12V Batteries Combined in SERIES - Different Size/Capacity OK?

EL JEFE

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Found a great deal on 2 Optima Blue top batteries from a private seller. Great price....new, never used, two months old.

The sizes are D31M and a D27M

Combining them in SERIES for 24volts.....

Does this pose any problem running my Motorguide 24V trolling motor, in terms of amps, capacity, etc?
How about with my ProMariner 20PLUS battery charger....issues with charging?

Batteries:

https://www.optimabatteries.com/en-us/bluetop-dual-purpose-deep-cycle-and-starting

Thanks!
 

karlow

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Kinda, Its OK when they are fully charged, but as you drain they, it can become an issue. One will be strong and the other one will become overly discharged.Over discharging is very bad for batteries. If you charge them is series it will be worse. after the smaller one becomes fully charged, the charger will continue to drive it as it bring the large one to full charge. The smaller one will have to dump the extra current as heat. not good.
 
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EL JEFE

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My Promariner charger is smart in that it charges each battery independently.
Yes, I was concerned about discharge rate affecting smaller battery as you suggest.

I would have to have separate volt meters to measure each battery independently.....and stop using trolling motor when smaller battery reaches 50% discharge of capacity.

Thoughts?
 
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dcarlisle

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Highly advise against it because of what was said before. The small one will likely kill the big one prematurely.
Also, Optimas aren’t as great as their marketing program would like you to believe. I unfortunately have to install them on every Navy boat (about 200 batteries per year) and am not impressed. They sure have the Government in their pocket though.
 
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gecsr1

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no, one will over charge or one will never fully charge ...
buy them both , sell one then buy another the same size...
 
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SouthBayKiller

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Also, Optimas aren’t as great as their marketing program would like you to believe. I unfortunately have to install them on every Navy boat (about 200 batteries per year) and am not impressed. They sure have the Government in their pocket though.
Not sure what changed, they were very good initially and have not held to that standard. I see the same thing at my work as well.
 
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EL JEFE

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no, one will over charge or one will never fully charge ...
buy them both , sell one then buy another the same size...

How can one overcharge if each is being independently charged by the Promar Pro Sport Plus charger?
 
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EL JEFE

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Highly advise against it because of what was said before. The small one will likely kill the big one prematurely.
Also, Optimas aren’t as great as their marketing program would like you to believe. I unfortunately have to install them on every Navy boat (about 200 batteries per year) and am not impressed. They sure have the Government in their pocket though.

Any batteries you would recommend for this 24v trolling motor application? Interestste, Walmart, etc.....

Aren't most made by Johnson Controls? If so, which are higher grade with heavier plates, etc? Not looking to spend $300+ per battery

Thanks
 
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hbouldin1216

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Costco sells the Interstate batteries and I've never had problems with them. Warranty and easy of exchange is why I buy them.

You could run those optimas in parallel for a house bank and have whatever the combined a/h is but charging will still be an issue when they don't match
 
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aguachico

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I like AGM deep cycle if they are buried on a place with not a lot of venting, so I went with Lifeline for the house. They are pricey, but have a good warranty and rep. Group27, parallel buried in the bow.

I use a group 29 deep cycle for camping and solar. It's an interstate battery. Wet battery. Good specs, great price and I like interstate's service.

I have two starter batteries in my boat, Interstate wet batteries. I again the price and specs are great.

I have two batteries in my truck. Interstate!

Only downfall with wet batteries is you need to maintain them
 
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gecsr1

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How can one overcharge if each is being independently charged by the Promar Pro Sport Plus charger?
he indicated the were being charged wired in series, if both are charged independent the as you say, it should be okay....
 
