Re: (ET) batteries

["David Roden" <etpost drmm net>]

On 5 May 2007 at 12:50, Nick Skinner wrote:

> In an ideal world, I suppose this would
> be done after every bulk charge.

Some charge algorithms do that, but I don't agree.  I think it leads to 
too 
much overcharge of the fully charged cells.  The net effect is to shorten 
the battery's life.  Anybody who has a Sparrow electric car is familiar 
with 
this problem.

The ideal would be to measure the SOC (by SG if possible) of the 
individual 
cells and apply an equalization charge when the SOC varies beyond some 
tolerance.  The amount of tolerance would have to be computed so as to 
balance the OC damage to the more-charged cells with the UC damage to the 
lower-charged cells.  

The manfacturer's recommendations are (or should be) the final word on 
equalization techniques, but here are some rough guides.

Experience shows that if you're using the battery every day (I'm talking 
about cyclic use here, not a starting battery), once every 2-4 weeks is 
about the right equalizing interval.  (US Battery recommends equalizing 
once 
a week if the battery is used daily.)

I use my ET about every 2-3 weeks on average, and I perform a very low 
current equalization charge (0.5 amp, which is about C/170) about twice a 
year.  So I'm actually equalizing about every 10 cycles.  That seems to 
work 
OK, but I'm not going to claim it's an ideal interval.  It's just my 
(slighly informed) guess and I won't know for many years how well it's 
working.  Note that I use six 12v gel batteries in series-parallel, and 
normally charge them with three 12v chargers, so this may not apply to 
other 
more "normal" ET situations.

Equalization current depends on the battery.  A flooded battery can be 
equalized at 2-4% of the capacity expressed in amps (C/50 to C/25).  Use a 
constant current charger.  Charge until the voltage stops rising or until 
it 
reaches about 2.65 volts per cell.  

A gel or AGM battery may have trouble handling that much current because 
the 
gassing may exceed its maximum recombination rate, so C/100 to C/50 is 
better for these batteries.  Charge until the voltage stops rising or 
reaches 2.5 volts per cell.  

If you hear any of the pressure release valves on a VRR battery clicking 
or 
hissing, I say turn the charger down fast.  

But not everyone agrees!  Some people will tell you that a certain amount 
of 
venting is necessary to get all the cells properly charged.  One person 
who's articulated this argument is John Olson, who designed the Optima AGM 
battery. He believes that some amount of equalization is necessary on 
every 
charge, and argues that without sufficient overcharge the negative 
electrodes remain undercharged and sulfation results.  He says that if you 
overcharge the right amount - he recommends 7% for Optimas - you get the 
longest life because sulfation and water loss are balanced.  That is, the 
battery dies from water loss at about the same time it would die from 
sulfation.

As you can see, not all batteries are the same, and there's both science 
and 
art in battery maintenance.  ;-)


David Roden - Akron, Ohio, USA

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