Wednesday, May 11, 2011

Back on the road again!

Last night I finished buttoning up both packs after soldering in all the connection wires between the groups of cells and adding in a fuse holder to each pack. I also had to create new carry straps. My first design was a fail as it had metal catches and was designed to be used for a load tie-down. Unfortunately, there is not enough room between the batteries for this to work properly and I nearly had a situation where I get to batteries in the car and could not remove them. After some thought, I went back to the original Twike design which is a knotted loop of rope. I did do a wrap of duct tape on top and bottom to prevent it from dislodging. That worked great. I charged the batteries for the first time and did some tentative drives around the neighborhood. I don't think the cells are balanced yet as they are in various states of discharge when I joined them. The car will do a balance charge when doing a normal charge (jog charge is at a faster rate and avoid the balance phase). I put on 8 miles in the evening giving my kids rides. If it all goes well, I will put in the back shelf cover and try to drive it to work tomorrow (about 16 miles round-trip). If that all goes well, I will attempt to take it to the OEVA meeting in Portland tomorrow night. Only about 9 miles or so but I do have a elevation climb of about 1200 feet (at night with the lights and wipers on :-) Wish me luck.

Wednesday, April 20, 2011

Pack Assembly

I've spent two sessions now with the battery tab welder. The first was to create 5 cell groups. I then had to take the 5 cell groups and bend the tabs so the cells were end to end. I used a PVC pipe from Home Depot which was a slight oversize on the cells to assist in aligning them when bending the tabs. This 5 cell group was then wrapped in a tough 5 mil heat shrink to form a "stick". I assembled all these sticks (112 of them) into 4 stick groups. Each of the 4 stick groups has two tabs welded to the bottom, and 3 on the top. It was difficult to find, but I finally found a place to get 10 mil nickel tabs (and did not force me to order 5kg worth). Previously, I could only get 5 mil tabs which I think are a bit small for the top and bottom tabs as they see much harsher physical stresses. I checked each of the groups with a voltmeter and all were within 0.2 volts of each other. This means I have no dead cells (opens or shorts). I did build 2 extra 4 stick groups in case I need to swap them out and I will keep them as a future spare parts. What remains is to install the 2 new Battery Management Boards, install the removable fuse, replace the cover (may think about replacing the fans as some point as they do not move much air) and test the packs in the Twike. I hope to be back on the road soon (and just in time as gas heads for $4/gallon). Expected range will be 30 to 40 miles per charge. I might think about building a mount for my third pack as well as the car supports it (program BMS to bat3 and plug it in). I'm concerned that packs of mixed capacity might cause the charger some problems but maybe not as it can open the charge relay on a per pack basis. I'll leave that to a future project.