Two Battery Set-ups in Parallel

I run two batteries on my boat hooked up in parallel. For those of you that are also doing that are you using two multipurpose batteries or a combination of deep cycle and starting?

I have my big motor connected to one battery by itself (but that battery is connected to the other). And then have all my other stuff (TM, electronics, bilge, etc.) on the other (but connected).

In this manner does it matter whether one battery is starting or deep cycle or not? Whatever set-up I am using is working but am contemplating getting fresh new batteries and getting some battery boxes to tidy everything up back there.

I noticed in wal marks the other day that you can get 2 ga battery cables for 11 bucks. Right now I am using some lighter gauge wire for connecting the two batteries. I think I will upgrade there as well.

You want them to be the same type and size of battery if you connect in parallel.

If your 2 batteries are tied together then every thing you connect to either one is in effect connected to both, unless you have some type of diode in line to prevent current flow in one direction.
When you hook 2 batteries in parallel you basically have one BIG 12 volt battery and it does not matter where you connect things to it.
Both batteries need to be as close as possible in type, size, and age. If they aren't you get uneven discharge over the two batteries.
I ran 2 deep cycle batteries in parallel on my motor home so I got to learn all kind of stuff, some the hard way.

Help us out here just a bit, besides cranking what all do you have hooked to this setup? If just a couple of finders, bilge, lights, livewell pump, areator just normal stuff. I see no need for 2 batteries. Just one good deep cycle group 27 would work just fine. And if you were worried at all a group 29.

Well....the battery your TM is on should be a deep cycle because of deep discharging and recharging....deep cycles are made for this. Regular batteries are not made to withstand deep discharges and therefor would have a short life. I recommend you put your big motor on a a battery of its own and connect your bilge pump....live well and electronics to it. and Get a deep cycle battery for your TM only....if you want more run time on TM then get a group 31 deep cycle for it and run nothing but the trolling motor with it.

I am thinking you want the two batteries matched for capacity and construction. Sort of like having matched horses on a yoke. So the power is as evenly distributed as possible. If you go ahead and get the dual purpose type then supposedly the quick amp draw of the starter and long duration draw of the other stuff will be okay. I would watch the amp hour and reserve rating and try to get the highest I could. Two 27 class would be better than two 24 and so on and so forth. But the weight difference and price has to be considered. I would think two 27 class dual purpose with high ratings would do it for the average guy. A short wire will carry more than a long wire. I think anything over a 6 gauge on a crossover wire of a few inches may be a little overkill.

Your gonna want to charge that with a 12 to 15 amp charger at least I think.

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Originally Posted by cray

Help us out here just a bit, besides cranking what all do you have hooked to this setup? If just a couple of finders, bilge, lights, livewell pump, areator just normal stuff. I see no need for 2 batteries. Just one good deep cycle group 27 would work just fine. And if you were worried at all a group 29.

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Cranking
Trolling Motor
Unit at Console
Unit on front deck
Running lights
Bilge pump
Aerator pump to nowhere (no live well but I can fill my boat with water if I want)
Spot light for running the river in the winter

Boat always had tow batteries in it. One for cranking and one for other stuff. I guess the last time I had to buy a battery I bought two multipurpose batteries and tied them together.

It is probably 70% a duck hunting boat so I suppose I felt like I needed all the juice I could muster to turn the motor over when it is really cold out.


I had a little folding cash in my pocket so I thought it might be a good time to re-do the bilge area and clean it up with battery boxes and while I was at it get some fresh batteries and clean up some of the wiring. It is bird's nest at the moment. The engineer in me hates it.

Do yourself a great big favor. Put 2 batteries in. One for trolling and accessories and one just for cranking and electronics. The reason for putting electronics on cranking is sometimes you get interference when hooked to trolling battery. That way if you did have a cranking problem you could always jump to trolling battery. Several ways to accomplish that. Could just move motor cables or rig a jumper of heavy cable between batteries. I do not like jumper cables in boat around gas tank for safety reasons. Just me I guess but fire in boat is scary.

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Originally Posted by "G"

Well....the battery your TM is on should be a deep cycle because of deep discharging and recharging....deep cycles are made for this. Regular batteries are not made to withstand deep discharges and therefor would have a short life. I recommend you put your big motor on a a battery of its own and connect your bilge pump....live well and electronics to it. and Get a deep cycle battery for your TM only....if you want more run time on TM then get a group 31 deep cycle for it and run nothing but the trolling motor with it.

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how many hours will the group 31 run...on almost full speed?

Depends on boat hull size and total boat weight with persons and gear etc......also depends on what trolling motor you have on mine full speed is about 3 mph....Two group 31 in series for 24 volts will run my 80 lb. Terrova 8 hrs. with power left at a continious 1.7-1.8 mph. This is with just a slight wind......If your trolling head on into much wind it will make a difference in your run time also.

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Originally Posted by "G"

Depends on boat hull size and total boat weight with persons and gear etc......also depends on what trolling motor you have on mine full speed is about 3 mph....Two group 31 in series for 24 volts will run my 80 lb. Terrova 8 hrs. with power left at a continious 1.7-1.8 mph. This is with just a slight wind......If your trolling head on into much wind it will make a difference in your run time also.

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i have a 17 ft grizzly tracker with a mg salt water version tm...modified v hull...the battery which i have now is 3 yrs old and i might...might get 3 hrs if the crank bait bite is on.....light wind.my option is to get 2 batts and run em parallel to get mo run time....depending on what you think about a single gr 31 single batt.. Thanks G.

Tal is your TM a 12 volt ? What lb. thrust is it ?

