I'm installing a dual batt setup in my truck, much like the one Roger has on his site, starting battery(Red Top) runs the starter, winch and OE fuse box, Aux is for emergency jump starts/acc fuse box, stuff like inverter, aux lights, stereo amp, etc. im using a 200A continuous duty solenoid with a 3 position switch like he has, the only thing left is to get the actual battery. from what ive read, using batteries with an dissimilarities will shorten battery life, even when the engine is running and batts. are charging. The obvious choice would be an Optima Yellow or Blue top, but at close to $200, i really dont want to spend that much. anyone have any experience with using differing batteries in their setup or a suggestion as to a reasonably priced GOOD deep cycle battery? please and thank you in advance <img src="/forums/images/graemlins/smile.gif" alt="" />

Only issue with dissimilar batteries is supposedly when they are connected and not being charged. As long as you primarily use the batteries disconnected when the engine is off, then should be no issues.

thanks roger...also, do i need an isolator? alot of people seem to run them, ive heard they can help if you drain one battery and the other is charged, keeps the current from melting wires or charging one batt too much too fast from the charged battery. ive also heard that alot of people have probs with them, they burn up easily and theyre expensive. any opinions?

I don't run an isolator. You basically run that or a solenoid (or switch). Never had any trouble connecting a dead battery to a fully charged one. Just be sure to use large enough cables to handle the charging current. I use 1/0 welding cable in mine. Isolators work by adding large diodes to each battery to keep them separate under discharge but still let them charge together.

Which battery?
AGM + AGM without isolator, switch to disconnect.
AGM + standard wet cell, use an isolator.

Those red and yellow topped batteries are AGMs and barely vary in internal design.





Depends on the isolator. Some use FETs. Some relays. I'd go for a heavy duty relay based one, but I bet most short on the size of the relay. If I was making one I'd try to find a 300+ Amp DC relay. Note, they are not cheap. I guess another design may work well. That is to use large diodes for separation when the voltage difference is high, then close a relay when they get close in voltage. A pair of 200+ amp low voltage drop diodes in half-pack cases should only cost $50 plus heat sink (needs to be isolated), and misc parts. For a hundred bucks a robust diode based isolator could be made.

ditto.. mine's exactly the same way.. 1/0 welding cable, wet battery up front, 100AH AGM gel cell at the rear of the vehicle.. and using a solenoid to parallel the two only when vehicle running.. Does have emergency override to parallel the batteries if the front one were to go completely dead.

As mentioned above, issue of connecting a run down battery to fully charged one is overrated.. There will be high current flow for a brief period of time, but it tapers down rather quickly. But you certainly don't want to skimp on the size of the wiring between the two. And you want the sysem to disconnect the parallel contection when the vehicle is not running.

We deal with commercial batteries one of which has an 11,000 amp short circuit capability (1700AH & weighs 6500lbs) and we have had to parallel those into old systems with weak batteries. No problem as long as you know what you are doing.

Yep, no different than hooking up a set of jumper cables. I like the single solenoid setup, it works very well, it is very simple, very flexible and very reliable. In normal operation, I just leave the switch in the automatic-on position and everything just works like stock. Only when I kill the main battery do I need to flip the switch to the jump start position and then start up the engine. Now, if I ever get all my auxiliary loads re-wired to run off the 2nd battery, I may not need to even mess around with that, since I should not be killing my main battery any more while camping.

I'm curious as to the reason for disconnecting the batteries when they are not being charged (other than eliminating the chance of killing the main battery due to loads on the auxillary battery or vice-versa).

I run a similar setup as yours and during cold weather I often leave the two batteries manually connected together to aid in cold starting. So far I have not seen any problems.

Supposedly if you have two batteries, and there is a slight voltage difference between the two as they self-discharge, the lower voltage one can pull current from the higher voltage one and end up discharging both of them.

I've not seen this happen, but with my solenoid setup, it automatically disconnects them when the ignition is turned off. And as soon as the ignition turns on, the solenoid kicks in and parallels both batteries for starting.

This is a valid problem and bites one the most when one of the paralleled batteries gets old near it's life expectancy. Note, this is only a problem when the current draw from the paralleled batteries is not strong enough to draw the combined voltage at or below the resting voltage of the weaker battery. Most often that is when nothing is drawing on the batteries, but it can still happen even when there is only a small draw. If the batteries are always being charged or discharged it isn't an issue. However if they spend allot of time idle, then it can be an issue. How much of an issue, I don't know.