Re: (ET) newer batteries are tougher?

["David Roden (Akron OH USA)" <roden ald net>]

> a difference in 5 hours vs. 19
> hours charging, for a couple tenths of a volt?

For a new battery, the difference in open circuit voltage between 25% 
charged 
and 100% charged is only a little over a tenth of a volt per cell - 
roughly 
0.35 volts on a 6 volt battery, just under 2 volts for the entire 36v 
pack.  
As a battery ages, its fully-charged voltage declines along with its 
capacity. 
 So an old battery, in effect, can't be charged beyond (say) 50% or 75%.

A fixed-voltage charger is a less than optimum device with which to charge 
aging batteries.  Even though the battery's maximum voltage is lower, the 
charger still hammers it with the same voltage as when it was new.  Thus, 
finish current goes up, the battery is further degraded by overcharging, 
and 
the depreciation process accelerates.  Add to this the fact that each 
discharge cycle also uses a greater percentage of the battery's capacity 
(if 
mowing used to require 50%, now it takes 70%, for example), and you can 
see 
why batteries sometimes seem to go down fast at the ends of their lives.

The GE strategy for handling this is, like their charger, fairly crude: 
guess 
that the batteries' capacity will degrade at some fixed rate per year, and 
tell the operator to set the timer to shut the charger off sooner.  

You really should adjust the finish voltage down on your charger as the 
battery ages.  (It should also be temperature compensated, as this also 
affects voltage.)  Unfortunately, I don't know of any way to do either 
with 
the GE charger.

To take better care of your batteries, you could change to a charger 
that's 
controlled by a programmable microprocessor, and reprogram its "brain" 
periodically as the batteries age.  Better yet, you could use a smart 
charger 
with a DV/DT algorithm, such as a Lestronic golf car charger.  This 
algorithm 
monitors the charge and waits for the on-charge voltage to stop rising, 
without regard to exactly what that voltage is (as long as it is above a 
reasonable threshold).

I don't use the GE charger.  Does it really have that wide a spread in 
charging times?


David Roden - Akron, Ohio, USA
1991 Solectria Force 144vac
1991 Ford Escort Green/EV 128vdc
1970 GE Elec-trak E15 36vdc
1974 Avco New Idea 36vdc
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The illegal we do immediately. The unconstitutional takes a 
little longer. 

                                        -- Henry Kissinger

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