12 volt drops low when ignition turned off

[djvivid]

Weird one this, can't find a solution using Search Engines, they all presume a battery issue.


So the Vans 12V on everything, radio, hazards, central locking and the auxiliary sockets etc.. are fine when the ignition is turned on (engine not started), about 12.6V the same as the battery, but as soon as I turn the ignition off the Volts start to drop. When I first noticed it a few weeks ago it was quite slow, maybe a few minutes, but now it drops from 12.6 to 5 or 6 volts in a few seconds. When I turn the ignition back on it restores the voltage to 12.6.(again, not starting engine, although when I do and rev, I get a healthy 14.3).

The battery Is fine, it cranks the engine like a fresh new battery, and I did a drain (amp) test between the negative battery terminal and the lead, that was well under 0.1 amp. So no drain or shorts detected.

Van: Ford Transit 350 LWB 2.4 D Sept 2010 (60 plate)
Single battery under seat

[blue estate]

You should have 2 batteries in a LWB RWD not 1
Any spare odd leads under there ?


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[Altransit]

The symptoms you're describing above, all point to a dying front battery. Turning on the ignition connects the rear battery into the circuit, which restores the 12.6v.

Try leaving the ignition on for a couple of minutes, you should hear a loud click as the connecting relay drops out, and the voltage will fall again

[djvivid]

Thanks guys,
I was sure there was only one battery, but sure enough there was a second hidden battery further back behind that one out of sight.

[Altransit]

Removing the drivers seat makes it easier to get at the batteries.
It's the one nearest to the pedals that's dying, but if they're both 2010 originals, it will be best to replace them both

[djvivid]

Thanks again Altransit.

I managed to get them both out with out removing the driver seat, Undoing the three bolts on the retaining plate was tricky though, but once the rear battery was out the front battery slid back nicely.

And yes the dead front battery was indeed a original Ford 2010. I'm impressed it lasted so long. The rear battery is 2 years old and cranking well So I've ordered a Yuasa Silver Calcium 80Ah/720CCA to replace the rear, and I'll stick the 2 y/o battery in the front

[dumper]

I don’t like only changing one battery in a twin battery set up I’ve seen many times on 24v symptoms putting one new battery in with a old battery has killed the new battery in a short space of time at work they stopped doing this and only replaced as a pair