How Can You tell if the HV System is Live or Not?

High Voltage is provided to the system from the HV Battery, this we know. And is controlled by operating some relays inside the HV battery to connect and disconnect from the rest of the system. Normally (traditionally) manufacturers’ have programmed the system to operate these relays whenever the driver switches the system on (starting the engine on a conventional vehicle) usually with a push button saying Start/Stop or ‘Power’. And the opposite happens when the driver wants to stop using the vehicle by repressing the button and the battery is disconnected form the rest of the system. Then the indicator (Ready light) reflects the condition of the system.

However, for some manufacturers despite the indicator telling the driver this, the system keeps the HV battery connected to the system despite the power button being operated to shut the system off!

Why is this? I hear you ask, surely this could be potentially dangerous if we want to work on the vehicle by carrying out a shutdown procedure!

Now at this point, I’d like to point out if you are considering working on a Electric Vehicle you must attend some appropriate training, get qualified and follow manufacturers’ instructions to the letter (if available).

So why would a system want to put out the 'Ready’ light but keep the HV battery switch on, surely this is misleading.

Well, firstly we understand what the HV battery connects to when it’s switched on.

The HV battery, once switched on supplies everything relating to High Voltage. But one significant component is the DC-DC Converter. So why is this significant?

This component is responsible for maintaining and charging the 12-volt system, just like the alternator on a conventional vehicle. So, when the system is switched on the DC-DC Converter will start up and start charging the 12-volt system. The charging voltage is approximately the same as a conventional vehicle around 14 14.5 volts. Therefore, if you want to see if the DC-DC Converter is working properly, put the vehicle into ‘Ready’ mode, get your Multimeter out and test the 12-volt battery to see if the voltage is around the same as the charging voltage i.e. 14 – 14.5 volts. If it is, then the DC-DC converter is doing its job.

Likewise, when we take the system out of ‘Ready’ mode the DC-DC converter should stop charging, as the HV battery is disconnected during the ‘Shut-down’ procedure.

But, as mentioned earlier some manufacturers want to keep the HV Battery connected despite the ‘Ready’ light being distinguished. This is to continue to charge the 12-volt battery and/or other components such as the air-conditioning compressor. It’s just the way they want to do it….

So, if with consider this to be a function on some electric vehicles we can use this to understand if the HV battery is still connected once the ‘Power’ button has been operated to switch the system off and the ‘Ready’ light is NOT illuminated. Just again, put a Multimeter across the 12-volt battery, if it’s reading a charging voltage, then the HV battery must be still connected!

How else would the DC-DC be able to operate without the HV battery? It can’t.

So, take this tip and next time you carry out a full shutdown of the HV system, take a minute to check the 12-volt battery. You never know. It helps you understand what’s going on and a safer methodology!