It’s common practice to add a second battery to motor homes, 4WDs, caravans and so on, so that any electrical or electronic devices used while stationary do not drain the main vehicle battery. It’s important at the best of times but can become a matter of life and death half way up the Oodnadatta Track!

There have been all manner of schemes "invented" to connect the second battery, ranging from simple permanent paralleling (definitely not recommended!), isolating switches and many "electronic" solutions.

This is one of the latter but it is different to most, in that it uses a latching relay – which we’ll explain shortly – to do the switching. This results in a very low standby current – less than 500mA – which can be even further reduced, to just 50mA, by eliminating the indicator LED. If, for example, you are using solar cells for long-term battery charging and you’re the other side of Woop-Woop, every microamp is sacred (with apologies to Monty Python).

Just connect this between your main and auxiliary batteries and never be caught with a flat main battery again!

By the way, the reason that permanent paralleling is not recommended is that it is all-too-easy to flatten both batteries to the point where they won’t start the vehicle. And a manual isolating switch is not an ideal solution to the problem because it is just that: manual. Too many times we’ve heard of flat main batteries because someone forgot to disconnect them, or flat auxiliary batteries because someone forgot to connect them.

Our circuit does it automatically for you by connecting the two batteries whenever the main battery is charged to a high enough voltage – say 13.5V – to allow this to be done safely. Almost invariably, that is when the motor is running and the main battery is being charged from the alternator. (It could, of course, also be when the main battery is connected to a battery charger.)

If you connect a charged main battery to a relatively flat auxiliary battery, a quite large current can flow for a short time from one to the other, resulting in a short-term voltage drop in the main battery.