Battery Bank

[EEstudent]

I have a few questions in regards to battery banks. And I know most of them shouldn't happen but in theory how would it be dealt with?


So i know that putting batteries in the same bank that have different amp hour ratings is bad, but I am not entirely sure why it is bad. Could someone explain or point me in the direction of a site or book that does explain this well? Or a good way to deal with this? (there is a thread about this posted sometime in 2009 but it didn't explain why it was bad, i searched through the storage section for a while)


And I'm pretty sure there is no way to fix this by it's self and that it is just a bad idea but lets say I have a 24V charging source and I am charging 2 12V batteries in series that are at different are at different charge levels. Is there a way to tell when one is fully charged to swap it out with another battery? Or a way to avoid damage being done to the battery that is now fully charged while the other battery finishes charging?

[Tink]

EEstudent,

By mixing batteries of different amh ratings will cause the 'bank' to be no better than the weakest (lowest rated) battery. This has the same effect as using a battery of very low specific gravity in a string of higher specific gravity batteries as explained below. This is solved by using a battery switch to separate the different amh rated batteries into seperate banks.

When charging batteries in series there will always be one that is of a lower charge than another. The charger will look at the batteries in a string as one battery and therefore charge all of them up to the maximum charge. If one battery is of a very different specific gravity it should be taken out of service so it won't bring the other batteries down to it's level. By leaving the weaker battery in service the other batteries have to work harder and overcharge to try to bring up the weaker battery and therefore shortening the live of the better batteries.
There are many books on this topic. One I found that enplanes battery theory is:

Battery Book for Your PV Home
by New England Solar Electric Inc.

I don't remember where I got it but a Goggle search should find it.

Good luck
Tink

[ghurd]

A lot of information is twisted because people try to apply the theory based on series batteries to parallel batteries.

I have read where people believed a 12V 10AH battery paralleled with a 12V 100AH battery will drain the 10AH first.
Somehow they based this line of thought on the load amps being evenly split between the 2 batteries.  It seemed to become a self perpetuating wife's tale for a while.
In fact, the parallel batteries will always be at the same voltage. 
They are in parallel, so they have to be the same voltage, meaning one is just as charged or dead as the other.  (as long as they are the same type)

Series batteries of different AH is not a good idea.
Could use two 12V dump load controllers I suppose.  One on each battery.

Swapping out 12V batteries from a 24V bank begs the question "Why have a 24V charging system with separate 12V loads, and who wants to baby sit and swap batteries?"
G-

[jlt]

You dont say what the 24v power supply is .you could  Try to charge them in parallel by using a resister to keep the amps down. 

[EEstudent]

First off I want to have a 24V system because the set up that I will end up having changing it to 12V will create too much current and I don't want to deal with that much current. I'm looking at a situation where I will have other people's batteries in the bank and they will from time to time come get their batteries and swap it out with another one that they were using. All the batteries are 12V batteries but they are not all the same amp hour rating. Which is why I'm wondering if there effective way to deal with this.

So correct me if i'm wrong, if i have two 12V 10Ah batteries in series in parallel with two 12V 100Ah that are series, does that mean the two 100Ah batteries will draw more current than the 10Ah batteries?


Tink:
I know better than to put batteries in series with different AH ratings but I don't understand what you mean by "will cause the 'bank' to be no better than the weakest (lowest rated) battery" so the weakest battery will cause all the batteries in the bank to charge at the same rate as it? Or am i misunderstanding what you are saying?

And I found the book that you suggested, thanks

[ghurd]

So correct me if i'm wrong, if i have two 12V 10Ah batteries in series in parallel with two 12V 100Ah that are series, does that mean the two 100Ah batteries will draw more current than the 10Ah batteries?

Correct.
Basically, if both are half full, all 4 at the same voltage, when they are all full, the 10AH series pair will have taken in 5AH, and the 100AH pair will have taken in 50AH.
Overly simplified, but thats the general idea.
G-

[Tink]

EEstudent,
I just got home from work, a long day, and will have to give that some thought...I may have misstated the facts. I'll get back to you on that one. Well I thought about it and I may be wrong about this but if you have a lower capacity batter in a string (series) it may overcharge trying to keep up with the larger capacity battery in that string. Anyone's input on this is welcome... I'm just going by my own experience. By the way if yo want the best hydrometer get the one made by Freas Glass Works in Conshohocken, PA. Their 'No.1' is the one to get.
Tink

[isoutar]

This was well stated by one reply ... when batteries are in parallel their current rating simply add together.    Balancing is not an issue.

If the bank were nickel iron then you could actually add a brand new battery to a bank of 30 year old ones.    With nickel iron batteries they are completely tolerant of over charging.   Aging does not reduce their current storing capacity (at least aging < 30 years does not affect them.

If anyone knows a source of the original Edison Storage Battery Co. batteries ... I really want to buy some antique ones and add them to my house.    They are a bit more tough than the modern NiFe cells.

Ian Soutar