Re: (ET) (no subject)

["David Roden" <etpost drmm net>]

On 12 Jul 2006 at 21:42, David C Robie wrote:

> These are self regulating under the premise of the old '5% rule, in that
> anything under 5%  of a batteries normal fast charge rate will not 
> damage it.

Well now, I'm not so sure I can agree with that rule.

For lead-acid batteries, I guess it depends in part on how you define 
"damage."  A battery on constant charge *will* overcharge, and it *will* 
fail eventually from positive grid corrosion.  How long that takes depends 
on the charging rate and ambient temperature, but even a battery on true 
float degrades.  That's why you have to replace the battery in your 
computer 
UPS now and then, even if it's never cycled.

If the charge rate is low enough, and the application not too demanding, 
it 
may take 5 years or more for the battery's capacity to decline enough to 
matter.  Or, the battery may fail or fall below usable capacity from other 
stresses first.  But I don't agree that there's some arbitrary amount of 
overcharging that doesn't damage a lead battery.  Any overcharging does.

My other concern here is : what is a "normal fast charge rate"?  A lead 
battery can be charged to 80% SOC as fast as it can be discharged - 
meaning 
hundreds or (for some batteries) thousands of amps - as long as 
temperature 
is monitored.

A charge rate in the range of C/50 (that is, the amp hour capacity 
expressed 
in amps, divided by 50) can be used for a long time - on the order of 
several days - without serious harm.  This is useful for equalizing.

But for charging that never stops, the rule of thumb I'm familiar with for 
lead batteries is that if the battery is to be under charge all the time, 
it 
should be held at float voltage on a regulated, temperature-compensating 
charger.  The float voltage will vary a bit by battery manufacturer, but a 
typical recommendation is 2.2 to 2.3 volts per cell at 20 deg C.


David Roden - Akron, Ohio, USA

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