Need help on wire gauge for second battery

[anthman]

Hi all, I have a Tohatsu m40d2 (a two stroke 40ho outboard).

I've bought a second battery as a backup and looking to hook it up to a vsr and a switch so that it can be maintained by the 11amp alternator in the outboard and at the turn of a switch, provide alternative power to start the engine in case the primary battery dies.

Ive bought a vsr already and about to buy a switch (four positions: primary, secondary, both or off). 

I'm not sure of the wiring between engine, primary battery, secondary battery, vsr and switch and I also don't know which wire gauge I need for which connections (or of they overlap - eg. Charging over the same cable as starting the engine).

Can anyone share some experience based in the above requirements? I couldn't find the amp draw of the engine so not sure about the wire requirements. Distance would be all very close to the engine, so would not expect more than 1.5m between connecting points...

 

Thanking in advance!

[552Evo]

Does your VSR have a model number or description that says how many amps it’s rated at ? That would guide you to the cable gauge required between the he VSR and each battery. Example also is a “start Isolation” wire and a “no current/storage” sensing wire - see the attached image that suggests 16gauge  

As far as from each battery to the isolator switch id be using at least what’s already rigged up to your first/original battery or bigger. 

6gauge minimum or better 4gauge. 

Without seeing how much room you’ve got and terminal size limitations  basically/within reason the bigger the cable gauge the better for reason of the cranking ability of the outboard. 

 

 

[anthman]

Thabks for the reply!

The VSR I bought is rated for 140a.

I've also attached the wiring diagram that's associated with the VSR - looks a bit different to your diagram...

[552Evo]

No worries anthman, the basic ide of both diagrams are the same, the diagram I used incorporates a manual changeover switch like you mentioned earlier. 

The other 2 small gauge wires in my diagram are optional. 

At 140 Amps (your VSR capacity) you could even go bigger than 4 gauge, like 2 gauge but I imagine that would be overkill for a 40 hp motor. 

Do you know what gauge cable your existing battery is wired up with to the motor ? 

[anthman]

I've just checked - it's 22mm2 (squared) , which looks like it equates to 3B&S?

[anthman]

Btw I just found a really good price on 2 gauge wire, so bought some of that - should be sufficient I think. Will have a go at planning out where things go this weekend - certain I'll have more questions!

[anthman]

Hello all, my dad has taken a go at drawing up a circuit diagram of how the wiring might go. Can someone please validate? Our objectives for it are:

1. Have the second battery as a backup to the starter. Don't have any accessories attached to it, so it's basically sitting there until we need it. Not sure how isolated the second battery is in how we have drawn it up in the diagram.

2. Use the VSR to keep both batteries topped up as needed when driving.

3. Use the switch - probably left in position 1 (starter battery) until we get home and set it to off. Not sure if the second battery can be charged whilst we have the switch on 1 (starter battery).