Headway Cell Testing - A Preliminary Study

Cross posted from the NEDRA list:

As promised by James here at EV Components we now have 500 Headway 30120-P cells
in the warehouse for install into the Pinto. Over
the last few days I have been doing some initial stress testing of these cells
to see just what I can expect in the Pinto. If you
remember the Pinto weighs in at approximately 3200 lbs which includes its 848 lb
lead acid Enersys pack. My preliminary tests on
these headway cells indicate I can run 4 strings of 120 cells each for a total
pack weight of 316 lbs for a weight savings of 532
lbs. If we change nothing else on the car other than the batteries we are
looking at weighing in at an approximate 2668 lbs :-)

The weight savings may even be a little better because I'm likely to take out
the front battery tray to which through a mistake at
the fab shop came out weighing way too much. So there will definitely be an
additional 40 lb weight savings ;-) So lets make the
anticipated 2010 Racing season weight target 2628 lbs.

Now to the fun stuff. I'm still trying to find my datalogger in my packed up
stuff, but I wanted at least pull some juice from
these things and I did find my 500 amp carbon pile load tester. So I rigged up
4 Headway -P test cells in series (for the 12V load
tester). I put meters on each cell as well as a current shunt and a K-
thermocouple temp probe in the middle of the cell pack. I
also check with a Greenlee clamp on ammeter.

I noticed after the first couple nights of torture (I mean the interrogation)
that the first run of a modest 200 amps would pull
the cells down to and a little below the 2.0v cutoff limit I'm holding the
testing to. This would quickly heat the cells up from
20*C up to about 35*C. At 35*C and up they would hold the full load of the load
tester which with the knob fully tightened in the
clockwise position was 428 amps.

Now let me back up a little and keep you thinking about that 428 amp number. My
testing conditions are as follows. Cells start in
a fully charged condition. The test starts by dialing the load tester up to the
desired amps and the timer starts. The test ends
when either 15 seconds has elapsed or one of the cell voltages drops to 2.0V.
At the conclusion of a test the sagged cell voltages,
the max and min current readings, cell temp and the resting voltage of the cells
at least 1 minute after the test is complete are
recorded. There is actually a lag time reading the cell temp as the temperature
probe still senses increase in temp for several
minutes after a pull. The cells would then be recharged back to 4.2V each (which
took about 10 minutes for a recharge with
individual 10A chargers).

So back to the numbers. After doing a brief low current pull that puts a little
heat into the cells I would commence doing fixed
current pulls. In one series of runs I would make the current 300 amps for each
pull until I was comfortable the cells could
sustain that for in excess of my 15 second tests. In another set of runs I
would max out the load tester to see just what the max
current the cells would output. In one series where I did max current pulls I
found that the current was temperature related (no
big surprise). Also I suspect with the load tester in the max position I just
cant get the pile resistance any lower. I may need
a bigger load tester ;-)

So here are some initial results. They are certainly not fully conclusive and I
do not yet have enough runs of the tests to say the
cells are going to hold up to continued abuse. But here they are for what they
are worth.

Max draw Test:
Test #1 320A dropped to 300Amps in 15 seconds, cells generally down to 2.0V
temp 35*C
Test #2 350A dropped to 320Amps in 15 seconds, cells generally down to 2.21V
temp 47*C
Test #3 397A dropped to 320Amps in 15 seconds, cells generally down to 2.11V
temp 57*C
Test #4 410A dropped to 340Amps in 15 seconds, cells generally down to 2.21
temp 61*C
Test #5 400A dropped to 340Amps in 15 seconds, cells generally down to 2.18V
temp 61*C
Test #6 415A dropped to 360Amps in 15 seconds, cells generally down to 2.21V
temp 64*C
Test #7 428A dropped to 360Amps in 15 seconds, cells generally down to 2.26V
temp 69*C
(Test #3 - 6 cooling was applied to the cells prior to making the next test)
Test #8 held current at a steady 350 amps for 20 seconds, cells generally
dropped to 2.30 V temp 77*C
Test#9 was a repeat 350amp steady draw without recharging and I could not get
enough load to keep the current from dropping to 320
amps, and the cell voltages dipped to 2.0V before the 15 seconds was up.

This is one sampling of the data that I've recorded and will get several
different measurements including time to cutoff voltage at
different discharge rates. But I need my data logging equipment unpacked for
that.

I am optimistic that if I can get reliable 350A discharge rates consistently
then I can run 4 strings of these for 1400 battery
amps. If I can hook up with that much battery current with the reduced weight
and I need more current I may have to add in another
79 lb string of 120 cells.

I still have more playing around with the cells to do, and some design work to
figure out how and where they will fit in the car.
Space isn't an issue but weight placement is. So I'll add some detail as I go.

Oh yeah.... a few teaser pics here
http://www.thefoat.com/fa/m-photos_album/album_id-555/member_id-cp%2BndsLnd2IM3o\
hQzYqz0w%3D%3D

Enjoy
Mike