Why are there two batteries?

[Laserbee]

I went to look into hooking up a new interior light circuit from the fusebox under the seat, and was quite surprised to find two batteries down there. One of them was nearly flat and the other was okay.
Wass all that about then?

[Guy88]

assuming MK7?

Rear is the starting battery, front is auxiliary

If the front is flat it needs replacing

Your interior lights should be wired to the take off points on the back of the seat base

[Laserbee]

Ahh, thanks Guy.
That would explain why the curtesy lights were so dim yet the starter motor was ticking over fine- and you're right, it was the front battery that was nearly flat and it only just started started...

...so would I be right in thinking that if the auxiliary battery is totally flat or removed, it won't start? And if thats the case, why not have both batteries in parallel (if one is no good without the other)? Is the starter battery for the starter motor alone?

Also Guy (sorry for all the questions!), I guess I saw the take off point hiding under a glimpse of red plastic that looks teally hard to get to. Is that going to be wired up before or after the battery saving relay? That's the puppy I want to bypass and use my own time delay light switch that I can turn on or off whenever.

I really appreciate your help with this Guy, and looking at your number of posts, I reckon a lot of other people must be thankful for your advice on this forum too!

[ake]

If the front battery is totally dead then there won't be enough power to wake the ecu up, however that's fairly unlikely.

If they were in parallel the front battery would be pulling the rear battery down, and the van probably wouldn't start, that's why they are separated, so that in theory the rear battery always has enough charge to start the van, although under normal circumstances, ie both batteries fully charged, they are connected together at start up, so both start the van!

Yes the connection points behind the seat are fed by the front battery, and are live all the time, so bypass the battery save relay. If you can't get to them from behind the seat, although it can be done, you can remove the batteries, and remove the pre fuse box, connect the new cable, then reassemble, sounds a pain, but in reality will take less than an hour!

[Laserbee]

Blimey. Thread closed!

Thank you both so much for that essential information. I'm sorted!

You two guys have nearly 50 thousand[u] posts between you....
... and so let it be said that through your kindness, "Our Ford" will look kindly upon you my children! - (Google out "Brave New World" by Aldous Huxley...)

Thanks again!

[ake]

Happy to help

[Altransit]

Happy to help

Nice to get a thank you for a change eh

[ake]

Happy to help

Nice to get a thank you for a change eh

Makes all seem a little more worthwhile doesn't it

[APH]

Happy to help

Nice to get a thank you for a change eh

You too must be honoured or something...

[Laserbee]

Sorry Guys - I've still got a problem here

I pulled out both batteries and had to literally snap off that fuse cover as it wouldn't open (seen similar comment on another thread) and got finally my auxilliary wiring fitted. I also replaced the front battery with a new one as suggested. Before re-fitting the last battery connector, I thought I'd check the current being pulled with everything switched off and this is what I found...

There was quite a spark re-connecting the terminal (front battery) and a motor "whir" for about a second somewhere in the dash area. The current then settled at about 0.8A. I shut the doors and everything was off. Looking through the window, 0.8A was still being pulled from something. Surely the ECU wouldn't pull this much current. I only use the van about once a month so this battery will be flat every time.

What is the brief whirring noise?
What could be pulling that current?

...and that was my Saturday
All good fun isn't it

[blue estate]

CD player is the noise

[Laserbee]

Hmm. Not sure if it was the CD player. It seemed nearer the steering wheel area. Also the size of the spark suggested it was a bigger inductive load ,like a proper motor somewhere???

[limitedblack]

Most circuits have noise filters on them now, meaning they have smoothing capacitors, these will draw a large load on power-up then when charged they reduce current. This is almost instant but will cause a spark.

Most manufacturers recommend the vehicle is not left longer than 2 weeks now without a maintenance charger.

I would leave the current check on for 1/2 hour where you can read the draw from outside the van, mine keeps the radio and clock powered up for about 20 minutes after the van is locked.
See if it has dropped by then.

[Guy88]

solar charger is the way to go

[duvet]

id second the need to take it for a run every 2 weeks.......

working on a solar charger now