Sunstead
Sign in
Battery questions
Battery capacity, charge and health are different
A battery can be fully charged and still hold less energy than it did when new. “100%” needs a denominator.
Give each label its own question
State of charge asks how full the battery is relative to the basis used by the reporting device. Remaining charge may be expressed in Ah. Full capacity describes an estimated or rated amount the battery can hold under stated conditions. State of health is another assessment whose definition depends on the system.
Do not translate every percentage into “battery health.” A monitor reporting 80% SOC usually describes current charge, not a claim that the battery has lost 20% of its original capability.
Keep design and current capacity apart
A simplified label exercise
Imagine a device reports design capacity of 100 Ah, an estimated full capacity of 90 Ah and remaining charge of 45 Ah. Relative to the estimated full capacity, the remaining charge is 50%. Relative to the original design capacity, it is 45%.
The 90/100 ratio is 90%, but whether a manufacturer calls that “state of health” depends on its definition and measurement method. These invented figures illustrate labels; they do not validate a battery-health algorithm.
Copy the device’s exact field names into your notes before interpreting them. “Capacity” by itself is too ambiguous for a useful comparison.
Ask how the capacity was established
A value may come from a product rating, a user-entered setting, a BMS estimate or a controlled assessment. Those are different kinds of evidence. A configured capacity does not become a measured result simply because it appears next to live current.
Victron’s capacity discussion explains that discharge conditions affect available capacity. Keep the stated conditions when comparing ratings or observations. A demanding load and a gentle test can produce different usable results without either being a simple arithmetic error.
Use operating history without overclaiming
Shorter observed runtime may reflect more load, less starting charge, different temperature, a changed reserve or a capacity change. Check those inputs before concluding the battery has degraded. A runtime field is especially sensitive to its current-load assumption.
Likewise, a cycle count alone is not a universal remaining-life calculator. The meaning of a cycle and the relevant warranty or test conditions belong to the manufacturer. Avoid predicting a replacement date from one unlabeled counter.
Build a label dictionary for your installation
- SOC: source, percentage basis and freshness.
- Remaining Ah: reported or calculated, and from which capacity.
- Full/design capacity: exact label, unit and origin.
- Energy estimate: voltage basis and included losses.
- Health: manufacturer definition, if actually reported.
Sunstead distinguishes reported measurements from available calculated values and cannot create a reliable health assessment from a voltage snapshot. If a field is unavailable, identify what evidence would be needed rather than substituting a related-looking number.
Keep this dictionary with the equipment handover. It prevents the next reader from treating a configured 200 Ah as an independently measured capacity or a full-charge percentage as a health certificate.
Sources & further reading
Documentation checked October 10, 2026.
Your next step
Understand SOC versus voltage