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Posts: 793
Location: Ames, Iowa | Read in an earlier thread that if two batteries are in parallel they should be the same age. I want to get a new battery parallel with my 3 year old battery for my 12 volt trolling motor. Why do they need to be of the same age? Is it so that one does not fully charge before the other?
Thanks,
Don |
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Posts: 2323
Location: Stevens Point, WI | I don't think it technically matters, but it is better to have two batteries the same age because when one goes, usually the other goes. You get stuck one day when one battery is on it's last leg and don't have enough power, you will be sorry. But otherwise, I don't think there is a sin for running a 3 year old battery with a new one. But I could be wrong. |
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Posts: 2754
Location: Mauston, Wisconsin | Merckid is right- there is no sin in connecting batteries of different ages in parallel. Just don't expect the older battery to last as long as the new one.
Have fun!
Al |
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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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Posts: 941
Location: Freedom, WI | Jono is right. I am a heavy equipment mechanic and a new and old battery in a 24V system will shorten the life of the new battery. It is not as big a deal in 12v parallel system but it will shorten the life also. If the old battery is not to old it would probably be ok. If you mix new and old make sure you charge them separatly as the battery charger (automatic ones) will go off of the higher charged one (which would be the new one)or use an on board duel bank charger. All lead acid batterys sulfate over time and use until they can not hold a charge. I have had good batterys last 12 years (not deep cycle) and cheap ones 2 years. |
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Posts: 2754
Location: Mauston, Wisconsin | Without getting into battery chemistry and calculus. Urban legends abound about batteries.
1. When a newer battery is connected in parallel with an older battery, the older battery will not shorten the life of the newer battery. This was the fundamental question posed in this thread.
Therefore my first post-> "there is no sin in connecting batteries of different ages in parallel. Just don't expect the older battery to last as long as the new one."
2. If adding a new battery to a series string (24V system) the older battery can shorten the liife of the new battery, ONLY IF they are charged as a series string.
If they are charged individually (dual bank charger, etc.) there is no danger to the newer battery.
3. Using an A-B switch on two paralleled batteries is good, if you understand the advantages of the A-B switch.
A. It can save the day if you have dual batteries for your engine starting batteries. You can run off one battery and keep the other in reserve.
B. It can allow you to charge each battery individually if you don't have a dual bank charger.
C. If using A-B switch for your trolling motor always run it in the A+B position (the batteries are then connected in paralel). This will give you ~3 times the runtime as using a single battery. Whereas Juno's sugestion will only get you 2 x the runtime, i.e., because you are discharging them individually at a higher load current.
Ok- trying to simplify this. Some folks think about batteries like gas tanks, i.e., that they hold a specific amount of fuel/power. That is true if they are used individually. However, if I start paralleling them-> connecting the tanks together some strange things start happening. They no longer hold the same volume of fuel to power the same engine. The volume of available fuel increases. Yet the physical volume of the tanks is the same!
F. ex. If your trolling motor draws 30 amps at full power, and if you connect it to the batteries individually each battery can only discharge 30 amps for x time. However, there is this funny little rule about lead-acid battery electrochemistry where: If you reduce the load current by 50% you will get 3 times the original runtime. By connecting another battery in parallel we effectively reduce the load current to 50% on each battery in comparison to an individual battery. This means each battery is now supplying 15 amps and 2 x 15 = 30A.
Lets say my existing battery gives me ~ 3 hours at full power. If I connect another same size battery in parallel, I will get 9 hours. Whereas if I discharge them individually I will only get 6 hours (3+3=6).
I hope this helps.
Al |
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