split charge system

This is very easy. Use a relay that operates from the ignition switch. This only energises when the engine is running and connects the second battery to the main battery for charging. When the engine stops and the ignition is switched off the relay disconnects the second battery from the first and prevents it discharging the main battery when using the second battery. Heavy duty relays for this purpose are available from caravan shops. Cheers, Peter

I want to build a split charge system for my series 3 diesel. Does anyone have plans for such a system?

Peter Blaabjerg

It is better to take a feed of the alternator. That way the two batteries are connected after the engine has been started (and the alternator starts to charge). If you take the feed for the relay of the ignition switch the two batteries will be connected before the engine is started and part of the current for the starter motor will be taken from the second battery. Since this often is a leisure battery (for low currents but deep discharge) it is not made for these large currents and will quickly deteriorate.

Pieter