# charging deep cycle batteries connected in parallel



## RMK (Feb 21, 2017)

i currently have 2 12V size 29 deep cycle marine batteries connected in parallel and a 3rd size 27 12V deep cycle marine cranking battery in the back of the boat. I am currently charging all 3 batteries with a 2 bank 15 amp onboard charger. 1 bank to the cranking battery and the other bank hooked to the 2 big batteries (charger connected positive to one battery in the circuit and negative to the negative of the other battery in the circuit) 

I m in the process of adding another 12v size 29 deep cycle marine battery farther forward in the boat that will be dedicated power source for 2 fish finders at the bow of the boat. 

i ordered a new 20 amp 3 bank onboard charger and planned to charge the 3 batteries in the back of the boat with the new charger and move the old charger farther forward to charge the new fish finder battery. 

Question is can i charge the 2 batteries connected in parellel each with their own bank of the new 3 bank charger? 1 bank to each of the 3 batteries in the back of the boat. Or should i have just bought a new 1 bank charger for the new fish finder battery and left the back of the boat alone? i was hoping with the higher amp and 3 bank (newly ordered onboard charger) i would be able to charge the 3 batteries in the back of the boat more efficiently and use my old 2 bank to charge the new fishfinder battery. (obviously only using of the of the charging banks )

I know somebody on here knows, if they are able to follow my rambling...


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## KaGee (Sep 8, 2006)




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## RMK (Feb 21, 2017)

i saw that. only confusing part was they said you could charge each battery individually in 12v mode but then when they demonstrated that in the animation they only hooked the charger up to one battery (the one on the left) when they did that. demonstration error or not? i m wanting to hook up all 3 batteries to 1 one 3 bank charger, each battery on their own bank, including the 2 batteries that i have wired in parallel.


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## leeabu (Apr 10, 2004)

RMK said:


> i saw that. only confusing part was they said you could charge each battery individually in 12v mode but then when they demonstrated that in the animation they only hooked the charger up to one battery (the one on the left) when they did that. demonstration error or not? i m wanting to hook up all 3 batteries to 1 one 3 bank charger, each battery on their own bank, including the 2 batteries that i have wired in parallel.


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## crappiedude (Mar 12, 2006)

I have the same 3 batteries in my boat and they are hooked up the same as yours.
2 series 29 for the TM (ran parallel for 12v TM) and 1 series 27 for starting/electronics.
I have a 3 bank charger and 1 set of leads go to each battery.
Unless I'm fishing, the charger stays plugged in.


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## RMK (Feb 21, 2017)

crappiedude said:


> I have the same 3 batteries in my boat and they are hooked up the same as yours.
> 2 series 29 for the TM (ran parallel for 12v TM) and 1 series 27 for starting/electronics.
> I have a 3 bank charger and 1 set of leads go to each battery.
> Unless I'm fishing, the charger stays plugged in.


Perfect! Thanks alot. I will proceed with my plan. It made since to me but I m not an electrician.


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## MuskyFan (Sep 21, 2016)

Crappiedude’s set up is the same as I had in my bass boat with a 12 volt TM and two batteries in parallel. Works the same way when they are in series as my current 24 volt system is set up. One charger bank to each battery. 

When using a multi bank charger you need to read the instructions. Many say you need to have all leads connected, even if only connected to one battery. So I would connect both sets of leads to the one standalone battery for your fish finder.


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## crappiedude (Mar 12, 2006)

MuskyFan said:


> Many say you need to have all leads connected, even if only connected to one battery. So I would connect both sets of leads to the one standalone battery for your fish finder.


I do believe BPS & maybe Pro Mariner charges say to do that but I believe Minn Kota chargers say not too.
Most manufactures have a website with Frequently Asked Questions. Some manufactures cover this in there. I would check there 1st to verify and if they didn't cover it I would email them just to be certain.


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## MuskyFan (Sep 21, 2016)

crappiedude said:


> I do believe BPS & maybe Pro Mariner charges say to do that but I believe Minn Kota chargers say not too.
> Most manufactures have a website with Frequently Asked Questions. Some manufactures cover this in there. I would check there 1st to verify and if they didn't cover it I would email them just to be certain.


My older Cabela’s one said to ensure all are connected. I used to use one car battery charger and hook it to one battery for my TM setup until I got the Cabelas one. 

You’re right about reading the directions, which no
man ever does until something goes wrong.


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## RMK (Feb 21, 2017)

I was also wondering about the spare bank/wires ill have when i hook the old 2 bank up to the single new finder battery. It is a few year old cabelas model. I planned on leaving the extra bank free but I ll look into it now that you guys mentioned it.


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## crappiedude (Mar 12, 2006)

Back in the winter I did a lot of research on batteries, chargers and TM's. Everything on the front of the boat was dead or dying so I put everything in new. One concern I had was if I stayed with a 12v TM I was contemplating going to 1 vs 2 batteries. Since I needed to replace my batteries before I decided on which TM I was going to get I also looked into what I should do with the extra leads on the charger should I decide to go with 1 vs 2 batteries.
I always assumed and have read on some chargers to just hook up all the leads from the charger. There was no reason to leave any lead loose. I just looked on the MK website and it said it was okay too. One charger I looked at specifically said not to.

I would imagine you'd be okay hooking both leads to a single battery. If it's a 10 amp charger, you'll get all 10 amps going to that single battery rather than getting 5 amps to 2 separate batteries.


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## DAVELEE (Apr 19, 2004)

the trouble with two batteries is if they are at a different amp draw, say one is almost dead and the other is only 50% they will both see the same amount of current this can be hard on both the charger and the batteries... I usually just disconnect and charge individually.


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## walleyechaser (Apr 13, 2004)

You can use a battery isolator to charge both batteries with one source by using a battery isolator like this one from auto zone.
https://www.autozone.com/batteries-.../east-penn-95-amp-battery-isolator/379952_0_0
This alows charging 2 batteries with one charger. The charger goes to the center post and each battery connects to the other posts. This allows the battery at the lowest voltage to charge first then shut off and charge the other battery without over charging. I have used these for years in my trailer to charge the camper battery without overcharging the vechicle battery. The current rating in amps is the max rating of this unit. It will charge at lower current rating like a battery charger.
You can also use a switch like this to combine or isolate batteries. I like this switch because it is a make before break switch. It does not disconnect the first battery untill the second battery has made connection.
https://www.solidsignal.com/pview.asp?p=6007200&msclkid=cd23e022790214cc5a82bf7703f37bdf
Just some ideas to solve a problem not the only solution.


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## RMK (Feb 21, 2017)

Thank you guys for your help. Just need to tidy up a few wires and the boat is ready for cumberland next week.


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