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.