- Details
More Battery Box Work
I finished painting all the battery boxes with the bed liner material. I had to touch up some of the sides of the rear seat boxes. Once all the painting was done I could start on the next phase of battery box preparation - the lining of the insides of the box with rubber sheeting. This is done to provide both a thermal as well as a electrical insulator layer. The material I chose was a Buna-n rubber, medium strength, 1/4" thick. The rubber comes in different hardness, I chose the 60A which is medium hard. That seemed like the best compromise for stiffness and elasticity. It is available in 12" wide sheets in many lengths, which is perfect because all my battery boxes are 12" tall. The material can be found from many sources, I bought some from both McMaster Carr and Ebay. The price on Ebay was 20 to 30% less than McMaster Carr, but there is a limited selection that had an adhesive back. I am not sure how important that is, except for assembly purposes (it is possible to apply your own adhesive). The battery boxes were designed to use this 1/4" rubber sheeting. I first installed the rubber sheets in one of the rear seat boxes. The material can be cut with a box cutter, but it is not easy - pretty tough material. After I got the material on both of the long sides I decided to test the fit of the batteries. Until now I could not do that measurement. I could slide two batteries side-by-side very tightly in the middle of the box but, unfortunately, I found I could not slide two batteries at either end. It looked like there was only as small offset that was preventing them from being inserted (less than a 1/16"!). Probably the box has a very small dimension variation at the corners because of the fabrication technique. This is one of the problems with battery boxes. The batteries have a fixed size, so if the box is too small the batteries cannot be inserted. Also the thickness on the rubber sheet can vary +/- 0.031". Fortunately with the rubber sheeting there is an easy solution. The material is available in 3/16" thickness. I ordered one sheet to test it out and it worked! Batteries can be inserted at both ends of the box. The still are very tight, but that is what is desired. Another part of the battery box preparation is to install a silicon heating pad in the bottom of all the boxes. The heating pad is designed to heat the batteries when their temperature drops below 0C because they will not take a charge at that temperature. The pad does not get very hot and is just designed to warm the batteries. I found a Chinese Ebay vendor that offers custom size heating pads. They not only can customize the pad size, but also the wattage and voltage of the pad. I chose 400W and 400V because I plan to power the pads with the battery pack voltage. This seemed the most simple electrical solution, with the wattage of the pads they will only draw an amp of current. I plan to use 22AGW wire so the wire will act as a fuse if there is a short. I also plan to have a high voltage 5A fuse in circuit as a failsafe. Each of the pads will have a thermistor - I don't plan to control the temperature but rather just have a upper limit for control. All the pads will be wired in parallel so they see the same voltage. The pads will sit on some of the rubber sheet material on the bottom of the box. This is done to prevent the heat from the pad being conducted away by the battery box. With all the sides of my boxes being insulated there is no reason to heat the box. The heating will be directly into the battery cases.
Other work on the interior of the car has progressed. I was able to remove all the underlayment material and install new underlayment. The next step is to install the new carpeting. That should happen soon, but first I need to finish running all the wiring to the engine compartment. Now I have heaters in all the boxes there are another 4 cables to run. I also want to get the rear seat boxes fully installed before the carpeting goes in.
No news on the Siemens spline adapter from EV West. Should arrive any day.
video of all this fun can be found here.
- Details
Interior Blues and Painting Fun
In the last video before EVCCON I showed a new carpet that I received for the interior of the 320i. I finally got around to ripping out the old carpeting. From the first day I got the car over a year ago it always had a musty smell to the interior. I assumed that was due to its age but when I was routing battery cables and had to pull up part of the carpeting and I found the padding underneath was wet. I assumed the carpet was moldy so I ordered the new carpeting. The old carpet was really in good shape, in fact I am sure it was replaced at least once and it had no worn spots on it. It took a lot of work to get the old carpet out, the e-brake handle had to be removed, the whole console removed and gas pedal. The carpet was also glued in some places so it hard to removed. Once I got the carpet out I found that the carpet was not really that moldy, but all the padding on both the driver and passenger sides was totally soaked. That really is amazing since the car has been in the garage since last January and at least in the winter the garage is very dry. What is interesting is the padding was not the typical jute padding usually used with carpeting but rather an urethane foam padding. From the shape of the it definitely was OEM, because it was cut to fit the floorboards and the holes for the gear shift and e-brake. The padding really was not that moldy either. Underneath the padding was what looked like a thermal insulation underlayment. That was also totally soaked and when I tried to remove it , the layer just fell apart. And what a terrible smell! I guess the fibers used for the insulation must have rotted and that is what was producing the smell in the car. The other fun part of this insulation is that it was really well glued to the floor board. I had to use a paint scraper to scrape it off and for parts that would not scrape I used paint thinner to dissolve the glue. The amazing part of all this is that the metal under all this wet crap was not even slightly rusted. A tribute to the BMW undercoating. I found on a website for company called TP Tools that they make a similar insulation underlayment so I ordered enough for the all the floorboards. I also had received insulation and padding with the new carpet. Comparing the old carpet to the new one shows there are a lot of holes to cut in the new carpet. I am sure that the old carpet was OEM because the holes were all die cut, at least for the gear shift and e-brake. It is going to be quite a task to get all those holes cut in the correct places.
