Bob Simpson provides us with a look at his BMW 325i converted with a Lithium pack and AC drive system.
Name, Age, Location?
Bob Simpson, 53. Banks, Oregon.
What is your primary occupation?
Electrical design engineer. Going on 33 years now at the same company and still loving it (but ready to move on to EVs full time though).
What got you interested in electric cars Bob?
A combination of my weekday commuting (40 miles/day), a growing desire to get off gas and my discovery about the Tesla project details. Being an engineer that loves wrenching on bikes and cars for fun and racing, it all came together in early 2007. I had my next project in the making, a full performance electric conversion.
Your goals for this project were no compromises and to match or exceed the performance of the cars original form, how did the 325i help you in this regard?
The volume of the 325i’s engine (now motor) compartment was perfect for the fit of the motor and for the quantity of cells needed to give the range goal of 50 miles. The transmission was nicely tucked into the tunnel so everything fit well together . I decided to trade off a small efficiency loss for a wide speed range and am very happy I did. The weight of this transmission, under 40 pounds, is completely insignificant, so no loss there especially with regen(erative) braking.
What do you think the most critical part of the conversion process was?
There was a mechanical challenge in this compact design where the motor and transmission mate that was my biggest unknown at first. I found a company that knows how to couple things that transfer lots of torque/power and I farmed out the adapter attachment to them. It worked out perfectly and now I know how to do it.
In the electrical side of things, the AC motor inverter was, and actually still is, the problem. I am happily using a 100kW inverter now but original design has a 200kW inverter, which I am just weeks away from getting the remaining piece I am told.
Can you tell us where you sourced some of the parts?
I got most of my components from www.metricmind.com
How did you handle weight distribution? I’m sure a lot of effort went into creating a well balanced conversion.
I figured if I matched the weight and location of the gas engine, the vehicle must handle as BMW intended. The initial planning was all around the battery pack design and how much weight and volume it took to get my range of 50 miles. To my delight, the pack weight came in at ~350 pounds and the motor at 200 pounds. That total matched the 550 pound ICE package just pulled out. I weighed all the components to keep track of things.
Can you describe the advantages an Alternating Current system has over a Direct Current system and why you chose AC?
I really wanted to use brushless drive for this high mileage commuter and secondly, wanted to have higher RPM potential for better use of the BMW drive train. Water cooling was also a motivating factor, I had a overkill of water cooling resources already built into the chassis I wanted to take advantage of. I knew it would cost more but I was confident that it would payoff with high capacity/continuous performance with the ability to last well over a decade because of no wear elements.
You have plans for installing a 5kw Photovoltaic array for charging the car, how is that going?
The installation was held up for months because of the wet weather we had in early summer. To get the Oregon energy trust grant money for the installation ($1.75 per Watt) we needed to meet the 75% exposure factor. Being nestled into 130 foot fir trees, we had to lay down a few truck loads of these trees. Once the weather dried up to get trucks in, the mills were inundated and shut down for a month to let the market stabilize. They just re-opened this week so logs just went out today. Once the final trees are down, a measurement will be taken to confirm we meet the 75% exposure level, and install will begin shortly thereafter.
I will be data logging the incoming solar power as well as the BMW power consumption and show a graph of the annual sum of the two. I am aiming to hit net zero in annual period.
Could you describe your battery pack and its BMS?
I started to design my pack and control system in late 2007, but at the time there was nothing out there to use as reference or buy. Once again, I had to figure it out myself, but had the advantage of having had a career in electronic design. After I researched the care and feeding of lithium ion batteries, I knew what I needed to do.
I did the circuit design and layout on my PC and once finished, I pushed the order button and the circuit boards arrived a few days later. I decided to create the pack as a module system for simple access and allowing for different depths, depending on where the module was. Above the motor, I used short 5 cell modules, in front of the motor is a group of 10 cell modules, etc. The BMS boards fit the same in each size module with two 5 cell boards stacked for the 10 cell modules. The voltage of the largest modules was only 33 volts, so handling was safe and easy. It only got hazardous once I linked the modules together at the end.
