# Why/how do batteries become unbalanced?



## rankhornjp (Nov 26, 2007)

I've read alot about BMS and how batteries become unbalanced and then the weak ones die before the rest. What I don't understand is how/why this is. My searching skills haven't answered the question.

If you have a charged battery and you hook it to a dead battery, won't this charge the dead battery (isn't this how dump charging works?) In my mind, I'm looking at this like a series of air tanks hooked together. If you open all the valves they will equalize at the same pressure/volume, given that all the tanks are the same size.

I guess my basic question is: Why don't batteries balance themselves?

Thanks for the help.


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## MN Driver (Sep 29, 2009)

If you hook batteries in parallel, their voltages will balance but even with the same voltage, that doesn't mean that they will have a similar state of charge.

When batteries are connected in series, they don't have a connection to 'balance together'. If you connect an empty AA battery's negative terminal to another cells positive terminal, they don't balance. See what I'm getting at? I might not have the best words to describe this but that is more or less why they don't balance. ...of course if I'm using the wrong words it won't get interpreted right, which happens often here.


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## rmay635703 (Oct 23, 2008)

rankhornjp said:


> I guess my basic question is: Why don't batteries balance themselves?
> 
> Thanks for the help.


FLA do if you perform regular and controlled equlizing charges.

The reason they become unbalanced is because no 2 identical model lead acid batteries (or insert your prefered tech here) are really exactly the same. In fact over time the middle cells in an FLA battery age faster due to more heat in the middle causing imbalance within the battery.

SO to answer your question there are small variations in every battery in the same line and model that cause them to have small capacity variations which over time grow until you equalize. However as they age these small variations also can grow leading to one battery in a string not getting charged as much and another overcharged resulting in further deviation.


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## rankhornjp (Nov 26, 2007)

Thanks for the replies. Really clear it up for me.


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## Sunking (Aug 10, 2009)

Simple answer is no two batteries are the same. In a series string th eweaker ones will never receive a full charge, and th eeffect is accumulative.


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