Thread: Hooking batteries in parallel

Originally Posted by Nutbush

I have a dilemma. I am wanting to start pulling/pushing cranks. My current setup is a battery for my motor and a battery for my trolling motor, 12v. I am pretty sure that to be able to keep up the constant speed that I will need, that I will probably get about 3-4 hours of run time out of the one battery. Also, I have two finders running off of my trolling battery. I currently have two choices: run a separate cable from my start battery to my finders, or hook up the two batteries in parallel. My future plans are to run the trolling motor off of two batteries parallel and the one dedicated to the motor, but I am going to wait till one of the batteries is dead so i can buy those two at the same time. If you were in my situation what would you do?

Run your finders and anything else off cranking battery. Don't hook the 2 together. If one is weaker than the other it will also draw on good battery. Also you stand a good chance of running down cranking battery to the point motor won't crank. If I was in your situation I'd buy 2 batteries and be done with it, But not knowing your financial situation in this case think I would buy 1 good battery then hook troller to it and use existing battery as back up without hooking together. Kinda goes against what I would normally say about buying 2 matching batteries but sometimes finances dictate otherwise.

As I understand it ... hooking up two batteries together that are different series, &/or age, is not a good idea.

I would go ahead & buy the two new TM/deep cycle batteries (as large a series as will fit) and the connecting cable.
Check the power cord of the two depth finders and you may find several feet of excess cord rolled up & bound ... if so, I'd unbind that cord and move the connecting ends over to the starting battery. (I'm assuming, of course, that you're thinking you have to run a "separate cable from my start battery to my finders" because the power cables wouldn't reach the start battery). But, if your finders aren't getting interference from being hooked to the TM battery, now ... then there may not be any reason to move them to the start battery anyway.

It might help to know : boat length/style - trolling motor # thrust - current TM battery size (series) ... as well as what is the max speed you can currently maintain. Parallel battery hookup will give you more "time", but not more "speed".

I'm no expert in any of this, so I defer/refer you to those that are more experienced & knowledgeable on the subject. Maybe CatFan will chime in !!

I wouldnt run anything off my trolling motor batteries except the trolling motor. Everything else in the boat should be connected to your starting battery. Batteries that are wired in parallel should be same brand...type...group size...and age. I recommend group size 31 deep cycle batteries for your TM.

Thanks for the replies. I fish out of a 17' bass tracker. The trolling motor has plenty of speed, its like a 45 mk. I found some 12# wire so I think I'm going to run me a separate power supply for the finders off of my start motor for the time being until I can get another battery.

Originally Posted by "G"

I wouldnt run anything off my trolling motor batteries except the trolling motor. Everything else in the boat should be connected to your starting battery. Batteries that are wired in parallel should be same brand...type...group size...and age. I recommend group size 31 deep cycle batteries for your TM.

I agree with everyone. I asked the same question couple months back and got same answers. I did what they are telling you and I have ran my trolling motor for 2 full days trolling and still had plenty of juice in them. I had 2 batteries that were bought for a semi that a guy didn't pay me for so I put them on my trolling motor and so far it has been better than people told me. If you can afford it do what they are telling you and you will be glad you did.

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Nutbush, my set up is the same as yours. I have a Tracker with a 12v TM and added the 2nd battery to add additional trolling time.
Like everyone said only TM on TM dedicated batteries, everything else goes to starting battery. With todays newer electronics I had to increase my starting battery in order to handle the extra load that the extra units put on the main battery. Original battery was a 24 series and I increased it to a 29 series.
Both of my TM batteries are the same age and they go bad & get replaced at the same time. My starting battery is not on the same replacing schedule as the TM batteries and it lasts longer than my TM batteries. I've had this set up for maybe 10/12 years.

One thing I didn't see mentioned in your plan is to add 3 bank smart charger. At the end of the day you just plug in the boat and everything is ready to go the next time you need it. FWIW I was a little short on cash when I added mine and I added a 5/5/5 BPS charger. It works great and in the 5/6 years I've had it I've never had it not charge my batteries overnight.

One other thing. I would increase your wire size to you electronics if you can from the 12 ga to 10 ga. Lot's of guys will have their electronics shut off when they start their outboards and usually it is because of insufficient wire size from the battery. I run 3 units so maybe if you're just running 1 the 12 ga may be big enough.

Good luck with your new set up.

Thanks for the replies. Yesterday I ran a separate wire from my start battery to my finders. I run two finders, a small DI Pirahanamax and a HB 859. I will see tomorrow if I will need to upgrade my cable from my start battery. When this one trolling battery goes bad I am going to buy two of the same and run the tm off of them.

You might consider a separate battery for you finders. I run a 7AH Gel Cell for a Helix 5 DI GPS at the console and an older 2D Lowrance up front and can fish all day without ever running the Gel Cell down. I charge it with a 2A trickle charger overnight. The Gel Cell is small and can be mounted just about anywhere.