11.2 v battery, Jag S-TYPE XS

[Dan909]

Hello, I recently bought a jag s type, has a few electrical issues (one of 4 break lights not working despite receiving voltage, restricted performance has come on twice in a week dissapears on restart, weak radio signal, etc etc). Through some reading here, it seems most of these can occur due to a faulty battery.

I have no error codes (using Fasdiag JD-101) and a battery voltage of 11.2v resting, around 13.8 - 14v immediately after cold start. Ive attached the battery test information in the below images. Essentially, should I be after a new battery?

Thanks for any help you can give.

[GreenV8S]

I assume that's a conventional lead-acid battery. That should read about 12.6V at rest if fully charged, down to about 12.0V if it is flat. Your reading is obviously well below that and makes me think the battery is either flat or knackered or both. Most places that sell batteries will test it for you to confirm whether it actually needs replacing, and there's an outside chance that you have some other problem and are getting a false reading at the ECU, but most likely you just need a new battery. If you do get it replaced, get the charging system and residual current checked at the same time in case the battery failure was caused by some other fault.

[E-bmw]

^^^^ Wot 'e said.

11.2 volts is flatter than the flattest flat thing with a high rise building on its head!

A car 12v battery is 6 x 2.1 volt cells in SERIES so should be at 12.6v when at rest if charged, every 0.1 volt below that is around 20% charge lost.

13.8/14v at idle shows good charge from your alternator, which is good.

Start with basics & get that right, then see where that gets you.

Bulbs have a soft solder bottom connection that can get pretty poor at making a connection, especially if the sprung centre connection is as bit slack, try bending the contact out slightly & cleaning the bulb & holder.

[Dan909]

Thanks for the help, new battery is in and the car starts really nicely now. Still issues with the break light and every now and then restricted performance (turning the car off and on again solves it).

I'm gonna clean the light housing, bend the contacts out, and do some research into the Ecu. I think I read somewhere that could be a problem.

[E-bmw]

Good starting (pun not intended) point then, well done.

Have you a mate with a code reader in case you can borrow it for when "reduced performance" kicks in to see what codes are stored?