# [EVDL] Why do some AGM batteries just... go... bad?



## EVDL List (Jul 27, 2007)

This is the problem with gang cells in 6, 8 and 12 volt batteries. You are 
equalization the total voltage of one battery to another, but not the 
individual cells to each other.

One cell could be low and the other cells are over charge. If a 12 volt 
battery full charge at rest reads 12.74 volts at 100% SOC, then ideally each 
cell should read 12.74/6 = 2.123V each, but I have found that they may read 
from 1.9 to 2.8 volts by checking each cell voltage. If the cells are out of 
balance by this much, then in time the battery will lose performance.

In a open cell flooded battery, you can read each cell voltage by inserting 
a cadmium plate electrodes into two adjacent cells. This is known as the 
Cadmium Battery Test. You can actually get a cadmium battery testers to do 
this type of test.

My first set of batteries was a 40 cell and a 50 cell Tudor Traction battery 
in the 300 AH size. These size batteries have each cell lead link together 
where you can test or even charge each cell if required.

If one cell went bad, this was normally cause by the oxide sediment shorting 
out the grids at the bottom, or a separator was bridge by two adjacent grids 
or the electrolyte was saturated with too much oxide from the positive plate 
causing a increase in conduction.

I then can pull that one cell by using a special post cutter that looks like 
a drill that is use to make wood plugs. Then I can remove the links and the 
cell which is now call a jar when its out of the battery. Pull the grids 
out of the jar container and quickly immersed them in a large glass 
container of distill water which sets on a vibration table.

After cleaning, the plates are then immersed in the same electrolyte 
solution that read the same when it was in the original battery cell. If 
the specific gravity reading was 1.200 when it was in the battery, then you 
only add 1.200 sg to it and then it is bench charge separately until it 
comes up to 1.277 sg.

If the cell did not charge or did not hold it charge under load, then I 
would slid in new separators while pushing the other ones out. Test it again 
and 99 percent of the time, the cell will perform as normal. The other 1 
percent, the grids had too much material loss.

This is the reason I only use floods. I tested out a AGM 12 volt deep cycle 
accessory battery that a auto parts store gave to me. They said they will 
not handle these types of battery again, I said why? He took me to the back 
room where they had stacks of pallets that of all which have fail under 
warranty.

Roland





----- Original Message ----- 
From: "Christopher Zach" <[email protected]>
To: "Electric Vehicle Discussion List" <[email protected]>
Sent: Sunday, July 04, 2010 8:06 PM
Subject: [EVDL] Why do some AGM batteries just... go... bad?


> So I am curious here: I have my Prizm pack down and as is normal, about
> 6 batteries out of 50 have gone "bad". Bad is defined as "battery reads
> 12.6 or so without a load, but drops to 8-9 volts under load".
>
> The question is: Why?
>
> Two of the batteries failed pretty much together. I had Paktrakrs
> watching them, nothing unusual, no overcharge or undercharge to speak
> of. Just one day they decided to drop to 7-8 volts while the rest of the
> pack was at 11 under load.
>
> Same with the others, nothing too odd. They just seem to up and die.
>
> I would say this is a Universal battery issue, but the same thing seems
> to happen with Hawkers. Over two years, about 6 out of 50 batteries will
> just drop dead like this.
>
> Any idea why? Batteries have zener regs on them and there's nothing that
> unusual about them.
>
> Thanks
> Chris
>
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