Hi David,
I've also got a 914, with 108 CALB 60ah batteries in 3 locations: above
the motor, where the gas tank used to be, and in the front trunk.
I just added heating to my rear pack because the BMS (minibms) was
complaining when the batteries were too cold. Just in time for summer,
too. :)
Cheers, Peter
On 5/4/13 11:39 PM, David Nelson wrote:
Ben,
I assume you have LiFePO4 cells in your car? A study was referenced in
http://blog.evtv.me/2013/04/2177/ where cells which were cycled at 35°C had
a longer life than cells cycled at 23°C. Might be worth reading the blog
post and the referenced study. I'm thinking I need to add heating to my
pack and I also won't worry about cooling my pack. It just doesn't get very
warm.
On Sat, May 4, 2013 at 8:56 PM, Ben Apollonio <[email protected]> wrote:
Well, it's spring again in New England, which means I have the 914 back on
the road, freshly upgraded with a Z1K, an Elcon DC/DC, and several tweaks.
I've started experimenting with different driving techniques and made a
few surprising discoveries.
The first is that my car actually seems to be more efficient when I'm lazy
and leave it in 3rd gear all the time! This is rather the opposite of what
I'd expect given I^2R losses and the efficiency curves of the motor (ADC
9"). However, I've noticed my tranny getting quite hot after a long drive,
and I suspect its losses at high RPM are enough to reverse any gains from
running the motor more efficiently. Now, I could probably get the best of
both worlds using 2nd gear for more efficient starts and 3rd for cruising,
but who's counting?
Second is that brush timing seems to have more of an effect on torque than
I realized. When I was starting in 2nd gear all the time, the car was off
the line so fast I never noticed any subtleties in acceleration. However,
driving around in 3rd, I notice the car feeling sluggish starting out even
at 1000A, but I can feel the acceleration pick up as my speed increases.
It goes from ho-hum to pushed back harder and harder into my seat. I also
never experienced this as much because the ol' T-rex did a
throttle-to-volts control whereas the Zilla does throttle-to-amps, so I can
see from the needle that I "should" be getting constant torque based on
T=kI^2. I suspect this is because my motor has advanced brush timing; I
just never expected the effect to be so noticeable! Makes me wish I had an
AC or BLDC motor where I could adjust the timing electronically...
Third (not so surprising really) is that there's a definite variation in
cell performance depending on where they're located in the car. I have
batteries distributed in 3 areas: the front trunk where the gas tank used
to live, the engine compartment lower rack (quite exposed), and the engine
compartment upper rack (less exposed). My BMS streams data via bluetooth,
so I was able to log cell voltages on my way home today. It was a
reasonably warm, sunny day, but dropped to 40 by the time I left work,
around 8:00. Looking at my telemetry, there's a definite trend in
performance: the cells in the front trunk were noticeably higher voltage
and lower ESR than the bottom rack engine compartment. The top rack engine
compartment were in between. There's even a trend within the bottom bank,
where the ones most exposed to airflow were the weakest. The pack is quite
well balanced, so this is clearly due to temperature. It will be
interesting to see how it affects the aging of t
he pack, and which age faster: the high-resistance cells that are being
driven closer to their limit, or the lower resistance cells that are being
degraded faster by high temperature.
Anyway, thought I'd share. It's been a little too quiet around the list
lately...
Cheers
-Ben
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