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 > _______________________________________________ > UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > http://lists.evdl.org/listinfo.cgi/ev-evdl.org > For EV drag racing discussion, please use NEDRA ( > http://groups.yahoo.com/group/NEDRA) > > -- David D. Nelson http://evalbum.com/1328 http://www.levforum.com Nokia Lumia 920 Windows Phone 8 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20130504/7bb89ce8/attachment.htm> _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