Other work I have been doing is getting the battery boxes prepared to use. The fabrication was just the beginning. All the boxes need to be painted. I am also installing rubber sheeting on the inside of the boxes for thermal and electrical isolation. The best "paint" used by EVTV is this material used for lining pick-up truck beds. It is very durable and is black. To use the bed-liner material the boxes have to be primed with a special primer that etches the aluminum for a stronger bond. Fortunately I have a lot of experience painting and all the tools so it was not too difficult to get all the boxes primed with the special primer. The bed liner material is like undercoating material. The kit I bought came with a spray gun that screwed right to the bottle of material. The kit came with 4 quart bottles that have to be mixed individually with a catalyst material. That is good and bad. Good because the material does not have very long pot time once it is mixed. But if you have a big job you have to stop and mix another bottle. I only had to use two bottles to coat all three of the boxes. The fourth box, the one that goes in the engine compartment I decided to paint the color of the car. That way it will blend in. The car came with some touch-up paint when I bought it. The touch-up paint was premixed and in spray cans. Unfortunately the cans did not spray well. I got enough paint on the box so it look painted, but I ordered some more touch up paint that I could shoot with a real spray gun. I also might coat the lid with some clear coat to give it the same luster as the car paint.
And lastly I removed the Siemens motor once again. This time it was to remove the Rebirth spline adapter so I could send it to EV West. They have a solution for the Siemens spline and will be sending that to me shortly.
A video of all this fun can be found here.
- Details
EVCCON 2014
I attended the fourth annual Electric Vehicle Conversion Conference that was held in Cape Girardeau MO. Earlier this year I had planned on taking my BMW 320i to the 2014 EVCCON. Unfortunately it just took too much time and it could not be completed in time. The conference was last week, from August 11 - 17th. This was the third year for me. It was great to see all my EV enthusiast friends that I have met in previous years. Part of the conference is a car show and many of the cars from last year were there along with a few new notable ones. I really look forward to getting my car there next year. Another part of the conference is that there are presentations by people on various subjects about EVs. I presented a talk on the Battery Monitoring System or BMS that I have developed with the Sendyne module. I have shown this design in earlier videos, but his is the first public presentation I have made on the subject.
- Details
Battery Boxes Received
I received the aluminum battery boxes I had fabricated by New Hampshire Precision Metal Fabricators in Londonderry, NH, who are about 20 miles from my house. They did a fantastic job on the fit and finish of the boxes. There is one box for the front of the car, two for the rear seat area and one for the trunk. They had given me one of the rear seat boxes before it was welded to check the size of the holes I had cut in the rear seat deck. Those boxes were 1/4" wider than I had originally modeled. The reason for the discrepancy is that I plan to line the inside of the boxes with neoprene rubber sheeting for both electrical and thermal isolation. Here in the northland you need to worry more about keeping the batteries warm than cool. I did not catch outside dimension had grown because of the sheeting on the inside of the box. My friend that did the CAD drawings forgot to inform me of that change also. However, the increased size of the box was not a big deal. I just cut more metal out of the rear seat deck. The only tough spot was the back right corner. The box just fits before a support rail. On the one side the weld at the bottom of the rail was too big to get the box in so I had to use a cut-off saw to cut away part of the weld. That was almost the 1/8" that kills battery box designs. The second rear seat box only took about 30 min to get the hole cut to the correct size, because I had the first one to measure from. Another issue with a battery box was with the box for the front of the engine compartment. Brackets were designed to attach the box on the inside of the area where the radiator once resided. I has assumed the radiator was in the center of the engine compartment but after I got the box I found the area was not in the center. It is displaced by over two inches to the left, looking out from the engine compartment. So one bracket fit because it was 1 inch outside, but the other one did not because the 1 inch overlap interfered with the rest of the metal around the radiator area. I had to have the bracket removed and a new one welded in the correct place. Fortunately NHPMF did that rework for no charge.
