Re: (ET) Gel cell charging

["David Roden (Akron OH USA)" <etpost drmm net>]

On 7 Nov 2006 at 8:43, tbamc wrote:

> You mentioned you used the Cliplight chargers: ...
> (basic cycle-dropping charge control at 14.3 and 14.8 volts) ...
> 
> In another post you said bulk phase termination V of 14.7 was too high 
> for 'any' gel cell.
> 
> 1.  Are these contradictory or am I not understanding something?

The chargers I have are switchable.  I don't recall the legends on the 
front 
panels, but in one position they clamp at 14.3 volts, and in the other 
they 
clamp at 14.8 volts.

> 
> 2.  In general, can AGM's and gel's be thought of as similar, for 
> charging purposes, or not?

It depends on the battery and its design.  

What gels and AGMs have in common is that both are valve regulated 
recombinant (VRR) designs.  This means that when hydrogen and oxygen 
are evolved late in the charge, the batteries keep these gases inside the 
cells.  Instead of being vented to the atmosphere, they're recombined into 
water. 

For this to work, current must be relatively low when the batteries are 
above 
the gassing voltage in the final 20% of charge.  If the charge current is 
too 
high, gassing will occur faster than the recombination reaction can 
accommodate it, and gases will have to be vented (to prevent excess 
pressure and possibly explosion).  This means that water is lost.  Because 
the cells are permanently closed, it can't be replaced; eventually they 
will 
literally dry out.

So, one must limit the end-of-charge current.  The simplest way to do this 
is 
by using constant-voltage charging during the absorption phase.  

The electrochemical design of each battery differs, but each design has a 
more or less fixed voltage at which the "natural" charge current will be 
within 
the safe range.  

I've never seen a 12v gel battery for which manufacturer's specs called 
for a 
constant-voltage absorption voltage any higher than 14.3 volts.  That 
doesn't 
mean none exists, just that I've never encountered one.  So I probably 
should 
have said that 14.7 volts is "too high for any gel cell with which I'm 
acquainted."

Some AGMs, such as Concorde brand, specify 14.2 to 14.4 volts (very 
similar to gel range).  But others, such as Optima and Hawker brands, 
specify 14.7 to 15 volts.  

Optima even recommends allowing the voltage to increase *without limit* 
under some circumstances, as long as current is limited (constant current 
charging).  I wouldn't want to do that without very close supervision.

Does that make sense?