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MYNomad

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What I am about to write is just my analysis, so I am not sure it is right. But here goes, addressing charging and discharging (load handling) separately:

Load Handling. Let's say the load requires 5 amps and 24 volts. If the batteries were identical, each battery will be required to put out 5 amps at 12 volts. As the batteries discharge, their voltage will decrease and their amperage will increase (assuming the load hasn't changed). If the batteries are not identical, they will not discharge at the same rate. But neither battery "knows" anything other than the load it is under. so, if one battery discharges faster than the other, the other battery will see a greater load as a result, and will discharge with a correspondingly greater current flow, but that is not damaging. On the discharge side, damage occurs from discharging too rapidly or from discharging below about10.8 volts. But that is true whether the two batteries are identical or not, and the "weaker" battery will get their first. Further discharging will damage the weaker battery, but no damage will occur the the stronger battery unless the current goes to high, or its voltage too low. But you have that problem even with two identical batteries -- you can't run them down too far or too fast without damage. If my analysis is correct, the only consequence of using two non-identical batteries is that the capacity is effectively limited to the capacity of the weaker battery (times 2).

On the Charging side, things become a little different and much more complicated. Charging the batteries independently is best and in that circumstance I don't see any potential for damage to either battery. But if they are charged together, the charger, if it is a smart charger, is going to monitor voltage and amperage to quickly charge the batteries to full capacity, without damage. So, a good charger may charge at 14 volts until the acceptance current drops to a point at which charging has significantly slowed (because it is nearing full charge), at which time the charging voltage is slowly dropped, ultimately to a trickle at about 12.8 volts. But, with two batteries, if the charger is charging at 28 volts (14 volts won't work), and one becomes fully charged, the acceptance rate of the other may not be enough to trigger the charger's reduction in voltage, so the weak (but now fully charged) battery will be overcharged until the stronger battery catches up. That will further damage the weak battery.

Note that this is for wiring batteries in series. Parallel wiring presents a different set of problems that require a different analysis. And, I may well be wrong in my conclusions, but my analysis makes sense to me.
 
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dcarlisle

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Not sure what changed, they were very good initially and have not held to that standard. I see the same thing at my work as well.
Not sure they were ever that great.A while back I was at one of the Naval Shipyards that installs submarine lead acid batteries. The have a super sophisticated battery tester and we put a few brand new fully charged batteries on the tester. The Blue top optima performed at the bottom of the list below the Walmart specials.
 
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EL JEFE

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As mentioned, the discharge and amps/volts available will be based on smaller battery (to ensure no damage is done to smaller battery,

My charger charges each battery independently. See below. I think that with this charger, charging each battery is a non issue. Discharging the different size batteries is a concern.......to be safe and not damage smaller battery.....when smaller battery reaches approx 10.8 volts, then batteries are done for the day. This means a volt meter capable of reading volts of each battery (independently) would be nice to have.

http://www.promariner.com/en/43021

NOTE: All of this hoopla is because I have these two NEW Optima batteries (group 27 and 31) available to me for $300. Original purchaser paid close to $600.

Walmart batts (group 29 DC Maxx) are always out of stock., so not really an option

Next battery option would be Interstate SRM-29DC at $129.99 each

Thoughts?
 
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kindafishy

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As mentioned, the discharge and amps/volts available will be based on smaller battery (to ensure no damage is done to smaller battery,

My charger charges each battery independently. See below. I think that with this charger, charging each battery is a non issue. Discharging the different size batteries is a concern.......to be safe and not damage smaller battery.....when smaller battery reaches approx 10.8 volts, then batteries are done for the day. This means a volt meter capable of reading volts of each battery (independently) would be nice to have.

http://www.promariner.com/en/43021

NOTE: All of this hoopla is because I have these two NEW Optima batteries (group 27 and 31) available to me for $300. Original purchaser paid close to $600.

Walmart batts (group 29 DC Maxx) are always out of stock., so not really an option

Next battery option would be Interstate SRM-29DC at $129.99 each

Thoughts?

i think the problem is this......

"My charger charges each battery independently."

and this below

"Combining them in SERIES for 24volts...."

if you wire them in series they become a single 24 volt battery. thats unbalanced. charger will probably shut off once smaller battery is charged, leaving the larger one not fully charged. as i understand it, this will dramitly affect lifespan, since optimas are agms and have a very specific charging cycle.

batteries-series-vs-parallel.jpg
 
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EL JEFE

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i think the problem is this......

