Re: (ET) AGM

["David Roden" <etpost drmm net>]

On 4 Feb 2013 at 10:37, Robert wrote:

> I didn't read the article but I do know about batteries. AGM have to 
> follow 
> a specific charging algorithm. If you charge them over their max rate 
> the 
> one way valve can open from the heat developed to relieve the pressure 
> and 
> the battery is toast.

The main concern with AGMs (and other valve regulated recombinant [VRR] 
batteries such as gel) is in the final stage of charging.

The relief valve will open if the battery gasses faster than its internal  
recombinant reaction can turn the hydrogen and oxygen back into water.  
This 
will indeed happen if you use too high a finishing voltage (and thus too 
high a finishing current).  You can usually hear this - it's a sort of a 
ticking sound.  There may be some hissing too.

However, below the gassing voltage - in the bulk charging phase - this 
won't 
happen.  Essentially all the energy going into the battery will go into 
charging it, with just a little going into heat (the amount depends on the 
battery's internal resistance), and none into gassing.  This is a very 
efficient part of the charge.  

That's why you can charge an AGM to 80% SOC at about as much current as it 
can produce into a load - which can be a LOT.

Unless there's something terribly wrong with the battery, you only have to 
worry about hydrogen evolution in that last ~20% of the charge (which 
takes 
more than 20% of the time, BTW).


David Roden - Akron, Ohio, USA

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