12v battery drops to 5v when connected

[Phil2025]

I had my 12v battery replaced about a year ago but 2 days ago I came out to the car and it was showing 5v. I managed to unlock the car, but having removed the battery it was a healthy 12.9v. I charged it off the car and refitted it and checked the voltage, still healthy, but as soon as I connected the negative lead, it dropped again to 5v. Disconnecting the negative lead, shows the voltage back to 5v. Could it be as soon as the cars sytems are activated, the battery just can't maintain the voltage. The car is stuck on my sloping drive, in park with the electronic handbrake on, so it will be a devil to move. I'm hoping that it might just need a replacement battery.

 

[EvanstonI3]

I monitor my 12v battery all the time and while the resting voltage with no load is around 12.9 volts, it drops to around 12.4 when I turn the car "on" and there is some load on the battery. Dropping to 5 volts is not something I have seen before.
The 12v battery itself sounds fine, but something is drawing a large amount when the car is turned "on". You may have a short someplace, like a rodent chewed through a wire or something.

 

[mojo]

I charged it off the car and refitted it and checked the voltage, still healthy, but as soon as I connected the negative lead, it dropped again to 5v.

There are really only two possibilities here - either the battery is good but there's almost a dead short-circuit in the car, or the battery is shagged.

If the former, you would expect to see a *big* spark and a loud crack as you connect the negative terminal. A load that's low enough resistance to bring a 12V AGM battery down to 5V will be getting very very hot (and note - the car's interior heater does *not* run off the 12V battery).

So I would suspect a knackered battery until you know otherwise. If you can't take it somewhere to be tested, I would dig out an old car headlight lamp, hook it up across the battery and see how long it can supply a 55W lamp for.

 

[Phil2025]

I monitor my 12v battery all the time and while the resting voltage with no load is around 12.9 volts, it drops to around 12.4 when I turn the car "on" and there is some load on the battery. Dropping to 5 volts is not something I have seen before.
The 12v battery itself sounds fine, but something is drawing a large amount when the car is turned "on". You may have a short someplace, like a rodent chewed through a wire or something.

Thanks for the reply. I think it was wishful thinking it might be as simple as a new battery.
My car usually shows around 12.4v, but sometimes it dropped to 11.9v. I am wondering if the car hasn't been maintaining the charge.
Time to call an auto electrician.

 

[EvanstonI3]

Thanks for the reply. I think it was wishful thinking it might be as simple as a new battery.
My car usually shows around 12.4v, but sometimes it dropped to 11.9v. I am wondering if the car hasn't been maintaining the charge.
Time to call an auto electrician.

Are you monitoring the voltage with a plug-in voltmeter in one of the power ports? I've seen my voltage drop to 11.9 if I just have the doors open, but the car is "off" and I am messing around, opening/closing doors and whatnot. The car will come on at 12.0 volts to boost the 12v battery. If you are seeing 11.9v while the car is "on" and you are driving, that's not normal.

 

[Phil2025]

The behaviour I described is as you say, when I first open the car, but when the main battery kicks in it charges as normal. The latest problem is unusual. It is out of my skill level now, so I need to speak to the garage. How I get the car there is another matter. I suppose it will take a full lift.