Battery Cause of sudden flat battery

[littlespark]

My eldest girl has a 10 plate Kia picanto. Bought for her by her grandparents, and she was supposed to be taking it away today, up to Aberdeen to move into her new flat.
During lockdown, it hasn’t been used all that much, but it has been used.
A few weeks ago, I swapped out the radio for a cheap Chinese thing with a screen, and Bluetooth.
Last week, I replaced both sidelight bulbs, and yesterday one of the rear wheels was jammed on by the handbrake.
Today. Two cars packed with all her stuff, first the doors wouldn’t open with central locking, then the ignition did absolutely nothing. Checked the battery.... 2.5 volts!

I don’t want to just charge it up, take it up to her and for it to drain again.
I was wondering if replacing the bulbs had caused a leak through to earth.(if there was already a problem with the switch? Can’t see it, as the lights are only powered through ignition)
I thought it might have been the radio? As it needs a perm 12v to keep the memory (I had to swap the 12v and aux 12v in the loom to get it to work properly) But that was fitted weeks ago, the problem would have appeared before now.

 

[Vortigern]

I wonder if you not only took a + of the rear reversing cable and local earth as well connected the (pink?) cable at the front rear of the radio? If you did you would be duplicating the + at the rear and a drain would occur on the reversing light. i.e. you would see the reversing light on even when not in reverse.

 

[Lucien Nunes]

I wish I lived on a hill, then I could have bumped it

With the battery at 2.5V there wouldn't have been anything to power the ignition and engine management, so nothing would have happened. A rolling start only helps if the battery can supply enough for these functions but not the starter motor.

voltage was about 13.5v (just off my multimeter) Still the same this morning.

Don't forget that some failure modes of lead-acid batteries make them behave like good batteries of much smaller capacity. Whilst a good open-circuit voltage is necessary, it does not say anything about the capacity. I recently discharge-tested a 100Ah battery that was reading 13.8V off charge and would start a V8 engine, but was actually down to 15Ah due to sulphation.

it read .5A

Obviously a load this high is a problem if it continues after things should have timed-out. It could well be due to the radio, but had not previously shown up due to the particular mode it was left in, etc. A sneaky way a new radio can affect things is by making an incorrect connection to the CANbus or other control line via the radio harness, that confuses the vehicle's own electronics and keeps the body control module awake and consuming power even when the radio itself is not.

You might want to make a fuse breakout adaptor (e.g. by soldering leads onto the pins of a blown fuse) connected to your meter via a fuseholder. You can then connect to any suspect circuit, prop the meter where you can see it, close all the doors and bonnet etc. and wait for everything to go into standby.

 

[littlespark]

I’m buying a new battery tomorrow morning, so thanks all for the advice. I’m not going to risk it, it’s possibly the original 10yr old battery.

I didn’t fit the camera that came with the radio. It’s only a picanto.... it’s so small you can SMELL if someone’s standing behind you.