Re: (ET) batteries

["David Roden" <etpost drmm net>]

Hmm.  Not to dismiss your reference, but neither of those definitions fits 
the one I'm familiar with.

As I understand pulse charging as practiced today, it involves a high peak 
current applied for a brief period, followed by either a rest period or, 
in 
some algorithms, a brief partial discharge of the battery.  The alleged 
advantage is that it provides a high initial charge current while keeping 
the average current manageable.  

Not being an electrochemist I don't understand why this is, but some 
batteries need occasional high current charging for at least part of the 
charge cycle or they lose capacity.  This is true of Hawker Genesis 
batteries, for example.  Pulse charging provides this.  It's also claimed 
to 
make faster recharge possible, but I honestly don't see how - charge time 
still depends on the average current.  I don't know of any other benefits, 
but I may have missed some.  

Apart from this, I'm skeptical about an algorithm that tries to measure 
open 
circuit voltage in the manner described.  The voltage will be artificially 
high when the battery has just been removed from charge.  However, there 
may 
be some details of the algorithm that the description you quoted left out.

As for equalization, an equalizing charge could be thought of as "topping 
up."  It's impossible to manufacture cells so that they have exactly the 
same capacity.  A typical automatic charger will shut off when it detects 
a 
voltage or current indicative of full charge.  But when it shuts off, it's 
likely that one or more cells won't be fully charged, because of these 
variations.  

As the battery is cycled, these cells fall further and further behind.  
Eventually they can become sulfated from the undercharging.  Equalization 
helps to prevent this; it forces current through the fully-charged cells 
in 
order to bring the less-charged cells up to an equal state of charge.  
Although the overcharge isn't good for the fully charged cells, the 
undercharge isn't good for the others. You try to pick the current, 
voltage, 
and length of the equalization, and the frequency with which it's 
performed, 
so that more good than harm results.

Equalization is normally a constant current charge applied with either no 
voltage limit or with a rather high voltage limit, for a specified time.  
I'm not aware of any benefit in fully discharging a battery before 
applying 
an equalizing charge, and in fact such a practice risks cell reversal and 
the consequent damage.

Normally you'll perform equalization every 20 or 30 cycles, or when SG 
testing shows that the cells are getting out of balance.  "Semiannually" 
might apply to a battery used weekly, but I'd say that's overgeneralizing.

I guess your sources and mine just don't agree.  ;-)

I should point out that a full discharge and recharge is part of the 
recommissioning procedure for nickel cadmium batteries.  But that's quite 
different from equalization.


David Roden - Akron, Ohio, USA

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