dual battery switch

[ShaneJ]

Hi guys,

I just did this quick drawing. I hope it makes sense to you.

[Warren]

interesting shane, thanks mate

Warren

[ShaneJ]

With that arrangement theres no chance of your crank battery being discharged.

[matty]

I'm guessing that the relay in this set up would have to be a high amp relay for the starter motor to operate.

if so what size relay should someone be looking for?

[ShaneJ]

In any setup you should use the highest rated relay you can find. Just pop into your local auto electrician. They should have them in stock.

Stay away from these, they are rubbish:

[Worzel]

I just did this quick drawing. I hope it makes sense to you.

I'm a bit fuzzy on the lekky side of things,( as opposed to really fuzzy with the mechanicals ),

I have a solinoid between the two batteries, so i have two batteries to crank,and they both charge when the motor is running but two independant systems when the key is off, but it looks like they both achieve the same thing

[Warren]

Worzel

Shanes system will not allow any current to be drawn from the crank battery once the ignition is off.

your set up will stop either battery trying to "top up" the other but still allow both to have curent drawn from them, depending on what circuits are connected to each battery.

Both systems will have both batteries available for cranking and charging once the ignition is switched on.

Warren 

[ShaneJ]

Warren is correct. My setup completely disconnects the main battery when the ignition is off allowing you to run your stereo, head lights etc without the worry of not being able to crank her over.

[ben]

what about mixing good batteries with not so good batteries thats a no no yeh?

[Warren]

Benny, if you just have two dissimilar batteries running in parrallel they will try to find a common level, so the lower charged battery will draw power from the other battery.

Dual battery systems have an isolater that prevents this. They use diodes to isolate the two batteries (diodes are like a one way valve for lectrickery).

The main problem I see with Shanes system is that, if the house battery has dropped considerable voltage over night, when the engine starts and the altenator start recharging both the crancking and house battery a large amperage could be drawn, possible overloading the standard altenator.

In the older manual type systems the house battery would be switched off for the first 10 minutes or so to allow the cranking battery to be fully charged before trying to charge the house battery.

Note - as a battery gets closer to its fully charged state, the amount of current that it can take reduces exponentially.

Deep cycle batteries can take 8 to 10 hours to get back up to full charge, possibly longer.

I have heard that "Marine" batteries are a good (and cheaper) alternative to deep cycle batteries as they are a blend of cranking and deep cycle - I have not tested this though.   

Warren