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



## Tin Man (Aug 24, 2018)

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!


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## GTS225 (Aug 24, 2018)

As far as being detrimental to the motor, you don't have to worry. You'll be limited by the amp capacity of the smallest battery, but the TM will draw equally from both, until you stop using it and recharge.

Roger


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## clarkbre (Aug 27, 2018)

The motor won't care that the batteries are different capacities. As stated above, the motor will draw what it can and stop once there's not enough voltage.

However, the charger is where you will run into potential issues...but there's a way around it. If charged together as one, 24v unit, the batteries will show the charger a false full capacity. In short, the charger will average the capacity of the batteries and charge to that. The D31M is 66ah and the D27M is 75ah. So, the 24v charger will charge to 141ah but it will be divided equally to both batteries. Essentially, one will be under charged and the other will be over charged.

Now, how can you do this set up correctly? Split the middle. It should be ok running both batteries for the motor; however, when charging, they should be separated as two, 12v batteries that are charged individually. For this, you will need two chargers or a 2 bank charger and a mechanism to break the center connection between the positive and negative on the batteries.


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## Tin Man (Aug 27, 2018)

My charger charges each battery independtly, even when connected in series.

Here's my charger:

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

Still not sure if purchasing these two Optima batteries (private party, new, never used) for $300 is wise, seeing as they are different sizes???? These batts new at Optima dealer are about $330 and $280....about $600 for the pair!!!!

Or is preferred to purchase batteries of same size? If so I would purchase two new wet cell batteries such as Interstate or WalMart Everstart if they are ever back in stock.

Thoughts?


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## PGRChaplain (Sep 10, 2018)

The Optima's are good to go. Their AGM Construction, Absorbed Glass Matt, is resistant to Vibration. Lost of Drag Race guys use them.


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