Cause of sudden flat battery

[G1ZQC_David]

Your garage should test it for you, by applying
known load, and monitor the voltage drop.

 

[James]

I have found that the first time your car fails to start or is turning over slow in winter,
change the battery if it is more than 5 years old.

testing them costs more in time and money than replacing the dam thing, especialy if over 5 years because the best your likley to get is another 6 to 12 months anyway.

 

[littlespark]

I think the overall concensus is get a new one. Fair enough
I don't want to take the car up, just for her to ring in a months time to say its failed to start

 

[Jim_e_Jib]

I think the overall concensus is get a new one. Fair enough
I don't want to take the car up, just for her to ring in a months time to say its failed to start

If the battery voltage has dropped to 2.5v it will likely have been damaged as lead acid batteries don't like to be discharged too deeply.

As DPG suggested, it would be worth checking the quiescent current once everything has had time to power down and before you put the new battery in. I'm sure your multimeter is accurate enough to measure something of the order of half an amp and that is a very high drain-I'd be expecting less than a tenth of that.

If the reversing camera is new as part of the radio system, is that powering down properly?

 

[pc1966]

I'm sure your multimeter is accurate enough to measure something of the order of half an amp and that is a very high drain-I'd be expecting less than a tenth of that.

Try on high-current (e.g. 10A range) first, or only if you can, otherwise you might find it pops the fast-fuse for a lower current range with something surging on switch on (like a light bulb, or charging some electronics's input smoothing capacitors.

 

[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.