TexasBorn
Amarillo Texas
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If you put a new and an older battery together you will only get the amount of power the that the weakest one has right?
Does that make since?
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mosseater
Dillsburg, PA
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I`m no battery expert, but I wouldn`t do it, neither series nor parallel. Even the same model and size would have different internal resistances because of age and use disparities. I`ve always heard you should only connect identical batteries of similar age. Also, keep in mind when I say connected, I mean good connections. I`ve noticed a lot of folks have trouble doing that part. A poor connection between batteries can muck things up. Clean and tight with max surface area contact.
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Cliford
Whidbey Island , Washington
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Yes weak one drags the good one down.
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Polishnurse
Schodack, NY
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As mentioned above, they should be of same capacity. It's always best to purchase them as a pair and use them as such. To take one from here and another from there will only weaken and shorten the life of both. It can be done in an emergency like when jump starting but not as a general practice. Bill
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bcckphillips
Upstate NY
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You could install an isulator between the two batteries and then you won't have to worry about draining one battery to the other.
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TexasBorn
Amarillo Texas
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Ok for two new batteries at 12v they are hooked up + to +, - to - that will double my amp hours but will keep it at 12v right?
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prichardson
Lafayette, La
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Correct
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bryanl
Reno, NV
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in re the weak battery a drain myth:
The battery chemistry sets the voltages that control the current that will flow in discharging or charging.
Unless there is a shorted cell or other significant battery failure, the weak battery won't detract from the other batteries in a bank. It just won't be able to contribute much to the available capacity. It also won't take more charging than what it needs. You can see this in very old batteries that run down rapidly but also recharge just as rapidly.
It is still a good idea to try to match batteries in a bank. That helps to assure that all are exercised equally and wear evenly so you can get the most life and reliability from the entire bank.
When you add batteries in parallel, you will get a bit more than if you just add their specified capacities. See Understanding the peukert effect. For two like batteries, this can be a 15% or better increase in available capacity.
Bryan
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NCHornet1
Mt. Airy, NC.
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I have a dual battery diesel truck and it recommends changing both batteries at the same time. It is not a myth, I have seen it, the wealer battery will require the stronger battery to work harder and weaken it.
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bryanl
Reno, NV
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re "not a myth" - I think a few things are being confused here.
Yes, indeed, the healthy battery will "work harder" and that is what "exercised equally" is all about. It is a different concept from usage causing damage or reducing capabilities so that the total is less than that of the strong and weak added together (where the reality is that adding a strong and a weak battery in parallel will usually yield greater capacity than just adding actual capacities)
If you have two starting batteries, you have a good setup for a lot of current to a starting engine. This is why many diesel pickups have two 12v in parallel. They share the current. If one gets weak, it cannot provide its share of the current so the starting current is weaker than it would be with two healthy batteries. The key here is that engine starting current is very high - high enough so that the wiring resistance and internal battery resistance can cause significant voltage drops.
But that is not how an RV uses battery energy and does not illustrate the purported 'myth' is wrong. It just illustrates that one battery can't provide the current that two in parallel can. that's all.
There is no question about the fact that weak batteries in your battery bank will reduce the capabilities of your bank compared to if the weak battery was replaced by a healthy one. What is not true, however, is that a weak battery will lower the capabilities below what you'd get without it.
It is necessary to be sure to distinguish between a weak or old battery and a failed battery. For the knee jerk folks who post before thinking, I'd also mention that the context here is batteries in parallel in typical RV service. A series configuration has a bit different situation. There are many factors and circumstances that can be trotted out but I am also not talking about special cases but rather generalities.
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