Dual batteries isolator or not?

okay all with dual batteries anyone use an isolator? or what do you use too protect the back up from main an vice versa?

It is generally thought that you MUST run an isolator with a dual battery setup. This is not really the case. If you are using to identical batteries of the same age--IE=you go out and buy two brand new matching batteries--you can get away without HAVING to have one. When you use two dissimilar batteries they will "fight" internally with each other and drain each other down when sitting for long periods of time.


Now,would I CHOOSE to go that route? No. Go out to a stereo shop that sells Stinger products and ask them if they can get you a 200 amp relay. This will set you back around $70,but it's worth it,and it's cheaper than buying one from Wrangler or whoever else sells the dual battery isolators (and it's the same identical thing,too). It's a very simple deal :

Wire it between the batteries,and hook the switch poles to a power supply that is switched on only in the ignition position--AND IS 12+ ALL THE WAY THROUGH THE STARTING PROCESS!!!!! (and the other to ground) This will allow you to totally drain the "main" battery(if you wanted to do that--totally draining a battery is the WORST thing you can do to a battery! *unless it's a marine deep cycle*),while still having the other one there to get you going if it's needed.

Motorhomes and vehicles that pull travel trailers normally use a solid state battery isolator. Keeps you from killing the main battery while "camping".

What you need may depend on what the second battery is being isolated for. You certainly should have an isolator, but for some purposes that can be as simple as a big fat diode on a separate positive lead from the alternator. This would be quite adequate if the second battery is dedicated to something like a winch. A deep-cycle battery with its positive cable going only to the winch, and an isolated positive lead from the alternator would do just fine. For the occasional emergency boost, you could just get out the booster cables.

If you expect the second battery to be used for more than occasional cranking, you need more than that.

Years ago I had a Scout with a balky starter, and put in a dual battery setup. The positive side of battery 2 was connected to the alternator through a diode, and to the positive of battery 1 through a starter solenoid. When Battery 1 wasn't enough, I pushed the "magic button" on the dash, and got an instant boost.

If you ever owned a Scout 80 or 800, you'll understand why this was easier than replacing the starter. I eventually cut a hole in the floor to do the job.

If you only just a single diode to isolate the second battery it will never be fully charged. It will always be charged at 0.7 volts less than the main battery.

PaulJ said:

If you only just a single diode to isolate the second battery it will never be fully charged. It will always be charged at 0.7 volts less than the main battery.

Click to expand...

That is correct, but in the case above, I never had a problem, because the charging voltage was still well above the residual voltage of the battery (thus no harm to the battery), and the second battery was only put into use when the first battery was drawn down anyway (thus no likelihood of having the voltage of battery 1 feed back into battery 2).

A single diode should still work all right if the normal use of the second battery is entirely separate from the first and if you're sure the voltage drop isn't too much. But Paul's point is well taken if you expect to use them together regularly. Then you should get a more sophisticated setup so that the two batteries will float at the same voltage. And of course the initial cost of the equipment will be offset quickly if the alternative of a poorly designed home-brew destroys batteries.

I found a slew of daul battery info and sources on a defender 90 site, made for an informative read, scroll to the Battery and Split Charge sections - enjoy!

http://www.d-90.com/faq/Engine/EngCharge.html