We do not recommend using an OFF – 1 – Both – 2 type battery switch,(as shown) this switch requires input from the skipper and if you forget to isolate your start battery you could run that flat overnight.
Plus the big danger is switching with engine running and the contactors do not make contact, you could blow up your alternator diodes.
A better choice is individual battery switches,shown on left
And remember NEVER turn OFF the battery switch with the engine running, it can blow up the alternator and or the smart regulator.
As a suggestion, if you are running a start battery and a house battery, separate the two i.e. run separate bus bars and use a
The DVSR (Digital Voltage Sensing Relay) allows charging of two independent battery banks from a single charging source. When the voltage on the start battery rises to a charged level, the DVSR engages allowing the 2nd battery to charge. When charging stops and voltage falls, the DVSR will disengage, isolating the two batteries from each other. Dual sensing functionality enables the sensing of two battery banks, allowing two way charging.
Another advantage of BEP's VSR is that the house battery is completely isolated from the engine battery during the voltage-hungry starting procedure. This means that, as long as all electronics are powered by the house battery (as they almost certainly will be), they will not be subject to damaging voltage spikes during start up.
More information can be found at the BEP website including installation hints: http://www.bepmarine.com/product-mainmenu-0/product-714/digital-voltage-sensing-relay-dvsr
There are several options in set up in respect of which battery is charged first, please ensure you have the correct one for your needs.