# On Board Charging Two 12v Batteries in Parallel



## Flannel_Carp (Apr 7, 2014)

My dad just bought a 16 foot Lund and is wanting a second battery in parallel for his trolling motor. The boat came with one 12v battery already.

I want to get him an onboard charger so that he doesn't have to remove the batteries at all because of how physical a task it would be for him. 

My main question is, having the batteries hooked up in parallel, would I want a one bank charger that is hooked up to both batteries (pos on one battery and neg on another), or a two bank charger hooked up to both batteries (one bank to pos and neg on each battery)?

I am looking at a 10 amp charger either way.

Thanks!


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## Popspastime (Apr 1, 2014)

2 Bank is best. Chargers come in different charging capacities so a single would be very slow because of loading the and dividing the amperage. You want to charge each battery individually to properly maintain them. Also when you charge with a 2 bank you need to remove a parallel lead so it's actually charging 2 separate batteries.


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## fastwater (Apr 1, 2014)

Agree with Pops. Would go with the two bank onboard charger. Besides very equal rate of charge per each battery, your charger will automatically run a diagnostic check on each battery separately before charging begins. Two bank would just be more efficient.

You could wire a single bank 12v charger hooking a positive lead of the charger to one of the positive post on one battery and the negative lead of the charger to the negative post on the other battery and be able to plug and charge with no problem as well. But charge rate will very to each battery and your charge time will most likely increase.


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## crappiedude (Mar 12, 2006)

I also have a 12v TM and I'm using a 2 battery set-up for the TM (hooked up parallel). I do have a 3rd battery for starting/electronics so I'm using a 3 bank charger. I suspect the starting battery doesn't need a lot of charging overnight but todays electronics and accessories can draw more amps than yesteryears electronics did (especially with livewell &/or bilge pumps running too).

If you only have the 2 batteries or you're only concerned about the 2 TM batteries I would get a 2 bank charger and I would run leads to each battery. There is no need to unhook any leads while being charged. I have 5 amps per bank and I've never had the batteries not charged over night and be ready for another full day of fishing no matter how much I've fished the previous day.

If the boat also has a 3rd starting battery I would recommend getting a 3 bank charger. If for no other reason it would keep the main battery charged and maintained through the winter months.


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## Flannel_Carp (Apr 7, 2014)

crappiedude said:


> I also have a 12v TM and I'm using a 2 battery set-up for the TM (hooked up parallel). I do have a 3rd battery for starting/electronics so I'm using a 3 bank charger. I suspect the starting battery doesn't need a lot of charging overnight but todays electronics and accessories can draw more amps than yesteryears electronics did (especially with livewell &/or bilge pumps running too).
> 
> If you only have the 2 batteries or you're only concerned about the 2 TM batteries I would get a 2 bank charger and I would run leads to each battery. There is no need to unhook any leads while being charged. I have 5 amps per bank and I've never had the batteries not charged over night and be ready for another full day of fishing no matter how much I've fished the previous day.
> 
> If the boat also has a 3rd starting battery I would recommend getting a 3 bank charger. If for no other reason it would keep the main battery charged and maintained through the winter months.


Thank you! This is exactly the answer I have been looking for that even Minn Kota had trouble answering for me! There is a lot of info online about onboard chargers and info about batteries in parallel, but I had no luck finding anyone talking about using a two bank charger on two batteries in parallel.

Regarding an onboard charger, do you have a recommended brand?


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## crappiedude (Mar 12, 2006)

I don't really recommend a brand but what I have on my boat is a BPS XPSit 5/5/5. I've had this thing for years and it's served me well. I think they have a new model out now which is something like an XPSit2 5/5/5. If mine dies I will replace it with one of these.
The reason I don't recommend any one brand is I think they all function pretty similar. I know people with Guest, Minn Kota and Pro Mariner (Pro Mariner made mine) and no one has had any consistent issues. The only guy I know who's had a problem with an onboard charger had a pretty high dollars Guest but he's an idiot and 1 charger problem doesn't make it a trend. Him being an idiot could have led to the problem though. I read a lot of maintenance threads and no one brand seems that much better than another.
Just be sure it's a smart charger and it will charge, monitor and maintain the battery while it's plugged it. This has to be one of my better boat purchases in my life.


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## Popspastime (Apr 1, 2014)

Be carefull.. if you have 2 batteries in parallel trying to charge they are only as good as the worst one, and will only read as good as the lowest/worst one. If your charging and one battery is week the charger will keep trying to bring the batteries up to 13.9 there and a bouts under the load while charging and will never get there even thou one of your batteries are perfectly good, thus cooking the good battery. 
Just as a test, take a fully charged battery and read the voltage, should read around 12.6 - 12.5 guessing. Now read one thats run down to say 11.2 - 11.6 or so, then hook them in parallel and read the two leads, might read maybe 12.0. So all the time the charger is metering the 12.0 because it thinks it's reading and charging one battery, not 2 so it keeps cooking the good battery trying to peak at 13.9 or so charging before going to trickle, not good. 
Hope you understand what I'm trying to tell you to avoid cooking a battery, but to each his own.


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## crappiedude (Mar 12, 2006)

Popspastime said:


> Hope you understand what I'm trying to tell you to avoid cooking a battery, but to each his own.


