BMS Respin and Battery Box Mounting
I thought I was done designing the Battery Monitoring System or BMS board. The last set of boards had a flaw in the circuit for the thermistor measurement, but it was not a fatal flaw and only slightly affected the performance of the measurement. I discovered the fatal problem when I went to mount the board, with the CINCH header into the enclosure. To both my surprise and disappointment the header did not fit the enclosure. Upon checking the MODICE website I discovered I had designed the BMS board with a header for the Mini size enclosure, which has the 18 pins in the center of the header. The problem is the Mini enclosure does not accept a 4" pcb. The only enclosure that accepts a 4" pcb is the Small Enclosure or SE. That is the same enclosure that is used for the GEVCU that has the CINCH enclosure. So I ordered a new set of headers, with 18 pins that are for the SE enclosure. When I received them, to my surprise, the 18 pins on the header were not in the middle of the header, but displaced to the left side. After reviewing the design drawings for the header I realized that CINCH does that so they can have a header for 18 pins, 30 pins, or 48 pins. Unfortunately the 18 pin header with the displaced position of the pins will not work with the design of the BMS pcb because the holes for the header are in the middle of the board. I had to order yet another set of pcbs to fix this issue. I don't think I mentioned it before but I order the pcbs from a Chinese circuit board fabrication house called ITEAD. What they charge for 10 boards is less than half of what domestic fabs charge. The only downside is that it can sometimes take up to 2 weeks to get the boards. The other downside is that if there is a fatal flaw in the circuit board design then you have 10 pcbs that serve no other function, except that of a drink coaster. Since I had to order new boards I also fixed the issue with the thermistor Wheatstone bridge circuit. I received the boards in just over a week and built up a board and everything is working. Collin Kidder has worked on some code to read the board and a flux gate sensor, a CAB300 that will be used in conjunction with the BMS to make a State of Charge or SOC measurement of the battery pack.
I also worked the mounting system for the battery box that goes in the trunk. That box is intended to mount up against the rear seat, below the rear window deck. When the box is in that position it would be impossible to put any batteries in the box or make any electrical connections or measurements. My plan was to use some type of roller system that would allow the box to move back, towards the rear of the car, so batteries could be loaded and then moved back up against the rear seat for use. The problem is that the box will weigh over 250lbs with all the batteries in it. What I found when searching for some roller system that could handle that kind of weight is that they are very expensive. Doing some more research I found a system that does not use roller bearings but rather a linear plastic T-slot and grove system. On the bottom of the box I attached some bearing material called T-Slot UHMW linear bearing. Here are some pictures of the material. That material is used with an extruded aluminum strut that is made by a company called 80/20. Below is a link to their web site.
http://www.8020.net/
The aluminum strut has a grove in one side that mates with the raised T-slot in the bearing marterial. The T-slot material is fairly slippery so the 80/20 strut slides easily on it. The struts and bearing are rated at 100lbs so I used four on the box. I assembled everything and attached the struts to some angle iron I had bolted to the truck bottom. The four 80/20 struts span the cavity for the spare tire so with them in place, there will be no way to have a spare tire. I think for my first conversion that a flat tire is going to be the least of my worries. That spare tire well however, will make an ideal place to mount the Brusa charger and the DC/DC converter and maybe even the 12V battery.
A video of the BMS respin and the battery box mounting system can be found here.
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