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.Add a comment
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.
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.
Instrument Cluster Spoofing
Because I plan to make the 320i a restored car as much as possible I am not planning to add any electronic gauges or meters, at least none that are usually visible in the dashboard. My plan is to use the analog gauges in the instrument cluster like the fuel gauge, tachometer and temperature gauge to give information like battery capacity, voltage, current and battery temperature. The tachometer will also be used to show the motor speed. All of the signals that drive these analog gauges will have be generated by some other circuit or processor that gets the battery information from other sensors. My first test of the instrument cluster was the fuel gauge, because I had access to the input wires from the fuel level sensor. Those wires were underneath the area I cut out of the back seat for the battery boxes. Many old cars used a bi-metallic strip that the fuel sensor would regulate current through, based on the level of the fuel. The heating of the bi-metallic strip would move the gauge. That type of fuel gauge is very slow and very non-repeatable. To my surprise I found the fuel gauge in the 320i to not be a bi-metallic but rather a true galvanometric gauge. I found it only took 10mA at 400mV to get a full displacement of the gauge. Setting up a MOSFET with a voltage divider circuit I was able to drive the gauge from 0 to full by varying the duty cycle of a PWM gate signal into the MOSFET. I used a Microchip PIC processor to generate the PWM signal at 1k Hz. To access the other gauges I needed to remove the instrument cluster. It took a couple of tries but I finally was able to remove the complete instrument cluster from the dashboard. I took note of all the wire colors and positions on the connectors because I have a wiring diagram for the 320i that has the wire color coding. I removed the tachometer first. In BMWs the tachometer was driven by the coil signal to the distributor cap. I did not want to make a high voltage signal to drive the tach so I took the tachometer circuit apart. Again to my surprise the circuit driving the tachometer was more advanced. There was an integrated circuit called a "pulse shaper for revolution counters". It is a frequency to current converter. That meant this gauge is also a galvanometric gauge and it should take a PMW signal to move it. Connecting directly to the gauge, I was able to move the tachometer meter over the full range by varying the duty cycle of a PWM signal, using the MOSFET/Microchip PIC driving circuit and a voltage divider to give 5V. I found that if I took the PMW signal directly from the PIC I could also move the tachometer, because the PIC is running at 5V.
The temperature gauge proved to be the most difficult to spoof. The gauge is a three terminal device, where one connection was ground, one was +12V and one was the temperature sensor in the engine. The schematic for the temperature sensor circuit showed the other side of the temp sensor was grounded. I found that by using resistors from 22 to 100 ohms I could get the temperature gauge to move the full range. Because this gauge was also galvanometric, the resistors were varying the amount of current to move the gauge. Programming a resistance is possible but I wanted a simpler method and one that would be continuously variable. I found that if I varied the voltage on +12V input side of the gauge I could move the gauge. The gauge moved from low to high by adjusting the voltage from 0.6 to 3.8V with a 10 ohm resistor on the sensor leg. The voltage adjustment changed the current from 6ma to 30ma. By controlling the +12V voltage with PMW on the MOSFET I could control the gauge movement.
Having all the gauges move by varying the duty cycle of a PWM signal is a great result as the GEVCU or the BMS could be programmed to have these gauges display battery info, once the movement vs PWM duty cycle is calibrated. The 320i instrument cluster has one more gauge. In the bottom center of the tachometer there is a 4-digit red LED display which was a clock. I plan to replace this 4-digit display with another 4-digit display that is the same size that will be programmable through a serial interface. The display will be programmed to show the time as well as all the battery info - the only visible digital readout. I plan to use some other digital gauges, but those will be hidden from normal view. For instance I plan to put a JLD 404 amp-hour meter in the area behind where the ashtray is on the center console. With the ashtray closed, no meter visible.
Video of the instrument spoofing is here.
Images of the calibration curves for the PWM are here.Add a comment
Battery Box Designs
I made the most intrusive modification of the 320i yet - cutting big rectangular holes in the rear seat platform to make room for battery boxes. The old duel gas tanks were located under the rear seat and the area left after their removal is an ideal place for battery placement. It is low and near the rear axle. I cut two 9.5"x20" holes in each side of the rear seat platform that will make room for 24 batteries on each side. Cutting the metal proved to be challenging. I used a saber saw that has a 1/4" blade. It is very controllable and precise, but the blades for cutting this thickness of metal just do not last. I think I went through 5 or 6 blades cutting the two areas. I also used a saws-all that I have and that cut through the metal like butter, but it was very hard to make a precise cut. I wanted to make the rectangular holes for the boxes to be cut precisely so I would have less material to fill in later. The joint between the boxes and the rear seat platform will have to be sealed on the inside. I also had to be careful with the cutting because the rear brake lines run around the inside of the gas tank space. Unfortunately the batteries are so tall that the battery box extends 1 inch in the front and 4 inches in the back, because the rear seat platform slopes to the back of the car. The battery box has to stick out of the rear seat platform because the bottom of the boxes cannot extend beyond the level of the rear cross-member. The boxes will have to be supported below and I am thinking of putting a thick aluminum plate under each box to protect it. The rear seat cushion will have to be modified to fit over the battery boxes. From the thickness of the seat cushion it will be possible to cut the springs out of the cushion and the seat should fit over the battery boxes with that modification.
I also made progress on the drive train. As noted in my last blog, when spinning the rear wheels there was considerable vibration in the drive train. My friend Tim Catellier emailed me and said from the video of the driveshaft installation he could see that I installed the carrier bearing upside down. Tim has converted a BMW Z3 (http://evz3.blogspot.com/) and knows all about this two piece BMW driveshaft. I flipped the carrier bearing over and voila the drive train has no vibration!! I tested up to 65MPH. The only detectable vibration was right around 25MPH. It is possible the car always had that vibration - it would not be noticed as that is right around the shift point of 1st gear. I spun the wheels up in 4th gear. The video of this progress can be seen here.
Images of the drawings for the battery boxes are here.Add a comment