Re: (ET) NiCD powered Elec-trak report

[Christopher Zach <czach computer org>]

> They also maintain very stable voltage until they are almost flat, at which 
> point the voltage falls off a cliff.  This is both a blessing and a curse.  
> You have plenty of power for a long time.  However, when they're flat, 
> you're pretty much done.  With lead batteries a voltmeter (such as the ET 
> uses) tells you very roughly how much charge you have left.  With nicads a 
> voltmeter tells you that you're going to be walking. ;-)  Christopher is 
> using E-meters to deal with this matter.

I've actually noticed that the voltage does drop a bit as they get 
around 20 ah down. The voltage slips from 36 down to about 33-34 or so 
which is down near the bottom of the green bar on the Elec-trak's meter. 
What is interesting though is how little battery sag there is. You can 
sit there at a stop, throw the tractor into D, speed 4, then engage the 
mowers and go up a hill without the needle moving much at all.

When they finally hit empty, one pack typically goes slightly before the 
other. Then the power output starts to fall as you draw the last ah or 
two from the remaining pack. Not much possibility of cell reversal; the 
dead pack will only put out 4 amps and that's it :-)

> There is also very little "limp home" capability in nicads, unlike lead 
> batteries; with the latter you can let them rest a few minutes and you'll 
> generally regain just enough capacity to get back to the garage.  

True. I've thought about this one, and the probable solution is to carry 
out either a set of three Hawkers or a set of NiCDs in the rear battery 
box to get it home. So far though I haven't been able to be stranded.

> The flooded type especially are relatively easy to charge with a moderately 
> smart charger : just put back as many amp hours as you took out at about 
> C/5, then drop to C/20 and add another 20%.  Alternatively, set a timer to 
> zero, then charge constant current at C/5 until voltage reaches 1.6vpc.  
> Reduce current to C/25, reverse the timer, and charge until the timer counts 
> down to zero.  (Based on charge instructions for Saft STM range modules.)

So far it's even easier. Just turn on the Elec-trak charger and set the 
timer somewhere depending on how much you used them. The Trac charger 
has a very sharp current cutoff right around 44.2 volts, and the pack's 
"full" is about 45.2 volts. By that time the trac's charger is down to 
something like 2 amps (1 per string) which is enough to make them hiss a 
bit, but not overcharge.

I watered them a few days ago, first time since they went into service 
in the summer. To be honest they really didn't need it. 10ml or so of 
water per cell, now they are filled to the absolute gunwales. So now I 
don't need to think about it till spring.

The only problem I have right now is the batteries seem to want to 
cross-charge a bit when left alone. The E-meter keeps showing a .1 to 
.05amp draw with the tractor off, going from one pack to the other and 
back again. My guess is that NiCDs don't mind being charged and 
discharged together, but they wind up with a current flow when connected 
in parallel. Kinda odd.

Chris