Drained 12V Battery in a hybrid or EV? The Physics of Parasitic Drain - and what to do about it!

I was contacted by ITV and subsequently filmed on this subject, but the final footage was cut massively short by time, so I thought my notes would be helpful to others in full.The trigger point for recording was instances of people being locked out of or locked into hybrids or EVs when their car's 12V battery had run too low. I did a video on 12V battery drain a couple of years ago and it had come to the attention of the ITV reporting team.

My response 

Despite the concern, there really shouldn’t be any alarm from a safety standpoint. All car makers put in place failsafes for such an eventuality. E.g. when electronic door latches don’t work on a Toyota or Lexus hybrid, from the inside you can pull the door opening handle twice. The first pull unlocks a mechanism and the second pull physically opens the door latch. 

And from the outside, you can use the (usually hidden) mechanical key included in every key fob. This fits into the keyway on one of the front two doors. On a Toyota or Lexus, pulling on the small plastic latch lets you pull out the end of the fob, revealing the key blade.

What’s going on

It’s just a matter of physics. With hybrids and TVs not needing starter motors, there’s no need for a traditional large 12V battery, so capacities as low as 35Ah are being used. While this is fine for everyday use, there’s a huge factor which needs to be borne in mind when leaving the car for a while or driving it sparingly...

Parasitic drain

ALL cars suffer from this. Power drain that's happening when the car's nominally 'off' (it's not really). And the more modern the car, the more tech onboard, the worse the issue:

• Alarm and immobiliser
• Dash cam and other security features
• A permanent data connection to the manufacturer
• Sensors on doors and windows
• Searching for signals from the key fob

All of these draw power from the 12V battery system.

Traditional cars with starter-size batteries can handle all this, but hybrids and EVs, if left for long periods or not driven much can see their 12V batteries get low and hence possible issues with not being able to power up the car’s computers and thus bring the main traction battery online. Or indeed power door locks!

footnote: some brands: Kia, Hyundai, BMW, Jaguar, have manual or automatic systems (which jump in) to prevent all this happening. Well done to them, anyway.

Things you can do to prevent 12V battery drain causing problems

• An hour of being powered ON per week is usually enough for the larger traction battery to keep the 12V battery topped up. This is usually driving, but in fact, even powered on upon a driveway also works. The 12V battery powers on all the computers in a car, systems check out and then the traction battery is enabled, after which you have anything from 1kWh to 70kWh available, which then repays the favour of the boot up sequence by recharging the smaller 12V battery.
• When parked up, you can stop part of the parasitic drain by turning off the key fob (on a Toyota by holding the lock button down and then pressing unlock twice - LEDs will flash four times in all). The car then doesn’t keep trying to communicate with it, and that saves some power. Plus it means that thieves can’t intercept these signals to steal the car from your driveway. So that's two big reasons to bother turning off the fob.

Tips when leaving a hybrid or EV for long persiods

If a car has to be left for long periods or you know it won’t be driven for more than an hour in a week, then there are things you can do:

• at the very least, for the worst case scenario, have a jump start pack - these are £50 or so and are worth keeping in the glovebox. Cables from this clip onto the under-bonnet jump-start point and then your car’s 12V system and computers can power up and you’re off and running.
  
  
  
  
• get a 12V solar trickle charger, also about £50 - get one that plugs directly into the car’s under-dash OBD port and you’ll never, ever have to worry about any of this as long as your panel on top of the dash can see the sky.
• if at home and you trust a neighbour, get them to press the Power button and leave the car powered on for an hour once a week. In the case of a hybrid, the engine will cut in a few times to recharge the traction battery in turn, but this is normal.

PS. In terms of hybrid or EV design, why not power everything in a hybrid or EV from the larger traction battery? 

Safety. These batteries run at (potentially) hundreds of volts, which would be a safety nightmare for anyone working on a car. Plus all car accessories and parts are designed to run at 12V - it’s a sweet spot. Not dangerous but with enough oomph to do things.