VW Electrics tracing battery discharging cause

Disconnect the negative terminal from battery, join (12v bulb and holder from Maplins £1.90) test light between battery and terminal (end of cable), if you have a light showing, it means you have a drain. Remove each fuse in the fuse box one by one till the light goes out, this tells you which circuit the problem is on. it is then a case of finding the fault on that circuit

CovKid:

For those who like to 'battery watch', whether using a twin battery set up or just the one, why not fit an LCD voltage meter in the heater cowling? Theres ample space behind where I mounted mine and you can see at any given point, what the current voltage is, and it'll rise as the alternator kicks in, giving you some indication that it is in fact charging, These meters cost arounf £7 from a store in china (ebay) and take about a week to arrive. Very easy to fit. Just earth the black lead on the meter and extend the red to the live terminal of whichever battery you want to monitor. Useful gagdet and helpful when checking for gradual leaks, drains or just watching your alternator do its stuff.

Because the cowling is only made of plastic, a little drilling, use of a craft knife and a file makes it a breeze to cut the hole. You could fit two (one for each battery) or fit a funky switch to monitor a second battery on the same display. Back-lighting for the meter can run from your normal dash lights. I disconnected the inbuilt display light and simply glued an LED in its place, run from dash light circuit so it automatically comes on at night. A reassuring mod this one.

Clearly it won't tell you if a battery is any good (ie amps) but its still worth having to give you an idea of whats going on.

Adding a switch to watch either battery is easy. Black (negative) wire for meter goes to chassis earth since both batteries use it. The red wire from meter goes to centre terminal on a two-way switch. The outside terminals on switch go to positive starter battery and positive leisure battery respectively. All very simple if you have a modicum of interest in circuits (nothing complex) and even if you don't, it should be easy to work out.

Meter seen in this article available from http://stores.shop.ebay.co.uk/Asia-Engineer__W0QQ_armrsZ1 - takes about a week.

Search for "Blue LCD Volt Meter DC 7V-20V" - if correct one, it'll say "Doesn't Require Power" - you don't want a meter that needs its own power supply (defeats the object).

Above photo shows meter fitted without a switch (an option) but the diagrams below and subsequent videos show the meter with switch fitted and how it works in practice.

Simple wiring diagram - note red and black wires on meter:

Here's a meter (same supplier) fitted by member 'Cruz' with the switch (see above diagram) which allows him to switch from one battery to another. You can buy a range of switches to suit from electronic spares shops or via the internet in toggle, rocker or slide form. You may prefer to mount the meter in the back of your vehicle (or even have two meter!) but the heater cowling is a good place to mount it.

There are two videos associated with this which give you an idea how it works in practice:

http://www.youtube.com/watch?v=JVkR4EG2nO8

http://www.youtube.com/watch?v=LTM_lHC-Ba8