I also did some more battery discharging using another grid tie power inverter. I got a second one on Ebay for very little money. It is 1200W which is half the wattage of the first one I got that had failed. It can only can be used up to 50V input, but that is good for smaller groups of batteries. The inverter work without any issue and I discharged the rest of the battery pack. Now I have to build the bottom balancing systems. I have a design that uses a MOSFET to vary the load on the battery, so that the battery voltage can be precisely tuned.
A video of the battery boxes can be found here.
- Details
Battery Conditioning
The 120 Lithium-ion iron phosphate batteries that I am going to use need to be conditioned first - a process known as bottom balancing. During this process the batteries are all discharged to exactly the same open circuit voltage. Having the batteries all at the same discharge voltage is a preventative measure to protect the batteries in case of un-intentioned complete discharge when the batteries are in the car. If all the batteries are not at the same bottom level during a complete discharge then the lowest battery will be drained first, probably irreparably damaging the battery. When all the batteries discharge at the same rate there is a very good chance to recover all the batteries. Part of the process for new batteries to condition them for bottom balancing is to fullly charge them first. Once they are fully charged then all that energy in the battery has to be removed. Usually that is done by running the car or some accessory in the car like the heater. But my batteries are not in the car yet. I could use some large wattage resistors. I have a set I plan to use to do the final voltage tweaking. But using them to dump all that energy seems wasteful to me. So I came up with the idea to use a grid tie power inverter. These modules are used to convert PV solar DC energy into 120VAC that is coupled into a house power system. The inverters come in all shapes and sizes of wattage. The kind of inverter that I would use if I ever put solar panels up costs about $2 a watt. I found on Ebay there are these Chinese built inverters that cost $0.20 a watt. Of course these inverters are not UL certified but for my purposes that does not really matter. I don't plan to use these like they would be used on a PV solar system. The inverter I got is 2500 watts and can accept up to 88V DC. I had to change the strapping on the battery boxes to reduce it to 80 volts. The first set of batteries I discharged into the inverter worked great. The inverter drew about 29Amp from the battery pack and produce near 2.5KW of AC voltage. I have a whole house power meter and I could see the wattage reduction of my power usage when the batteries were being discharged. I connected a second set of batteries, fully charged them and then started the discharge process with the inverter. Nearly the exact same DC starting voltage. The inverter started up fine and I could see the power reduction on the whole house meter. The central air conditioning was running at the time so the house was pulling 8KW. The discharge process was working fine for about 2 minutes. I was video recording the process for my video record and suddenly there was a large electrical discharge sound. Now that is kind of scary because I had over 4KW of battery capacity connected to the inverter and if all the energy was released quickly that would be dangerous (the battery pack was protected by a fuse). The sound came from the inverter and it was obvious that was the problem because it had stopped converting power. I disconnected everything and took the inverter apart. This inverter is basically two 1200W inverters in one package, one board on top and one on the bottom. I found on the top board one of the power DC mosfets was totally burned. I checked the other mosfets and found about half of them were shorted. The DC fuses on both circuit boards were blown, but the AC fuses were still okay. From looking at the inverter circuitry I think this is a very marginal design, for instance the DC fuses were not rated for the voltage or current being used. People are probably using these cheap inverters to convert energy from their solar cells and putting their systems in danger of catching on fire if these inverters have a propensity to fail. After I experienced this failure I found on YouTube many videos for repairing these inverters. Along with the inverters on EBay there are also mosfet repair kits, so I think this is a common problem. I probably will repair this inverter because I still have a lot of batteries to condition. I don't know why the inverter failed, it worked great the first time. The main difference was that the central air conditioning was running when the inverter failed. These inverters work by phase matching the 60Hz 120VAC from the house. It is possible the reactive load of the air conditioner skewed the AC voltage so the inverter freaked out and drew too much DC current. I did notice a spike in the DC current on the battery monitor I was running right before the inverter failed, but who knows, could be anything with a marginal design.
A video of the discharge process can be found here. The video also shows the battery cable layout in the car.