"My charger charges each battery independently."

and this below

"Combining them in SERIES for 24volts...."

if you wire them in series they become a single 24 volt battery. thats unbalanced. charger will probably shut off once smaller battery is charged, leaving the larger one not fully charged. as i understand it, this will dramitly affect lifespan, since optimas are agms and have a very specific charging cycle.

batteries-series-vs-parallel.jpg

That would be true if there was ONE set of charger leads to these connected batteries. My 3 bank charger uses a set of leads to EACH battery. That is why I stated each battery is charged independently.
 
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kindafishy

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That would be true if there was ONE set of charger leads to these connected batteries. My 3 bank charger uses a set of leads to EACH battery. That is why I stated each battery is charged independently.

so the charger has 24v output. can you set each output independently, 12v on one, 24v on the other? or are you just going to use one output for the 24v bank, and a different charger for your 12v systems?
 
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MYNomad

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so the charger has 24v output. can you set each output independently, 12v on one, 24v on the other? or are you just going to use one output for the 24v bank, and a different charger for your 12v systems?
I had a charger designed to charge 24v circuits powered by 2 12-volt batteries wired in series (probably the most common way to provide 24 volts of battery). It had separate positive and negative leads for each battery, but each was putting out 12 (nominal) volts. Using a charger with a common negative on batteries wired in series would create a dead short. But, with separate leads for each battery, the fact that the batteries are wired in series is irrelevant to the charger -- it still looks and acts exactly like two isolated batteries.
 
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EL JEFE

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so the charger has 24v output. can you set each output independently, 12v on one, 24v on the other? or are you just going to use one output for the 24v bank, and a different charger for your 12v systems?


The charger is 20 amps for 12V batts. It is a 12V charger. 20 amps are distributed between 3 batts depending on need of each one. If one is topped off, amps will dedicated the other two as needed until all 3 are topped off.

The ProMariner charger I use is a pretty common onboard charger for smaller boats with up to 3 batts.
 
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kindafishy

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The charger is 20 amps for 12V batts. It is a 12V charger. 20 amps are distributed between 3 batts depending on need of each one. If one is topped off, amps will dedicated the other two as needed until all 3 are topped off.

The ProMariner charger I use is a pretty common onboard charger for smaller boats with up to 3 batts.

ok, so if its 12 volt only output it wont work. unless un remove the wire connecting them in series and charge as two separate 12 volts.
if you wired them in parralel it would be 2 batteries making 12 volts
once in series 2 batteries making 24 volts, as ,long as they are wired in series they are now 2 batteries acting as one. how do you intend to hook up the charger? maybe a diagram will help me understand better. cause i just went thru this (kinda) with a ACRnewcharger same as yours think, but mine is 2 6 volts in series making 12v.
 
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kindafishy

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I had a charger designed to charge 24v circuits powered by 2 12-volt batteries wired in series (probably the most common way to provide 24 volts of battery). It had separate positive and negative leads for each battery, but each was putting out 12 (nominal) volts. Using a charger with a common negative on batteries wired in series would create a dead short. But, with separate leads for each battery, the fact that the batteries are wired in series is irrelevant to the charger -- it still looks and acts exactly like two isolated batteries.

i must not be getting it? when a set of batteries is wired in series, the cable in the center......that connects the negative side of one battery to the positive side of the other is what....+ or -.....? and if these batteries are connected by this center cable + on one side - on the other and you hook up the + and - from the charger isnt it going to cook the charger? you see what im getting at? a pos from the charger will be dead shorted to a neg on the charger by the center batt. cable, right?
 
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rxm

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The charger is 20 amps for 12V batts. It is a 12V charger. 20 amps are distributed between 3 batts depending on need of each one. If one is topped off, amps will dedicated the other two as needed until all 3 are topped off.

The ProMariner charger I use is a pretty common onboard charger for smaller boats with up to 3 batts.

You need a 24 volt battery charger to charge a 24 volt battery bank.
 
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kindafishy

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