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Originally Posted by "G"

Tal is your TM a 12 volt ? What lb. thrust is it ?

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12 volt and 55 # thrust....

For the past 7 ys, have switched to small tin boats and 12 volt trolling motors with batteries in parallel. I started out with the largest agm Sears expensive deep cycle (1100 amps) and then added their smaller agm (880 amps) and trolling motors are 55lb thrust min kota's and have never had a problem getting 8 to even 10 hrs of running time running them in parallel with trolling motor attached to the largest strongest amp battery. I know this is against the so called rules of hooking them parallel and not having identical batteries hooked in series, but never had a problem of running trolling motor or length of time running, as I primarily long line and crank and trolling motor is on continuously. When the batteries finally go dead, will spend the money to get 2 that match for the trolling motor usage. I do have and run a third battery for gas motor and all other items in boat hooked to it. Comment why I see no decrease in power over 8-10 hrs or would I see a vast improvement if I upgraded to 2 uniform batteries now. I am now running a new min Kota v2 12v with co pilot and see no need to change my setup.

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Originally Posted by cevans

For the past 7 ys, have switched to small tin boats and 12 volt trolling motors with batteries in parallel. I started out with the largest agm Sears expensive deep cycle (1100 amps) and then added their smaller agm (880 amps) and trolling motors are 55lb thrust min kota's and have never had a problem getting 8 to even 10 hrs of running time running them in parallel with trolling motor attached to the largest strongest amp battery. I know this is against the so called rules of hooking them parallel and not having identical batteries hooked in series, but never had a problem of running trolling motor or length of time running, as I primarily long line and crank and trolling motor is on continuously. When the batteries finally go dead, will spend the money to get 2 that match for the trolling motor usage. I do have and run a third battery for gas motor and all other items in boat hooked to it. Comment why I see no decrease in power over 8-10 hrs or would I see a vast improvement if I upgraded to 2 uniform batteries now. I am now running a new min Kota v2 12v with co pilot and see no need to change my setup.

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Main reason is that your smaller battery is going to deplete first, then it will pull juice from stronger battery trying to equalize. Thus drawing down larger battery. Yes it works and has not given you problems but the ideal thing is running 2 that are same capacity. When buying new the dollar difference isn't that much.

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Originally Posted by fishervet

i have a 17 ft grizzly tracker with a mg salt water version tm...modified v hull...the battery which i have now is 3 yrs old and i might...might get 3 hrs if the crank bait bite is on.....light wind.my option is to get 2 batts and run em parallel to get mo run time....depending on what you think about a single gr 31 single batt.. Thanks G.

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Tal, I run a 55 Terrova on a 17 ft Wareagle with 2 group 27 batteries. I can get a good 4 hr's if not breezy pulling cranks. If you are going to keep that motor and try to pull cranks and you have the room bite the bullet and get at least 1 golf cart battery. Way more reserve capacity than even a group 31. And not a whole lot more dollars.

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Originally Posted by cray

Main reason is that your smaller battery is going to deplete first, then it will pull juice from stronger battery trying to equalize. Thus drawing down larger battery. Yes it works and has not given you problems but the ideal thing is running 2 that are same capacity. When buying new the dollar difference isn't that much.

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Will the v2 min Kota run faster and stronger with a two identical battery in parallel setup vs what I have now? I have been waiting in these Sears batteries to die and now in 6th year of running heck out of them.

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Originally Posted by cray

Tal, I run a 55 Terrova on a 17 ft Wareagle with 2 group 27 batteries. I can get a good 4 hr's if not breezy pulling cranks. If you are going to keep that motor and try to pull cranks and you have the room bite the bullet and get at least 1 golf cart battery. Way more reserve capacity than even a group 31. And not a whole lot more dollars.

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Do golf cart batteries come in diff. sizes? And best place to buy?

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Originally Posted by cevans

Do golf cart batteries come in diff. sizes? And best place to buy?

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:popcorn

They come in various sizes and voltages, like 6,8, and 12 volt. I bought 2 six volt Interstates hooked in series for 12 volts to for our motor home. This was on the advice of an Interstate rep at an RV show.
Reason for the 6 volts were that battery had a tremendous reserve amp capacity.
The difference was remarkable over the 2 12 volt deep cycle Interstates we had before.
We had a ceiling fan plus all the other stuff running off the batteries when we were parked without A/V hookups.
The percentage of use was about half what the previous ones were after 24 hours of running.
They were big boys though. I had to put an extension on the hold down brackets because these were about 2" taller than the 12 volt ones. Same length and width, just taller. Nice things was there was very little difference in price. If you are near a large RV dealer, go check them out at their store. This is everyday stuff for them.

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Originally Posted by cevans

Will the v2 min Kota run faster and stronger with a two identical battery in parallel setup vs what I have now? I have been waiting in these Sears batteries to die and now in 6th year of running heck out of them.

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No, it will run just the Same. Run time on new matched batteries would be longer. But I would not change anything until you notice a big drop off on what you have now. One thing I can tell you having run them before is they will run good up to the minute they die. When dead they are dead. No taking a half charge and run a couple of hr's.

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Originally Posted by cevans

Do golf cart batteries come in diff. sizes? And best place to buy?

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Like SeaRay said couple of inches taller the rest is same as group 31. Best place price wise is usually a commercial battery store. Lot's of times they will have blems or over run's you can get at a good price. They are substantially heavier than a 31 but if weight and space are not a issue they are the way to go.

Thanks to all, for your valuable input. Just going to wait either till these batteries quit or end of year to switch them out as they just keep going strong.