View Full Version : Dual battery setup

04-04-2007, 04:00 PM
Hi all, did a search and found myself more confused than before... isolators, combiners, perko, voltage meters, etc. Yikes!

I want to add a second battery to my '04 Mobius LS. I talked to a guy at my local auto parts store and he suggested just purchasing another battery and two cables and hooking them both up in parallel? He didn't seem to know anything about the combiners or isolators they sold. Seems simple enough? I don't think I need to go as complex as separating the starting system vs stereo, lights, etc. as we don't usually run the stereo or for hours on end. Just want a little extra piece of mind and simple-stupid installation that both charges on the alternator and doubles my battery time. Can someone clear this up for me and/or provide a few recommendations? Many thanks!

04-04-2007, 05:53 PM
my personal preference is a Perko Battery Selector switch. I do not have one on my Outback but would like to add. In my offshore fishing days, we always had that style. It allows you to turn all off, 1 on, #2 on or both.
This allows you to run accessories like the radio on just one battery and protect the other. You can switch to both while running to charge both or for added cranking power.
If you were not going to add a selector switch, my opinion is you would be just as well off buying one of those portable jumper batteries in case you went dead. If you had both batteries connected at all times, you could end up with 2 dead batteries... Just my opinions as I am not a tech pro... I have never installed one of the Perko switches so I am curious as to what is involved in the process. I would not mind doing so soon.


04-04-2007, 06:34 PM
I used a Stinger 80 amp relay that I picked up for $15 on ebay. It connects the two batterys together when the boat is running and separaes them when it's not. That way I always have a backup to start the boat. It's really easy.

04-04-2007, 07:36 PM
The relay system mentioned is similar to the motorhome type relays that can connect or isolate the two batteries and are cheap and reliable. The drawback is the relays are not manufactored for a flammable environment which means you need to have the relay away from gas fumes or where gas fumes could potentially exist due to the spark created when the contacts open and close. The battery isolators cost more but use electronic switching and would not have the arcing. You should isolate the batteries and connect your accessories to the backup battery.

04-04-2007, 10:56 PM
I installed a Perko switch and a second battery. This is the second boat I own with this set-up. The manual control (Perko) has its limitations. The worry-free way would be Zabooda's suggestion but is more expensive.

04-05-2007, 01:01 AM
Actually I am cheap and would probably go with the relay if I could ensure the storage area is sealed off from the bilge as it is in my boat if I made a tighter seal around the vent tube that goes from top side into the bilge. People have to make their own determination and risk based on the location they put the isolator.

04-06-2007, 10:53 AM
All good suggestions, many thanks...I still like the cheap and simple stupid idea of just buying pos and neg cables and running them in parallel. I highly doubt I'd end up w/ two dead batteries as I'd have 2x the power reserve and I've never drained even one before. Are there any technical reasons this won't work? Or, the jumper battery pack is also a good idea and fits my criteria.

04-06-2007, 11:29 AM
Keep in mind that when you run a bank of batteries in parallel, it is best start with two fresh batteries. If you don't the bank will only be as efficient as the weakest battery. Best way to kill a good battery is to attach it to a bad one. I had a Perko switch on my travel trailer, and it worked great. When I had two batteries in parallel, they died running the furnace fan overnight. I woke up to no power. After the switch was installed, I would select one battery at a time so I always had a spare. Switch it to both when you are charging and it will charge them up at the same time. The switch was only $30 and wasn't hard to install.


04-06-2007, 05:39 PM
Boat's got it right. Your system will only be as good as your weakest battery and the weakest battery will serve as a load for the other battery to attempt to equilize each other. You will have to replace both batteries at the same time with equal batteries. You would be better off with one large battery. If your intent is to run the stereo and other auxiliary stuff and not risk having a dead battery to start with then you need to need to isolate the two batteries when your parked.

04-08-2007, 10:47 AM
Excellent points. I think I'll either go w/ the jumper battery pack or the Perko. Thanks!