Rotary Battery selector switches and isolators
I was recently asked for my opinion on a rotary battery main switch being sold on E-Bay.
I am not at all keen on these rotary switches, I have seen too many problems with alternators routed through them blowing up and also from accidentally discharging both banks of batteries and being unable to start motors as a result. All too easy.
Yes they are used a lot, but Sod's Law rules, if you can turn the thing off and the charging is routed through it, pound to a penny it will happen. Same goes for leaving the switch on 'both' and draining all the batteries.. Been there, seen it a dozen times.... convivial atmosphere, a few beers and someone forgets to switch the blasted rotary switch back to battery 1 instead of both....
No, I prefer to route the charge from the alternator direct, through a splitting diode, to the batteries, no switches in that circuit. (You can fit an ammeter in the circuit, here, I have, so you can keep a check on the charge rate all the time.) With this system, if the master switch is turned off the alternator will still charge and not blow up any item it can still send volts to.. I was on my boat, back in the early 90's, when the VHF exploded, a battery switch was turned off whilst the motor was running, (the old Stuart turner) The alternator sent an estimated 110volts into the 12v radio.. Smoke was impressive.! Amazingly the radio survived, after a hefty bill for replacing the components blown... and survives to this day! (2007) I rewired the alternator very soon after that, on advice... it works! Since then I have always kept electrics as fool proof as possible!
The Diode splitter I have cost only £30.00, I added a second a little while later when I wanted to split the charge further to another bank of batteries for the fridge, so it is split 3 ways. No battery can back charge to another and the one that needs the most charge gets it. Today I would use one of the more modern splitters that does not cause a 0.8 volt drop. But as the ‘clever’ charge controller on my alternator compensates, it matters not that there is a drop. Without that controller you would be better with one of the more modern sort, little to no voltage drop.
For the Start and Services batteries, a heavy and simple 'Hella' switches fitted. (The sort with a red plastic key) cost less than £10.00. The keys come out and you can take them away, without a key you have to rewire the battery switch to start the motor. Anti theft as well.
The negatives will be connected all the time, via the earth on the motor, so if you do need a jump start it is simple to use a single red jump start cable from Start battery to services and hey presto you have both together. No chance of leaving the jump lead in place all night either, too obvious...
With this sort of set up I can be confident there cannot be a flat battery nor any damage through voltage spikes..