Q: I recently had a garage car, a Hyundai Kona EV, on loan while my 222-reg Enyaq was being repaired. The first evening, I went to start it, and everything was dead. It turns out it was fitted with an old-style wet battery, and this had died. The garage told me all EVs still have the old-style batteries, and these have a relatively short lifespan of approximately four years. I wasn’t aware of this, and on the few occasions I opened the bonnet of my Enyaq, failed to see one. – Brendan M, Co Wicklow
A: Yes, it’s true. Any electric car you buy has two batteries. The bigger one is the one that gets all the headlines – it’s the one with lots and lots of kilowatt-hours of energy storage, that takes ages to charge (at least from a slow AC charger), and that actually powers the wheels. It’s technically known as a traction battery for that very reason.
But there’s another, kinda secret, battery that lurks alongside the big traction battery, or indeed sometimes even within the structure of that big battery.
In your Enyaq, the 12-volt battery is located quite low down in the compartment under the bonnet, but the positive terminal is under a flap at the back and to the passenger’s side.
This second smaller battery will, in many cases, be a familiar sight to most motorists, and is often as you describe – a “wet” lead-acid battery, discharging 12 volts of current. This has been the standard design for batteries that have started up all our petrol and diesel cars since time immemorial (all right, since the 1950s…).
Why does a modern, high-power EV need such an extra battery, of such an old-tech design? Well, to be fair, not all of these 12-volt batteries are quite so old-school. While many of them are indeed old-fashioned lead-acid designs, many car-makers are now starting to use the same lithium-ion tech that powers the big traction battery, just with a lower-voltage output.
Such low-voltage lithium-ion batteries are generally much smaller and lighter than the older-style lead-acid units. If you’ve ever had to change the lead-acid battery in your car (be VERY careful if doing so), then you’ll know how weighty these boxes of electricity are.
So, why does a new EV need one? Simple – voltage. A modern electric car’s traction battery will run at a voltage output of 400 volts, while some newer, more powerful designs will run at 800 volts.
That’s great for charging up quickly and for big power outputs, but if you ever tried running any of your car’s systems and functions other than the main electric motor, you’d have a big problem. Basically, that 400 to 800 volts of power would instantly fry any of the 12-volt-based systems. And those systems are really important.
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Essentially, every electrical switch or touchscreen that you interact with in your car is running on a 12-volt system. Why? Because it’s the system that’s simplest and easiest to design and produce. Car-makers don’t fancy the challenge of redesigning everything from door locks to windscreen wipers to run on an 800-volt system when the cost and time spent on such research and development would be astronomical.
It’s also a safety consideration. Because the voltages in the big traction battery are so strong, if a driver is involved in a crash, that battery needs to be isolated promptly and shut down so as to make it safe.
Not only does the 12-volt battery on board do that job, it also means the rest of the car’s systems should continue to work – not least electric windows and the increasing number of electronic door lock releases.
It’s really important that the door locks, in particular, are powered by the 12-volt system, as you definitely don’t want 800 volts running through or near the door catches in the event of an emergency.
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However, we’re not sure about your garage telling you that the 12-volt battery will only last for four years. A 12-volt battery should last, with reasonable care and regular servicing, for at least a decade unless there’s some specific fault.
What can happen, and what has happened with the loan Hyundai you were driving, is that – just as with the 12-volt battery in an old petrol or diesel car – the battery can run flat. In a combustion-engine car, the 12-volt battery is kept charged by the car’s alternator. In an electric car, that’s not possible. Instead, that battery can be topped up by the bigger traction battery while driving, thanks to a DC-to-AC converter. That also allows the 12-volt battery to be topped up when the big battery is charging at home, or on a public charger.
Problems tend to arise when a car is left sitting for some time. New systems such as keyless entry and ignition require sensors and receivers to be switched on all the time, and if a car’s electrical and electronic subsystems aren’t well designed to work in low-power mode for long periods, or if there’s a fault, then the 12-volt battery can become drained very quickly indeed.
When that happens, your options may well become limited. Most doors can be opened with mechanical locks, as can most (but not all) charging covers, which should allow you to get access to the car and to plug in a charger.
However, you may have to resort to an older-school technique – jump starting. It is possible – although not with all vehicles, so make sure you check the owners’ manual first – to connect an EV’s 12-volt battery to another car’s, via good old-fashioned jump leads (or you can use a jump-start battery pack) and bring it back to life that way.
Some car-makers – notably Hyundai – are now starting to fit systems that allow the 12-volt battery to be “jumped” from the car’s big traction battery, and thankfully Hyundai has been clever enough to create a physical button which does this job, as the touchscreen is going to be dead in that scenario.
