Batteries

 

My earlier looks at making an electric Midget made it clear I had to use LiFePo4 batteries if I wanted a car that handled like the original Midget. Lead acid batteries would be too heavy to be able to use the original springs, and the overall weight would not seem like the light fun sports car a Midget inherently is. I wanted at least a 50 mile range since that would cover my commute to work, and the ability to run an errand or two between charges. I also wanted to be able to drive at 65 MPH up hills. The 65 MPH up a hill meant I needed to be able to drive enough amps into the motor at 3600 rpm (60 mph). To figure this entire thing out I turned to an EV calculator. There are a number of these on the Internet. The one I choose had some different motor and controllers modeled into the calculator. I choose a motor that I thought was similar to the Warp 9. At the time I thought my car was going to have a Warp 9 motor, but it turned out to be a Warp 9 Impulse. Lucky for me, using the Warp 9 motor for modeling meant I’d have plenty of volts to drive the Warp 9 Impulse. The Warp 9 Impulse has a lower torque constant than the Warp 9, which makes it easier to push current into it at higher rpms. I choose to go with 120 volts to meet my 65 MPH goals. The next part in picking the batteries is what range did I want. Like everyone, I wanted 100 miles of range, or more. The calculator quickly shows you that you’ll need a lot of batteries to get 100 mile range. This amount would be too heavy, and cost too much for what I wanted. At the time Evolve Electrics, which is located about 10 miles away in Boulder was offering some used Thundersky cells. These where the 160Ah cells, and only had about 20 charge cycles on them. At 10% off, I couldn’t resist trying to use these cells over the other choices. I settled on a range of 60 to 70 miles at 60 mph. The calculator estimates this once you’ve input your car’s Cd, frontal area, and rolling resistance estimates. So the 120 volt requirement meant 38 cells, and the range fell out of the fact that they where 160ah. Total capacity is 120volts X 160ah, which is 19.2 Kwh, and the weight was 475lbs. I was hoping for less weight, but the range was more important. In the BMS article, you’ll see the reason. The best way not to ruin your lithium batteries is to not push them at either side of the charge cycle. Don’t try to charge them right to the absolute maximum voltage, and don’t run them down to their lowest voltage. My real range requirements were more like 50 miles, so the extra 15 miles should allow me to stay away from the ends of the voltage range of my batteries. I also read that range anxiety is a common problem for owners of EVs. My thought was the extra range would keep me from going through that most of the time.

Getting the batteries ready for use

After picking up my batteries I set them up for balancing. Even though I was going to use a battery management system (BMS), I still needed to start the cells off balanced. Because of the BMS, I choose to top balance the cells. Top balancing means I get all the cells to the same voltage near the top of their voltage specification. The other choice is bottom balancing, and please read the BMS article if your interested in how that works. To top balance I hooked up half the cells (19) in parallel fashion. I used my buss bars to do this. It is important for the connections not to have any real resistance drop, or the batteries at the end of the circuit will not have the same voltage as the ones closest to the charger. I found this out when I had tied to use some wire for some of the batteries. After connecting them all together I used a variable power supply to set the voltage I wanted the cells to get to. I chose 3.65 volts for my final voltage, which is above the resting voltage of the cells, but below the max charging voltage. It took a couple of days to get the 19 cells up to that voltage. I watched the amps out of the supply to tell when they where close. I stopped the charge when the pack was only taking .15 amps. I then connected up the second group of batteries and repeated the process.

Placing the batteries in the car

I had thought that because the Midget was small, it would make an ideal electric car. It turns out that being that small made it tricky to fit all of my batteries in the car. I wanted to maintain the weight distribution of the car in order to preserve its normal handling characteristics. This meant trying to have as many batteries in the front of the car as the trunk. With li-on cells, you’d prefer to have them mounted in rows because it reduces the complexity of the wiring. Placing a couple of batteries here, and a couple there wasn’t an option. After many hours of studying the front of the car I decided I could fit 18 cells up front, and the rest (20) would go into the trunk. Again, I wanted the midget to look like a midget inside and out. Having batteries in the passenger compartment wasn’t an option for me. To see how the weight distribution worked out, see the article on the MG Midget’s weight. The front would consist of one string of 10 batteries mounted above the motor, and a string of 8 across the front of the car. The other issue I had was that the height of the cells wouldn’t allow them to be mounted vertically in the front of the car. After looking all over the Internet I decided that mounting li-on cells on their side should be Ok. There were people reporting they had done that and had no issues with it. If there are long-term affects, I’ll find out. For now they are working just fine. Li-on cells should be mounted in such away as to limit them from bulging during use. From what I read, they shouldn’t really bulge if you’re using them correctly, but if they get overcharged they might bulge. This can distort the inside of the cells and possibly hurt the cells. For my installation, I strapped 10 cells together for one front pack, and 8 for the other front pack. I choose to use steel strapping bands instead of the plastic ones. I was worried that over time and heat, the plastic bands would creep. I found a complete strapping kit on the Internet for a little over $100. I made the endplates out of a structural plywood. People might think that wood might not be strong enough, but we're interested in bending strength. Bending strength increases with the cube of the thickness. My wood pieces are close to 1/2 inch thick. I think the aluminum endplates are only 1/4 inch thick. I haven't checked on the modulus of wood Vs aluminum, so I don't know for sure if the wood is as strong, but I think it is close.  When it came to mounting the the other 20 in the trunk my plane was to strap two create sets of 10 cells. I made the mistake of making my trunk battery box just a little too small to accommodate the end plates required for strapping the batteries. This forced me to assemble the cells inside the box, and then put spacers in at the ends to provide the compression. I put threaded rod across the length of the box to provide the clamping force. So I ended up squeezing the entire box to get the compression. I don’t recommend this approach. If I have to get the cells out of the battery box, I’ll have to disassemble each cell. I should have put thin plates on the cells and still strapped them together, then used the shims and box to provide proper compression. Assuming that I have to remove the batteries in the future, this is what I’ll do when I replace them.

The rear battery box and front battery trays

For the rear battery box I decided to cut a hole in the trunk and build a box that dropped down to the level of the old gas tank. After driving a Midget for over 30 years, I knew that I never hit my gas tank, so the batteries should be safe at this position. I then had to decide how to build the box itself. Many people build a frame from angle iron, and the cover the frame with aluminum sheeting. This seemed like a lot of work to me. Just getting the frame to be square after welding it seemed hard to do. I kept looking around on the Internet for already made tubs or pans the correct size that I could then drop into the hole in my trunk. I never could find the exact size I wanted, and then I stumbled on the idea of using a pickup truck bed tool box as a starter for my box. They were the correct width, just too long. I figured if I could shorten it by cutting out the middle section it would work great. I found someone selling one for $65, and I bought it. It worked out great. These boxes are made of fairly thin diamond plate aluminum. They cut like butter with a metal skill saw blade. I cut out the middle section and attached the end back on using aluminum angle iron and pop rivets. I sealed all of the openings with caulk, and built in reinforcing ribs into the bottom so it wouldn’t sag with the weight of the batteries in it. The ends of the tool boxes have features to rest on the bed of the pickup truck. I used these features to rest on angle iron I welded onto the frame of the midget where my hole in the trunk was. The battery box rests on this angle iron. The other benefit of using the battery box was I had plenty of aluminum material to use when making mounting features for other components in the front of the car. I built a lid out of acrylic sheet so I could show off my batteries, but still be able to put things in my trunk. I stiffened the acrylic sheet with wood strips. This time I choose wood because if the lid collapsed, there would be no metal to short out my batteries with. The box ends are open, so there is good air flow over the batteries.

 

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