Posts: 726
Location: Eau Claire, WI | Hi, I am not a battery authority but this is what I've learned....
you are only as strong as the weakest link. if you have an older battery with an impaired abilty to hold charge, you won't utilize the full potential of the other in a parallel system. I believe the older battery will act as a resistor as it discharges which affects the ability to provide the current your troller wants. You will have an improvement though over the single system older batt. Not as important, in my opinion, with a 12v parallel but obviously a big deal in a 24v series system. I wouldn't recommend having mismatched batts in a 24v system.
Really it's not a matter of age, rather it's ability to hold a charge. Buying at the same time so they are the same age does up the odds that they have the same potential and will grow old together as a matched set. It's the best thing we can go on in trying to ensure "matched" sets short of a chemistry experiment in the battery aisle of the store. Buying brand A and brand B two years apart will be an impaired system. You might not notice it depending on your use but it's there.
Check your water levels, check the specific gravity. You should get a good idea of what the old battery is capable of. A battery that is still in good shape should no load rest at about 13v or higher. If you have a batt closer to 12v or less, you need to replace it.
At three years old, you already have a candidate for replacement.
You don't necessarily have to make it parallel. You could install an A/B switch and just jump to the other batt when the first one is discharged.
Jono
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