Auxiliary battery charging



The information on this page is illustrative. It is not to be used for training purposes or as guidance or instruction. It is also incomplete. A full version of this content can be found in our PicoScope 7 Automotive software, which is downloadable from here.



The purpose of the test is to evaluate the charging of the vehicle's Low Voltage (LV) auxiliary (12 V) battery system by its High Voltage (HV) system when it is not connected to Electric Vehicle Supply Equipment (EVSE).

Auxiliary battery charging measurements.

Auxiliary battery charging measurements.

Further information

After an EV has closed its HV contactors and entered its Ready state, a DC-DC converter will convert the high voltage supply from the HV battery and step it down to a low voltage supply (approximately 14 V - a common system voltage in alternator based charging systems) to charge the auxiliary battery.

In the waveform example above, a large current of approximately 74 A is initially delivered to charge the auxiliary battery. As the battery state of charge stabilizes the current is then gradually reduced.

The low voltage auxiliary battery is not always easily accessed on some vehicles. In these cases, you may find the cables connecting to the DC-DC converter easier to access. In such a case, please use the vehicle manufacturer's data to safely locate and access the low voltage output from the DC-DC converter (used to charge the auxiliary battery) to measure the voltage and current. If safe access is not possible, do not perform the test.

In the example waveform, the vehicle systems draw a significant current (just over 30 A) just before the vehicle has entered its Ready state. In some vehicles (see the Renault Zoe example described here) this might be a strategy to check the auxiliary battery's state of health before the vehicle controllers close the high voltage contactors. However, this strategy might not be common to all EVs.


This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.

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Guided test: Auxiliary battery charging