Auxiliary battery charging
The purpose of this 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).
High Voltage (HV) electrical systems have high voltage cabling, stored electrical energy, retained hazardous voltages, and the potential to affect medical devices.
Risk of damage to equipment and property, risk of electric shock, personal injury, or death, and risk of explosion and fire.
- Read carefully and understand the hazards and risks described above.
- Only work on HV systems if you are a qualified technician, have received appropriate type-specific training from a qualified electrician, and hold valid accreditation.
- Do not work with HV systems if you have been fitted with a pacemaker or other electronic medical devices.
- Always refer to the vehicle manufacturer’s information sources to identify the actions you must take to prevent danger.
- Use equipment and accessories in accordance with their safety guides.
- Always wear appropriate Personal Protective Equipment.
How to perform the test
- Disconnect the vehicle from any EVSE and take all precautions necessary to keep them disconnected throughout the test.
- Ensure the vehicle is switched off by checking the display on the vehicle's instrument panel.
- Connect a test lead to PicoScope Channel A and its probe and ground leads to the auxiliary battery positive and negative terminals, respectively.
- Connect the high amps clamp to PicoScope Channel B and around the cable between the auxiliary battery negative terminal and the chassis ground.
- Minimise the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
- Start the scope to see live data.
- Switch the vehicle on (the state is typically indicated by a Ready lamp in the instrument panel).
- With your waveform on the screen stop the scope.
- Use the Waveform Buffer, Zoom and Measurement tools to examine your waveform.
Further guidance
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.
Disclaimer
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.
