Re: (ET) Chris's comments re charging

["David Roden (Akron OH USA)" <etpost drmm net>]

On 18 Jul 2005 at 6:59, tbamc wrote:

> > However you should never run T105's down past 80% ...
> 
> Is that right?  For some reason I was thinking 50%.

Both are right, sort of.

The rule of thumb is that cycling between 50% SOC and 100% SOC is most 
economical in terms of cost per unit of usage.  That is, shallower 
discharges 
mean more cycle life, but less use per cycle.  When you do the math, it 
turns out that 50% is about optimum.  To look at it the other way: for 
most 
economical operation, you decide how much energy you need to do a day's 
worth of work, then you buy batteries that store twice that amount.

Now, the 80% rule: Discharges to 80% SOC are about as low as you should 
go for ^reasonable^ cycle life.  Life drops off rapidly when you exceed 
that 
80%.  

Also, because of manufacturing tolerances and inevitable pack imbalances 
(worse on the ET because of its tapped pack), discharges below 80% SOC 
risk reversing one or more cells.  In an ideal world, you'd monitor every 
cell 
and stop using the battery when any one cell falls below 1.75 volts ^under 
load^.  But hardly anybody does that.  At best they monitor individual 
modules, not cells.

> 
> >You really should charge the pack to 44 volts
> 
> That's measured several hours after finishing charging?

No, it's on-charge voltage - that is, measured at the battery terminals 
with the 
charger connected and operating.

As for the value, follow your battery manufacturer's instructions.  A 
typical on-
charge finish voltage  is 2.45 to 2.55 volts per cell.  For a 36 volt 
nominal lead 
pack this would be 44.1 to 45.9 volts.

US Battery recommends 2.583 volts per cell, the highest I know of.  This 
would be 46.5 volts for a nominal 36 volt pack.  

Note: the above numbers apply to brand new batteries at 80 deg F only!  
Finish voltage should be temperature compensated (based on battery 
internal 
temperature, not ambient temperature), and should also be reduced as 
batteries age.  

Obviously the GE charger does neither of these things, although the timer 
dial's unusual calibration makes an attempt at the second.  It does have a 
ferroresonant design, which stabilizes the voltage in spite of mains 
variations, 
but otherwise it's about as crude as they come.  

Between charger stupidity and the gross pack imbalance induced by tapping, 
the ET frankly abuses its batteries.  The reason they still give 
reasonable 
service is that flooded lead batteries are pretty forgiving of abuse. 

Besides, most ET users cycle their batteries only a few dozen times a 
year. 
With proper care golf car batteries should last for 750 cycles or so.  But 
even 
if you get only a third of that with your ET, you're still getting about 5 
years of 
service.

Because of its unsophisticated charge control, it's hard to see how the GE 
charger would take proper care of nicad or valve regulated lead batteries. 
 If 
Christopher had paid full price for those nicad cells, I'd guess that he 
wouldn't 
be trying to use the stock GE charger.  But he got them very cheaply, 
thanks 
to Uncle Sam - our tax dollars at work!  (Not that I wouldn't have done 
the 
same thing.  ;-)