Anil's NiMH Solectria Force

This is the story of my friend Anil and his two year long effort to convert a Solectria Force EV to NiMH batteries. Anil embodies the generous and giving spirit of the EV community. Rather than sell his priceless spare NiMH module for a significant sum of money (they do have a price on ebay) he donated them to a fellow budding EV'er, me! In return I've tried to help him as much as possible in constructing a cooling system for his rear battery box. Press read more for Anil's own words: December 2004 I justified to my wife that an electric vehicle could allow me to spend at least 10 minutes more in a day with her. For I would have access to one of those rare white (alternative energy) car-pool access stickers. While rolling her eyes, she hesitantly agreed. Although I love spending time with my beautiful wife, the real reason for my decision to buy a ’95 Solectria Force (don’t believe those range numbers in the link) was: 1. I was putting an unfulfilling 50+ freeway miles per day in my 97’ Integra Type R (#365 out of 500). 2. I had access to some used and discontinued Panasonic 95Ah Ni-MH batteries. So I bought a 95 Solectria Force from a former EV1 owner. He was not too happy with the performance of the Force, especially considering the problem with it not delivering peak torque. A problem that I thought, as a former Honda R&D engineer and at the time AeroVironment engineer (I now work at Tesla Motors) I could resolve. Unfortunately, I was wrong. The only thing I haven’t tried is replacing the speed sensor or the motor. January 2005 I was doing my 27-mile commute with the good old lead-acid batteries. One evening after charging at work I tried to play with the throttle POT to see if I can get more torque out of the system. I ended up learning that those class=SpellE>Brusa inverters are sure sensitive to feedback as I totally screwed up the throttle calibration. I limped home at a top speed of 25 MPH while calling my wife to help me navigate home on surface streets while she pulled the Thomas Guide out of her car. Through August 2005 The car was un-drivable for this period. I experienced the following problems: 1. I tried adjusting my POT with the old, inconsistent, DOS program monlog.exe program I got from href="http://www.brusa.biz/e_welcome.html">Brusa and an RS-485 to RS-232 translator. I discovered the POT was bad. Could that be what was causing the torque shuddering? 2. I finally received one of the last custom POTs from Solectria….it didn’t fix the shuddering. 3. I played with the speed sensor gap. I went too close once and scraped off the optical encoder markings and had to get a new encoder wheel. 4. The car got towed (the one and only time) after it wouldn’t start about 7 miles away from home one night. It turned out that there was a loose connection in my ignition box that I found about a year later. 5. I bought a new, err refurbished, $1,300 motor controller thinking that this was the source of all my problems. Now that “new” controller doesn’t work. 6. Discovered a short in the speed sensor ribbon cable. Nope, this didn’t fix the torque shuddering problems either. A Careless Day in August 2005 My 4-year old son would often keep me company while I worked on my car. I love him being around asking questions and assisting me with basic stuff. He always loves rolling down the windows and pushing the buttons. Of course the windshield wipers were always his favorite. Being an engineer, I love his inquisitive curiosity. [img_assist|nid=264|title=Future EV driver?|desc=|link=popup|align=left|width=200|height=150][img_assist|nid=265|title= |desc=|link=popup|align=left|width=200|height=150] ..................................... On this day I was in front of the car, with my son in the driver’s seat. The key was in the ignition with the power selector in the off position (until my son moved it). He would on many occasions tap the accelerator pretending to drive. Well this day he actually did! Right over and on my right foot! When the car lurched forward he was as surprised as I was. After I screamed in shock he ran inside the house and upstairs to his bedroom. He thought he would be getting timeout for the rest of the summer for this one. My wife came running coming out. I said “reverse the car! Aaaargh!” She said “I don’t know how to drive this thing.” She ended up helping me lift the car’s front suspension up to allow my foot to slide out. Who Needs SUVs (Through May 2006) We started a house expansion during this time and I took some vacation in India. So the car retrofitting was put on hold for awhile. One day, though, we needed to go to Lowes. Eager to put miles on this “Force” I convinced my wife to take the car (it has rear reclining seats) on this perfect, made for EV, 7-mile commute. Well, as typical of the home improvement shopper, we ended buying more than we originally intended and had a challenge packing the car up. [img_assist|nid=266|title=Who Needs an SUV|desc=|link=popup|align=left|width=450|height=338] Lead-acid batteries don’t like 80% DOD cycling with a huge thermal imbalances (between my front and rear battery packs). I occasionally drove, errr limped, the car to work during this time. The Solectria Force is configured with 5 batteries in the front and 8 batteries in the rear where the gas tank used to be. [img_assist|nid=268|title=The Front Lead Mine|desc=|link=popup|align=left|width=450|height=338] Through May 2007 I installed a discontinued Battery Management System (BMS) made by AeroVironment on existing lead-acid system. The BMS monitored the following: * Shunt current (I used the same shunt as the AH meter) * Pack voltage * Module Voltage and Temperature (13 each) * Key ignition * An “equalize” switch * A relay to know when the vehicle is plugged in With this BMS, I monitored the charge algorithm used by the Brusa lead-acid chargers (there are two paralleled chargers in the vehicle). The chargers were programmed to charge to over 15V/module. This is well over 100% SOC for Ni-MH batteries so I knew I could use this same charger for EV-95 batteries. I installed a Contactor to open the AC line into the charger on certain dT/dt, dV/DT and AH thresholds. I wrote a lot of software for this Motorola 332 system so it was pretty easy for me to do this. [img_assist|nid=269|title=Diagnostics App|desc=|link=popup|align=left|width=450|height=300] I used a custom Visual C++ diagnostic application (that I wrote) that communicates with the BMS over RS-232. I quickly realized that Solectria had little knowledge about the importance of thermal management to increase the life of your battery pack (checkout the insulation between the modules….yuck). The rear batteries in the Force run about 10 degree C warmer than the front batteries. The charger had no idea that it was constantly overcharging the rear-batteries and undercharging the front batteries, since there was no thermal feedback. The thermal imbalance was due to the heat from the chargers located in the trunk [img_assist|nid=267|title=The Rear Lead Mine|desc=|link=popup|align=left|width=450|height=338] The Ni-Mh batteries that I was planning to install would not tolerate this imbalance and I wasn’t going to spend all this effort to see my priceless Ni-Mh (they are discontinued) batteries die quickly. So I started to undertake the most challenging part of this retrofitting…to install an air-cooled system. June 2007 Through September 2007 I finally put the Ni-MH batteries in the vehicle. Only 4 modules fit up front. And since I could symmetrically lay them out, a relative simple cooling system was employed. I used a furnace filter on the inside of the pack. The href="http://en.wikipedia.org/wiki/Corrugated_plastic">corrugated plastic under cover prevents moisture and rocks (at least I hope) from entering the battery pack. [img_assist|nid=270|title=Front Battery Box|desc=|link=popup|align=left|width=450|height=338] Cooling the rear battery pack was not so simple. My main concern was safety. I frequently take my kids to their baseball practices, ballet class, grocery store, grand ma’s home. And with them in the rear I wanted to make the rear battery pack as safe as possible. I ended up setting the batteries as far forward as possible. Secondly I bolted down the modules using rivet nuts (there were under-carriage locations not accessible from underneath). [img_assist|nid=274|title=Rear Battery Box|desc=|link=popup|align=left|width=450|height=338] As far as a cooling system, I used some of href="http://www.avinc.com/">AeroVironment's proprietary “jet cooling” system which works great in un-symmetric packs like this one. Besides the safety aspect, filtering the air and keeping out water was another challenge. One of AeroVironment’s machinist said I started to learn enough about “machining” to get dangerous. Well with some training and help I built these nice replaceable filter cartridges. [img_assist|nid=275|title=Air Filter|desc=|link=popup|align=left|width=450|height=388] Final Results On my last day at AeroVironment I did a capacity test on the battery pack using and ABC-150. I discharged the battery pack to 75 Ah at a 1C rate (there was probably 10% still left). This corresponded to me driving at a constant 62 MPH for 60 miles. I enthusiastically told my neighbor this, and they responded pessimistically “only 60 miles.” I guess the masses still need a 100 mile range EV. Acknowledgements Many people help me with this project which I am ever grateful. * Nayna – my wife * My kids – for helping and sacrificing their time away from me * Chuck, John and Rex for helping me recycle these batteries * Peter, Scott, Gregg, Marc, Brad, Rich, Lindsay at AV for advice and a little sweat and time.

