Battery calculator

Battery Charger Size Calculator

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.

Minimum charger output16.7 A

Calculation breakdown

Calculation breakdown
Battery charging current	16.7 A
Active-load allowance	0 A
Charger rate	0.17 C
Approximate DC output	213 W
Finishing-stage allowance	30 min

Charger A = bulk Ah ÷ (bulk hours × efficiency) + active load
= 75 Ah ÷ (4.5 h × 100%) + 0 A
= 16.7 A

Round up to an available charger size, then confirm its voltage profile and current are approved by the battery manufacturer.

Before you rely on this

Use only a charger with the correct chemistry profile and voltage. Confirm the calculated current against battery, BMS, cabling and temperature limits.

Method

How the number is reached

The available window is split into a constant-current bulk period and a finishing period where current tapers. Only amp-hours below the chemistry's bulk-end SOC are divided by time.

Active DC loads are added because charger nameplate current must cover both the battery and equipment operating during charging.

Bulk Ah = capacity × bulk SOC span
Bulk hours = available hours − finishing allowance
Charger A = Bulk Ah ÷ (bulk hours × efficiency) + active load

Symbols

SOC: State of charge
C: Charger current divided by battery Ah

Worked examples

The same maths, applied

Example

100 Ah LiFePO4 from 20% to full in five hours

A lithium battery has 4.5 hours for bulk charging after a 30-minute finishing allowance.

Bulk Ah = 100 × (95% − 20%) = 75 Ah
Current = 75 Ah ÷ 4.5 h = 16.67 A

Result: 16.7 A minimum charger output.

Limits of the model

What it assumes, and where it stops

Assumptions

Charger current remains available throughout bulk charging.

The finishing allowance matches the battery and charger profile.

Not covered

Actual charger output can derate with AC input limits, temperature and cabling.

This does not select charging voltage or certify chemistry compatibility.

Sources

Where these figures come from

Blue Smart IP67 Charger — Operation and charge-current limitsVictron EnergyDocuments bulk-current sizing, staged charge time and typical maximum charge-rate guidance for lead-acid and LiFePO4 batteries.
Blue Smart IP65 Charger — Estimating charge timeVictron EnergyManufacturer guidance for staged charging: Ah divided by net charge current for the bulk stage, about 80% SOC before lead-acid absorption, above 95% for lithium, and the different finishing-stage durations.
BMV-700 — Charge efficiency factorVictron EnergyExplains amp-hour charge efficiency, why it varies with battery type and use, and the monitor's 95% default factor.

Next steps

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Last meaningful update: 2026-08-19. This date changes only when the model, the sources or the guidance change.