Also, you have an extremely well built battery box with its own thermal control system could you describe its construction and function?
This was one of those things that I designed while trying to get to sleep, quite literally. There was regular tight conversation between mech and elec engineer with occasional SW engineer stepping in, so I had some busy dream time for a while. The next day I would measure up my results from the previous evening’s thoughts and often it worked and became part of the design. This was a design with focus in 3D space utilization.
The water lines I plumbed into the base of the pack are primarily for cell balancing heat removal. Very little energy involved, but I wanted to err in favor of too much cooling capacity. The other motivation to do this was for winter heating. Whenever plugged into AC power the pack can be heated using the BMS controllers that put any requested amount of heat into its own base. When plug is pulled to drive, the little bit of heat from motor/inverter cooling takes over to maintain a moderate temperature.
What kind of braking system did you use?
The original BMW friction brakes are still used in original form, but I do most of my braking with regenerative type so the pads will last a very long time. Unless there is a need for panic braking, I apply little pull on a hand control so I don’t even need to lift my foot to brake (brake lights come on above a certain point), and nearly all this kinetic energy is returned to the pack as charge. The battery pack can take an enormous amount of current the same way it can deliver it with insignificant thermal rise, which is my evidence of its high efficiency.
Does the car have a steering assist ?
Yes, I used the original steering rack powered by a late model MR2 electro-hydraulic pump that fit right into the cavity in front of the drivers front wheel.
It is a quiet pump that has the ability to drop its power with increasing speed. I simply divide down the digital signal coming from the rear wheel abs system and feed it to the PS pump speed input pin. This makes the power level drop off as the use diminishes with increasing speed.
What is your Wh/Mile on average and what is your typical range?
My initial data shows 350 Whr/mile. This gives me a 45 mile range. I will be adding an aux pack in the rear where the gas tank and exhaust used to be to get over 80 miles later this year.
Which EV enthusiast sites do you visit ?
I like the technical discussions on greencarcongress.com
Which are some of your favorite conversions or available new EVs?
I am happy with both the Chevy Volt and the Nissan Leaf designs that are close to release, but would rather be driving a Tesla S or Fiskar.
What has your conversion experience taught you overall?
It has given me a sense of scale on the energy involved with moving/stopping a given mass. I feel like I have just graduated with a specialized degree. I was motivated to understand each and every detail in the system and now have the confidence to do much more. Just another element of return on my investment.
Do you have a favorite story about building or driving your EV?
This was completely a solo project (except for engine pull day) and sure I had a lot of fun little things happen along the way. A little fun we had at a gas station (my wife and I) early after getting it on the road was memorable. We went with the intent to get some gas in a 5 gallon container for my garden tractor. But for fun, I handed the gas pumper my card and said “fill er with regular”. I had my wife jump out earlier and capture the reaction with her camera. We got two classic candid shots of the guy reaching for the gas cap “click” then he stepped back with a tilted head “click” and said “what the….” I then jumped out and opened the trunk and pulled out the gas can. I had to put the pictures on my website, look for “Good fun at a gas station…”
Are there a few people you’d like to thank?
I owe Victor at Metric Mind a special thanks for his generous feedback during my contemplation of this project back in 2007. And the person that I owe my biggest thanks, my wife Carol. She was very supportive and accommodating which turns out to be critical on this scale of project.
You’ve also converted a motorcross bike to electric with the same aims in mind, have any other riders expressed interest in the idea?
This was actually my warm-up development that was just ahead of and for the sake of the BMW project. But because of the significant result, it got (and gets) a lot of attention from serious bikers. Turns out there is a growing list of parties lining up to license this design as well as the BMW. So again, a good investment indeed…
Is there anything else you’d like to mention?
Not only was this project immensely fun to design, fabricate and install in my own shop, it had a very refined result which is one of those things that is hard to imagine initially. So in some ways the project turned out better than planned or expected. Now, I have the experience to do a “serious” design for my next project…
Are there any EVs out there who you’d like to see on EV Gearbox?
Yes, the four that I mentioned above…