Explaining why the 12V battery in all hybrids and EVs depletes - and how to manage the problem!

(I answered this in video form, but I thought an article would help people find it too!)

As part of my research into buying an EV or hybrid, one complaint I heard was that, perhaps especially on the Toyota Yaris models, the 12V battery tended to die if the car wasn’t used for a few weeks, then requiring a jump start from a battery pack or another car.

And there’s a lot in this. Not least because the problem is endemic to all cars, though more so with more recent models, as there’s more going on - literally - under the hood.

Let’s start with the 12V battery itself, a lot smaller in hybrids and EVs than in traditional pure petrol cars because it doesn’t need to be beefy enough to crank a starter motor. Which is mainly why you don’t hear of 12V depletion in those cars - the larger batteries can withstand much longer periods of inactivity.

A 12V battery is used to run the car’s lights, computers, air conditioning (usually), cabin fan, engine pumps and fans and power steering, that sort of thing. The usual car stuff, running at the usual car voltage throughout.

Plus in a hybrid or EV, it’s used to activate the relays and safety systems that bring the much larger  - and potentially hazardous - battery, sitting in the back or underneath the car, online. This larger pack provides power for the electric motors as needed. And, crucially, also repays the favour by recharging the 12V battery continuously when the car is on.

It’s a circular arrangement and generally works very well indeed. 

Now, what happens when you turn the car off? Importantly, it’s never actually off. While parked on your drive, ostensibly off, the 12V system is still active and running:

• The alarm and associated door and window sensors
• The 4G data connection to the Internet and your manufacturer’s servers, for example to liaise with the app on your phone, so you can see the car’s status, location, and perform various remote functions.
• The wireless security pings, looking for your wireless keyfob
• The computer processors associated with all of the things just mentioned.

That’s quite a bit going on, even if all of these represent relatively low power drains individually, and it’s not surprising that they can add up to cause 12V battery low voltage after a week or two in this ‘off’ state.

This is true for all hybrids and EVs, though Toyotas seem a bit more susceptible - there has been some suspicion that they used low quality 12V batteries for a while, though this is to be confirmed.

Regardless, in normal operation, even driving the car for a few minutes once a week, is enough to fully recharge the 12V battery from the hybrid or EV battery. In an extreme case, you wouldn’t even need to drive the car. Merely powering it on, on the drive, is enough to get the batteries passing current - and, of course, if the hybrid’s main battery is low then the engine might kick in for a minute. 

So, thinking of when an extreme case might occur, for any hybrid or EV, let’s say you fly off on holiday for two weeks and don’t want to risk the 12V battery being too low on your return to kick-start the car’s systems. If this were me then I’d give the keyfob to a trusted neighbour or friend and ask them to pop in once, a week in, and turn the car on and leave it on, on the drive, for 20 minutes. Then turn it off again and lock up. Job done.

At worst case, if you leave a hybrid or EV too long without any operation, and the 12V battery does get too low, jump-starting in traditional fashion will work - there’s a red charging pin in most cars, somewhere near the fuses under the bonnet, for just such an occasion. If no donor car, then any starter battery pack from the likes of Halfords or Amazon will also do just fine. I keep one in my boot for emergencies, even though I doubt I’ll ever leave my lovely Yaris Cross along for long enough to need it.

So there we are. Don’t believe the doom and gloom. And it’s not a Toyota issue, it’s just physics and how all the tech works. And now you know the ins and outs too!

PS. If you like my work then think about buying me a beer at paypal.me/stevelitchfield - thanks!