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I'm curious i was told by a friend that i couldn't use to different size batteries because they would go back and forth and kill eachother. Is there any truth to this.
 

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just dont mix types of batteries (AGM, wet cell, etc..)
 

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What you're saying is sort of right. Long story short, the battery should be the same make and model (and ideally lot). Use an isolator if they're going to be different batteries, especially different types of batteries like a regular Die-Hard and an Optima.
 

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If you cherish your alternator, you should use an isolator to isolate the aux battery from the starting battery, regardless if the batteries are identical or not. The life of both batteries and the alternator will be extended, considerably.
 

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I find age is probably more of a factor in where they want to rest. Generally you don't want them resting at a different voltage or the lower will pull down the stronger to charge itself.

I have no idea who anybody uses a regulator though.
 

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I'm curious i was told by a friend that i couldn't use to different size batteries because they would go back and forth and kill eachother. Is there any truth to this.
I will do my best here.

There are two different ways to wire batteries, in series or parallel. Series is generally used for 6V batteries, parallel for 12V batteries, be careful not to wire 12V batteries in a series or you will double your voltage to 24V and will essentially power surge all your electronics.

Parallel wiring is positive to positive, negative to negative very simply (always good to remind ourselves).

Now for the real question, what happens when two different aged or CCA batteries are used, well here's a real world example.

My friend runs a 500 CCA green top interstate in the front, and a 750 CCA black top in the back. No battery isolator, this is what's effectively happening. The 750CCA battery is always charging the 500 CCA battery when the car is off, thus reducing its lifespan. It is charging it because it is weaker, that's why the closer you can get to identical batteries the better.

You can eliminate this problem entirely by simply using a battery isolator, just remember solenoids take the least power draw but don't last quite as long. More fancy ones can cause a SIGNIFICANT .3 to .4 voltage drop, big drawback to me and I wouldn't use it if it was the last battery isolator on earth. You're not going to kill your battery instantly, but it will reduce its lifespan, impossible to say how much.

Hope this helps.
 
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