Comments

NiMH Batteries

I was considering using the NiMH battery set from a Prius. In fact I purchased one to dis-assemble, and did. I posted some questions at the Solectria site at Yahoo. And got some very good answers regarding some of the engineering constraints that need to be observered - namely that cooling is important and that it is necessary to clamp the individual cells together to prevent them from "thickening" when being charged. Bottom line is that I have not gotten a good answer as to whether the Prius NiMH battery has potential as a conversion battery. The cost is excellent. Any thoughts?

re: NiMH batteries

It can be done! See www.electric7.com. The issue is that they are only 6.5Ah per pack. You'd need to parallel about ten packs to get some decent range, and there in lies the problem. NiMH batteries don't parallel well! There will always be a slight variation in capacity between batteries, so when you charge the paralleled packs, one pack will reach full before the others. When a NiMH battery hits full it heats up and the voltage across it drops a little. When this happens the packs that aren't quite full yet will have a higher voltage than the full one. They will then start discharging into the full pack; thus heating it up further and causing a runaway reaction. Before you know it one of your packs will be on fire. There are of course engineering solutions to this problem as evident by the electric7 mentioned above. You can parallel your prius packs through diodes and use an individual charger per prius pack, or a roaming charging solution. Other than that the prius battery has plenty of power and can be found for cheap, so as long as you're aware of the pitfalls it can be done and should work well. Hope this helps

Paralleling Ni-MH batteries

I previous did simulation and testing of parallel Ni-MH cells. It is no problem. because the impedance of the cells increase as you discharge, the cells will naturally balance.

Re: Paralleling NiMH

Interesting, I agree they would parallell well on discharge. It's the negative DV/Dt at the end of charge that I'd be concerned about. Did your simulation find this not to be a problem?

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