(ET) Kill the battery (Fall, leaf vacuum, etc)

[Chris Zach <cz alembic crystel com>]

So it's fall. Cooler, leaves are starting to land on the yard, and after 
a quick run with the leaf vacuum, tractor motor, and deck running at the 
same time I realized it was time to power cycle my batteries.

Note: I have 90 1.2v SAFT NiCd batteries in my tractor for a max 
capacity of 90 amps. NiCds have an interesting profile where they will 
provide pretty constant power till almost empty so when my tractor hits 
the bottom of the green bar on the volt meter it's going to go dead 
pretty quickly. However I did notice that once it hit that level things 
slowed down but did not collapse, which means the packs are probably out 
of sync.

So after running it down till it could barely move I parked it next to a 
power cord and left the deck and the blower running slower and slower 
till the deck cut out (the deck contactor drops out at about 20v or so) 
and let the blower spin till the pack read a good 8v or so. Then put it 
on charge and let it charge for 30m.

Then I drove it up my hill, parked it, and fed branches into the vacuum 
inlet until the blower started to slow. Then regenned it down the hill, 
ran the batteries again down to 6v under load and disconnected the 
blower (which was still spinning, but slowly. Nice new bearings make a 
big difference at lower voltages). Now it's back on charge and after 5m 
the charger was pulling 1,100 watts from the wall, power factor of .8.

In theory the pack should be (30ah*3*36)=3.240kw. Assuming a .9 charge 
efficiency from wall to pack (I really should hook up the E-meter again) 
that would mean if it takes 3.564kw from the wall it should be charged. 
I have a Watts up meter monitoring it now, will check again in an hour, 
3 hours, and 6 hours (probably in the rain) to see how it's doing.

That should give me a good idea of the state of the pack. Using the 
on-board charger is not perfect: technically it can only supply about 
30a into the batteries and NiCDs really should take 1C-2C charge rates 
at first (90a-180a) but that would require a 4 to 8kw charger.

Hm. You know I do have a spare Magnecharger system (paddle 
receiver+control logic+DC-DC charger and 9 temp probes for the pack) I 
wonder if it would be programmable to 36-45v. Then I could do a constant 
current to 42v, then constant voltage to 45v then a nice trickle 
overcharge as needed. I could even put the probes in the pack to watch 
for temp spikes. Hm hm hm. Maybe this should be my winter project.

It would be nice to dump charge the tractor in an hour...

C