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Battery Testing v.s. BMS readout

When the state of health of a battery needs to be determined, most manufacturers rely on the readout of the BMS. Although most BMS’s provided a number, it does not say how reliable it is.

The BMS state of health (SOH) value

Most battery packs have a BMS that is based on a standard IC solution. Well known suppliers are O2micro and Texas Instruments. They have documented to algorithm used to calculate the State of Health. The chips computer the FullChargeCapacity (FCC) value if a discharge from full to empty has been done.

Discharge graph
Discharge graph with full power peaks

Understandably, e-bike user will avoid a full discharge situation, so the FullChargeCapacity value will not be update at all, or maybe once in 1 or 2 years.
To avoid large errors due to measurement inaccuracies, chips will only apply a part of the capacity difference (i.e. 25%).
Example:

Full discharge Determined capacity FullChargeCapacity
1 jan 2013 Factory set 10Ah 10Ah
1 mar 2014 9Ah 9.75Ah
1 jan 2016 6Ah 8.8125Ah
1 jun 2016  BMS readout 8.8125Ah

Reading the FullChargeCapacity value from the BMS will give value that is not closely related to the actual capacity, is the pack is bad. If the pack is good, the readout is close to the factory value and difference is less. But it will be impossible to tell the difference between a good pack and a bad one.

A battery analyzer

A Battery analyzer will perform a test to measure the Ah of the pack. This will always give a correct reading at the first measurement. But beware! Measuring at low discharge currents (lower than the use on the e-bike), may conceal problems. The pack temperature will be lower, the mechanical deformation of cells will be slower, thus less and this may lead to a normal capacity readout. However, at higher current and peak loads the temperature and mechanical stress may cause cells to fail.
For this, the test should be done the continuous load of the application (250W for an e-bike) and regular short duration peak loads of 2 time the continuous load.

Warranty handling

If a manufacturer wants to rely on measurement reports of dealers, the report must be reliable and not been tampered with.
Reliable, requires that it is guaranteed that a pack was fully charged, before the test. If not, test result is to be expected. This cannot be used to evaluate a warranty request.
Analyzers from Battery Condition Test provide a test that includes the charger. It shows the charge voltage and the test results on in 1 PDF report. These testers are fully suitable for remote evaluation of test result, as required by manufacturers support and warranty handling.