Martynn Randall is technical editor at Haynes and has been with us for approaching 30 years. He's written more than 60 Haynes publications and has owned more than 85 cars and 60 motorbikes... so far!
If you drive a car, bus, van or motorcycle for long enough, you’ll eventually turn the key or press the starter button, only to hear the dreaded click.
Yup, a click, likely followed by a deafening silence as your engine greets the forthcoming journey with all the energy and excitement of a gravestone.
A flat battery means you can’t start the vehicle, making you late and meaning you need to either dig out jump leads or call in favours from friends and neighbours.
What is a hybrid battery?
Hybrid cars are different, because they have not one but two batteries, one of which is used to start the engine and provide power for the ancillaries such as the fan, the lights and the rear window demister. And don’t forget the radio. The other battery is the high-voltage battery, which is used to power the electric motor in the hybrid system.
Now, manufacturers tend to have different ways of approaching how the drive system works, but in essence, the battery powers the electric motor, which can either assist the engine or can drive the car on its own for short periods.
Then, when the vehicle is moving, it is able to recharge the high-voltage battery either through energy generated by the brakes when you slow down, or, in some models, by the drivetrain turning into a generator as you travel down hills without touching the accelerator.
All very modern, all very efficient. And the good news is that high-voltage batteries are generally expected to last the life of the vehicle.
However, we at Haynes know that life isn’t like that. So, what happens if your hybrid’s high-voltage battery goes the way of an overworked Ever Ready?
How will I know if my hybrid battery is dying?
The good news is that this should be comparatively obvious.
For a start, the vehicle will also be slower than it should be, and there will be a warning light on the dashboard.
You’ll probably also notice that your car is using much more fuel than it should be. And given that economy is one of the primary functions of a hybrid, this should raise a red flag.
You might also notice that the battery tends to lose charge extremely quickly, or that the amount of charge in the battery fluctuates randomly.
And if you notice that the battery fan is running constantly, this is a sure sign that the battery is unhealthy, because it will be getting too hot.
What if the battery dies completely?
There are different types of hybrid systems out there, which means slightly different advice about what to do if the high-voltage battery dies.
However, in the vast majority of cases, the advice is that you should not try to drive the car, and indeed, the car probably won’t run anyway. If you do manage to start it, the engine will run poorly, and the engine stop-start system won’t work as it should. You also risk causing significant damage to the hybrid system.
It’s best to sit by the side of the road and call your preferred breakdown recovery service. Given the range of hybrid systems on the market, any breakdown service is most likely to elect to place the vehicle on a truck and take it to a dealer.
Can I replace a hybrid battery myself?
Given the high voltages involved, and the fact that some specialised equipment is likely to be required, the law demands that anyone working on a hybrid vehicle be specially trained to a specific standard. To that end, we at Haynes would recommend that only a dedicated replacement service be used to remove and replace a dodgy high-voltage battery.