georgeh,
"...but this is too prone to human error for me, the chances of accidentally connecting the 2 batteries and draining the starting one is too high."
Well, the expensive packages are there to save you from exactly the problem you mention. If you want a simple A - B - A+B switch, you can get them from boat chandlers. An ON-OFF switch can be of the type used to disconnect batteries in emergency. Any car accessory shop shuld have them. Also, you can use an 70amp+ relay. Check the Vehicle Wiring Products website.
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Roman (UK)
www.overlandcruiser.info
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Roman (UK)
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