Do I need to replace my battery? Won't take a charge

[el_presidente]

Morning all - I think I know what's going on here but I would like a second opinion please if you would be so kind. I have an 85 1.6TD . Here is the history:

The battery has been flat a few times over the last couple of years, due to lights or stereo left on. I've never had any issue either jump starting or trickle charging it overnight.

First weekend in August took the bus away for the weekend, no issues whatsoever started on the button every time.

Two weeks later went to start it and no joy - starter motor won't even turnover. I think the stereo had been left on (I have a really annoying stereo that is not wired on the ignition and the on-off button also mutes the device, so you can think you have switched it off but actually you've only muted it). Anyway, I left it on trickle charge for a few hours, then it started fine. I drove it around for 30 mins to properly charge it and parked up. Next morning it wouldn't start again, again starter motor completely dead.

OK time to get a bit more scientific. Multimeter across the battery is reading 10.25V, obviously no good. Back on trickle charge overnight this time. Next morning it still won't start - battery is only reading 10.75V. I need the bus so I try to jump start - even after sitting on the jump leads for 5 mins or so the engine will only turn over with an arthritic wheeze and no hope whatsoever of starting.

I assume my battery is kaput and is even inhibiting the jump start process
a) do you agree with my diagnosis and if so
b) what battery should I buy? Or is there anything else I should try?

Many thanks

[itchyfeet]

you can't chareg a battery in half an hour it needs something like 12-14 hours on a proper battery charger that can provide a number of amps not a trickle charger.

Lead acid batteries don't like to be totally discharged and as you have done it many times may be knackered. get that stereo on the ignition feed.

I'd give it a full charge first to be sure before replacing , anything less than 12V is pretty much flat, should be 12.6 or 12.7 fully charged, leave it 2-3 hours after charge before measuring voltage or you get a false reading, if after the long charge the voltage is not up at 12.6 ish then it's on the way out.

[el_presidente]

Thanks - I gave it the full 12 hours charge the second time round and it only got up to 10.75V

The only thing I didn't understand really is why I couldn't jump start it - maybe I have crappy jump leads? (they came with the bus *hollow laugh*)

[itchyfeet]

OK it's knackered and probably shorting internally, if you disconnect the duff batt negative and fix the jump lead negative to the body or negative strap
Then leave the positive on the duff battery so the jump lead can be attached as usual and it's not going to accidently short to anything.

Then try it.

Also you should only jump start from a vehicle with the engine running and I would never leave it connected for 5 minuites, all you are doing is charging the dead battery from the other vehicle which you have already charged and so it's not helping.

You may however cause damage to the other vehicle if the battery is internally shorted because the voltage on the other vehicle can be pulled down and the alternator is put under alot of strain. connect it, if it won't start quickly disconnect it.

[el_presidente]

Good idea, I will give that a go tonight just to be certain it's the battery

You're right that leaving the jump leads connected for 5 minutes is a bad idea - it wasn't 5 minutes that I was running it for, I gave it a couple of goes then disconnected it, then tried again for another couple of goes so the whole process took 5 mins rather than leaving it connected. Thanks though

thanks for your help

[itchyfeet]

sorry I added a bit after you wrote that ^^^^

[el_presidente]

So did I