × (100% − 20%) = 80 Ah; 80 Ah ÷ (30 A × 0.97) ≈ 2.75 h

Why the last few percent take longer

Chargers use the CC/CV method: first the full current flows until the charge voltage is reached (constant-current or bulk phase). Then the charger holds the voltage constant and the current drops (absorption phase). For lead-acid batteries this phase begins at about 80% state of charge and takes a long time – the calculator allows three times the time for it. For LiFePO4 the CV phase is short; it is accounted for from 95% with a factor of 1.5.

The factors are approximations. Charger, charge voltage, temperature and age affect the real duration.

Worked example

A 200 Ah AGM battery is discharged to 50% and is to be fully recharged with a 25 A charger.

Inputs

Battery capacity: 200 Ah
Battery type: AGM (lead-acid)
Charger output current: 25 A
State of charge at start: 50 %
Target state of charge: 100 %
Charge efficiency: 85 %

Result

8 h 28 min

Estimated charge time

Charge to replace: 100 Ah
Constant-current phase: 2 h 49 min
Absorption/CV phase: 5 h 39 min
Charging current relative to capacity: 0.13C

Calculation

To recharge: 200 Ah × (100% − 50%) = 100 Ah
Effective charging current: 25 A × 85% = 21.3 A
Constant-current phase to 80%: 60 Ah ÷ 21.3 A = 2.82 h
Absorption phase from 80% (factor 3): 40 Ah ÷ 21.3 A × 3 = 5.65 h
Total: 8.47 h (8 h 28 min)

The variables explained

Battery capacity (Ah): Rated capacity per the data sheet.
Charging current (A): Maximum output current of the charger or DC-DC charger. The actual current may be limited by the BMS or the temperature.
State of charge (SoC): State of charge in percent. For lead-acid it can be derived from the resting voltage; for LiFePO4 only a battery monitor (shunt) is reliable.
Charge efficiency (%): Ratio of stored to supplied charge. LiFePO4 about 95–99%, lead-acid about 80–90%.

Common mistakes

Choosing a charger that's too small: a 10 A charger needs over eight hours for 80 Ah – no problem overnight on shore power, but a problem during the day on the road.
Forgetting the cold with LiFePO4: below 32 °F (0 °C) there's no charging (BMS lock-out) – plan for a heating pad or waiting time.
Calculating with the charger's current even though the BMS allows less.
Never fully charging lead-acid batteries: if you regularly cut the absorption phase short, you slowly lose capacity.

Assumptions and limits

Constant charging current in the constant-current phase; temperature compensation and the charge profiles of individual devices are not modeled.
Absorption factors: lead-acid from 80% SoC factor 3, LiFePO4 from 95% factor 1.5 (approximation).
Loads running in parallel during charging are not considered – they extend the charge time.

Frequently asked questions

How long does a DC-DC charger take to charge the battery while driving?

A 30 A DC-DC charger delivers about 29 Ah per hour of driving (at 97% efficiency) – a 100 Ah LiFePO4 is full from 20% after just under three hours of driving. The DC-DC charger calculator works that out including driving time.

Sources and background

CC/CV charging method (IU per DIN 41773); manufacturer data on charging currents and efficiencies (LiFePO4, AGM, gel)