[C320-list] Dual battery wiring and voltage drops

[Graeme Clark]

The wiring diagram in the manual isn't the easiest to follow and the actual wires, running under sole plates and through bulkheads etc even less so, so hopefully somebody can help with this.

As far as I know our fit is 'factory standard' for the build year (1996)

We have two batteries (under starboard saloon bench seat) and on the port side electrical panel we have a large round switch that enables us to select battery one, battery two, both batteries or all batteries off.

The automatic bilge pump is 'hot wired' to the batteries so that it will work even if the switch is set to OFF.

As far as I know, all other electrical services are "downstream' of this switch - is this correct?

The reason I ask is that last year we fitted a Raymarine e7 chart plotter and now, whenever I start the engine, it turns off. The local electronics expert tells me that this is because when you start the engine the battery voltage drops (more so if you use pre-heat) and the plotter is voltage sensitive so switches itself off when the voltage gets too low!

This is more than a little inconvenient as it then takes 30 secs to a minute to re-boot and find itself, As we tend to turn the engine on only when we approach a tricky shoreline this is not good!

I have had two suggestions to fix this:

I) Isolate the two battery systems so that Battery 1 can only be used for engine starting and battery 2 for other services (in extreme circumstances you could use one to jump the other with cables)

2) Fit some form of voltage regulator/uninterruptible power supply between the boat supply and the plotter

Solution 1 would require some extensive re-wiring and the fitting of another isolation switch to remove power to the engine circuit. Something else to forget to turn off - it seems to me to be an expensive an inelegant solution


Solution 2 seems good but I have no idea where and what to buy and with my limited knowledge of electronics I think I would need a mini 12V rechargeable battery as well, since I doubt there is a capacitor large enough to supply the plotter for long enough whilst the engine start takes place.

Raymarine specs state

Nominal voltage 13.8V
Operating voltage 10.8V to 15.6V
Power consumption 13.2W at max brightness


Has anyone else had this issue, how did you solve it and what would you recommend?

Thanks in advance!

Graeme,  "Simia:
Hull 303, Great Britain