Re: (ET) time for a new pack...

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

On 7 Aug 2001, at 12:29, Don Barry wrote:

> As always, the thing to remember is not
> charge up the batts until they're fairly well discharged.  That's the 
> basic 
> purpose of the sizing.

I want to clarify this so that no one gets the wrong idea.  

Don is correct that you should, if possible, size the battery (choose its 
capacity) so that your usual mowing or other service discharges it about 
50%.  That's the optimum use of a battery for lowest overall cost per 
amount of work accomplished (in a road EV, we'd say lowest cost per 
mile).

But don't get the idea from this that if you use only 20% of the battery 
capacity to mow your lawn, you ought to charge only every 2 or 3 times 
you mow.  Definitely not!

Lead batteries should be charged immediately after any use that 
discharges them more than a trivial amount, perhaps 10% or even 5%.

The reason for this is that the discharge reaction converts the active 
material (lead and lead dioxide) into lead sulfate.  As this compound 
sits, it crystallizes.  The longer the sulfate remains, the more 
difficult it is for the charging reaction to change it back into lead and 
lead dioxide.  The crystalline lead sulfate breaks away from the plates 
and falls to the bottom of the cells, where (in spite of what the 
"desulfator" salesmen claim) it is for all practical purposes lost 
forever. This process is called sulfation, and it's one reason for the 
loss of capacity as a battery ages.

What's more, the more deeply you discharge before charging, the shorter 
the battery's cycle life will be.

Don is right to the extent that the most economical operation of a lead 
battery is when you use about 50% of its capacity each time.  However, if 
you can't use that 50% in one session, charge it up anyway.  Letting it 
sit uncharged for 24 hours -- even 12 hours -- will negate any advantage 
of using half the capacity this way.


David Roden - Akron, Ohio, USA
1991 Solectria Force 144vac
1991 Ford Escort Green/EV 128vdc
1979 General Engines ElectroPed 24vdc
1970 GE Elec-trak E15 36vdc
= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =
Thou shalt not send me any thing which says unto thee, "send this to all 
thou knowest."  Neither shalt thou send me any spam, lest I smite thee.
= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =