Battery Voltage Sag Calculator

From inputs to result

For this worksheet, the governing relationship is Vload = Voc − sag. The entered quantities are Open-circuit voltage and Sag at load.

The initial scenario returns 12.10 V. Re-enter field data before using the answer in a design or comparison.

Calculate loaded voltage from open-circuit voltage and sag at load. Keep Battery Internal Resistance Calculator available for the companion calculation.

Measurements and entries

Use voltage and capacity values from the same battery condition. If operating conditions changed between readings, calculate separate cases.

A prefix error can move the result by several orders of magnitude.

Open-circuit voltage

Default example: 12.8 V. Enter open-circuit voltage in V.

Sag at load

Default example: 0.7 V. Enter sag at load in V.

Checks that catch bad inputs

A plausible-looking output does not prove that the entries are compatible.

Verify sign, scale, and physical meaning before accepting the number.

Assumptions and limitations

Transient and steady-state sag can differ.

Outside the entered variables, consider chemistry-specific limits and manufacturer test conditions. Do not hide omitted behavior inside an unexplained correction factor.

Use voltage and capacity values from the same battery condition.

Sensitivity check

Change Open-circuit voltage from 12.8 V to 15.36 V and preserve the rest of the case. This moves the answer from 12.10 V to 14.66 V.

This isolates one variable; a real system may change several at once.

Using this result in a larger analysis

Check the source of open-circuit voltage before calculating. A credible result requires every field to describe the same physical condition and unit basis.

After calculating loaded voltage, compare its magnitude with a warm, cold, new, and aged battery case where relevant. An independent order-of-magnitude check can reveal a misplaced decimal or unit prefix.

Document any effect handled outside this page, especially chemistry-specific limits and manufacturer test conditions. That note prevents a later reader from assuming the simple equation covered it.

Conditions that deserve another case

The calculator rejects values outside its stated field limits, but a numerically valid entry can still describe the wrong condition. Confirm open-circuit voltage and sag at load against the original source before saving the result.

Use an additional case for temperature, cell imbalance, BMS cutoffs, aging, and rate dependence when that behavior is not negligible. Keep the baseline intact so the effect of each changed assumption remains visible.

Questions about Battery Voltage Sag