With charger # 202 and 203 slated to ship tomorrow, and still not a lot of hard data to help us figure out how to rebuild a pack as cheaply and long lasting as possible, I wanted to get some answers, and while I would love to be tracing all 120 cells at the same time, the next best thing is to at least know what the individual subpacks are doing.
Yes the best solution is rebuilding the pack with new sticks, and yes if we have no way to determine where the sticks in the pack are in relation to each other or to a really good pack, we are going to have many false rebuilds with old sticks, but if we can start to really quantify the stick level condition under real world level testing, I expect that a pack built from used sticks that were matched properly could lase a lot more than 6 months.
An in car stick level datalogging system could go far to identify which packs are worth fixing and which are not. What is missing id the data, so this is the prototype of something better, as well as being a quick and dirty analysis tool.
I can tell you that there are many many people out there with an Insight or civic that they cannot afford to put a battery into, and cannot afford to get a new pack. They are willing to work hard to fix their old pack and just need the guidance on how to best determine the bad sticks, and to have a good source for sticks with known characteristics that will work within the rest of their pack sticks window.
The meters will be replaced with stick level isolated monitors that report to a central master that will examine the relationship of the 20 sticks under real world in car conditions, similar to a cell level BMS system, and I expect to learn a lot on the way to my A123 lithium BMS which will be the spinoff of the work.
As I mentioned in another post, After we build up the rest of the chargers (250 total), I will take a break from charger construction, and get going on the other hot projects that have been neglected.
2 days to design and build this fixture, and I can start learning with it. Not elegant, but fast and useful.
Always looking for a better window into what is happening in the batteries.
I have 2 old packs that will soon move to the tester as the sticks get replaced with the new MaxIMA sticks that I just ran two weeks of test on. A one page photo showing some full discharges at ~ 75A are compiled here:
At the bottom are discharge traces from two of the better sticks pulled from my first cell level test of my 170K original pack.
I have 60 or more traces of the 20 MaxIMA sticks, but they all are quite similar as you can see in the photo
Lots of info there, but you need to zoom in. The key data is that the MaxIma sticks all put out 6.15 to 6.3AH under this 5-6 minute complete draining of the stick at > 75A the whole time. The two stock sticks could not make it to 5AH under the same conditions, while showing similar temperature rises. The new sticks had better IR (voltage under discharge was higher).
On other traces of not so good old sticks, the higher IR caused the temp to rise to over 130F after only 2- 3AH.
So much to learn, so little time, but it is interesting and fun.
I did some interrupted discharges to let the process proceed with less temperature rise, and always dropped out at just about the same AH, so the delay does not change much.
The disabled discharge test which is shown in the new photo here:
Shows how the one stick was droping out much faster than the rest, and the only way to get below that point it to let it reverse, and see where the rest of the pack is.