I get what you're saying.
I've had this current set up since 2011 and have had no problems so far. Prior to this I had a 24v system. I can't remember but I did have a bad battery with this charger and but it wasn't much of an issue. It's been a while and I don't remember all the details but I think a fault indicator light lit up on the charger. It was no big deal.
Either way using this set up my current batteries are 5 years old and still going strong.


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## Fishtracker1 (Mar 29, 2009)

You may be interested in this. Thx

https://www.ohiogamefishing.com/threads/2-bank-10-amp-battery-charger.346041/


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## walleyechaser (Apr 13, 2004)

You could also use a marine switch connected to the batteries which would make the parrallel connection but also give you a way to separate the batteries and charge them independant without removing them from the boat. Connect the dual charger to each battery put the switch in off and let them charge. Put the switch in 1+2 and now they are in parallel. Change the switch to 1 or 2 and use each battery by itself. This link is and example of what type of switch I am talking about.
https://www.amazon.com/Blue-Sea-Sys...&sprefix=marine+battery+switch,aps,165&sr=8-4


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## c. j. stone (Sep 24, 2006)

fastwater said:


> Agree with Pops. Would go with the two bank onboard charger. Besides very equal rate of charge per each battery, your charger will automatically run a diagnostic check on each battery separately before charging begins. Two bank would just be more efficient.
> 
> You could wire a single bank 12v charger hooking a positive lead of the charger to one of the positive post on one battery and the negative lead of the charger to the negative post on the other battery and be able to plug and charge with no problem as well. But charge rate will very to each battery and your charge time will most likely increase.


Of course, as stated, a dual bank charger would ultimatey be best but if you had/have a "quality" 10 amp load sensing charger connected +- to either one of two batteries in parallel, the charger "sees one big 12 v. battery", and would charge both nicely overnight! It'd be sililar to my grandson's electric dirt bike which comes from factory with a connected dual sealed lead/acid battery pack and a basic "plug in the wall" type charger similar to your phone charger! This is not "rocket" science as some seem to think.


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## Popspastime (Apr 1, 2014)

If one of the batteries takes a dump it won't charge the other.. why is it some don't understand the theory?.


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## crappiedude (Mar 12, 2006)

Popspastime said:


> why is it some don't understand the theory?.


I think a lot of us understand the THEORY behind what you're saying but it's not an actual problem we're having.
Besides me using my set up, I have 2 other friend using the same set up I described and none of us have had any problems. Between us we have to have over 20 years of use and none of us are having problems. I know at some time we've all replaced a battery here and there but batteries don't last forever. If one TM battery dies, I may fish a day or 2 on the good single battery only if I need to but then I will replace both batteries at the same time.

Pops, is this something you're having problems with? OR...is it just theory?


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## Popspastime (Apr 1, 2014)

Crappiedude,
Nope all good, I used to race electric and batteries were the most important thing, they all have a peak thats different from the identical other. Snow plow parallel batteries were great for longevity until 1 got weak and both would stay low until the weak one was taken from the string. Your batteries and your choice what you do, but when someone asks we all try to tell how it really is and not the backyard hook-up thats been working for years. Do it yourself and see the outcome, charge 2 in-line and see how long it takes, does your charger go green? Then take a multi-tester and read the peak voltage in each battery seperated. Thats the point of my response.


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## crappiedude (Mar 12, 2006)

Popspastime said:


> I used to race electric and batteries were the most important thing


So what did you race? I'm curious.



Popspastime said:


> Your batteries and your choice what you do, *but when someone asks we all try to tell how it really is *


FWIW...that's why I answered. Like you I was simply trying to help the guy out with what has worked for me *using my experience* and some of my friends. I mean its working.
Again (and this is my experience) marine batteries generally seem to last 3 or 4 for years. After that something seems to go wrong with one or both of the batteries. AGAIN FOR ME, if something goes wrong anytime after 3 years I figure they served there purpose and anything after that point doesn't matter to me since I'm replacing both batteries anyway.
I'm no expert on batteries, not at all and don't claim to be but *I have always read that in any marine 2 battery set up it's best to change out both batteries at the same time.* (don't mix old and new batteries) With that said if either of my batteries don't take a charge, they're both coming out anyway. So whatever happens to the second battery once the 1st one fails...it doesn't matter.



Popspastime said:


> and not the backyard hook-up thats been working for years.[/QUOTE]
> 
> I believe my wiring was per the manufactures guidelines. I have no idea where the book is so it's hard to verify. If ya really think about it though...over 20 years and no problems that's pretty good.


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## crappiedude (Mar 12, 2006)

I also wanted to add...
What you're saying about the batteries makes a lot of sense. I don't doubt or question any of it at all. If I were having problems and trying to diagnose a set up, I see the point. If I was looking to maintain each component and keep every battery as long as possible regardless of age, it makes sense. If keeping an old battery and adding new battery was okay in a set up, it would matter. I'm not trying to imply that anything that you're saying isn't true, but I am suggesting that for the most part (in this application) it doesn't matter.

For me though the 2 batteries act as 1 unit.
If part of that unit fails, you replace the whole unit.


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## c. j. stone (Sep 24, 2006)

I once had an electric golf cart. I don't remember how many batteries it had but it was a bunch! They were all hooked(factory design) in parallel. One place to plug in the charger that charged them all at one time. Overnight did it. If one fails, the ones ahead of it got charged, the downstream ones did not. People who have these carts do not change all of them at once, you'd need to be Bill Gates to do that! You test and replace the bad ones! If one of your two batteries is bad, you will know pretty quickly


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