Hi all,
I've got 4 dewalt 36v packs, more precisely 4 sets of tabbed batteries from current-pille. I told the seller I want the cells and won't be using it's own BMS (which may be defective anyway), I just would like to have as many working cells as possible. He had sent me only the bare cells cross tabbed as in the pack, with no bms and plastic case. It was lighter and easier to ship, and he checked the cell voltages. Only one tab was broken, and every cell was between 3.2-3.4v. Seems like I have 40 working cells out of 40. Without case it's harder to say what year the pack is from, but from serial numbers it seems they are 2006/7 and 2006/9, so probably more than 3 years old.
I wanted to test all the cells for capacity and match them to make a 18s2p battery pack, but I ended it using a couple of cycles(4-5) wired 20s2p in the initial tabbed form.
Bulk charged almost full, then topped every single cell to 3.6v. I had went down 100% DOD 1 or 2 times. I think it was 100% DOD, because voltage started dropping whole volts in 5-10 seconds. Seconds after stopped using it, measured every cell, and lowest was around 2.4-2.6v. It was probably less then 2.0v under load for a couple of seconds, maybe I have stressed them a little, but after resting they came up to 2.8x volts.
Later I proceeded to capacity test and match the cells.
I cut the cells into single cells, keeping the tabs and bending them. Charged every cell to 3.6v in parallel. Almost all of the cells remained at >3.5v after 12 hours, 2-3 sat around 3.4x volts.
Then wired them series, and put under continuous load. When the weaker cells were going downto 2.8v, I have stopped the process, and started to capacity test every cell to 2.0v. The RC charger could do only 1A, so I have wired a car headlight in parallel adding another 7+ amps. So the end discharge was done at 8.x amps. So I had only relative capacity measures and those are not very precise, only within maybe 5% error.
Later I have precisely capacity tested the best cell and the 4 lowest ones with the RC charger @1A.
The results show the best delivered 2257mAh, the 4 weakest ones yielding 1824mAh, 1949mAh, 2037mAh, 2091mAh.
I have calculated later every cell capacity interpolating between these numbers according to how much they resisted at discharge end to 8.x A load.
The full capacity chart would be something like this:
The average for 40 cells turns out to be 2146mAh, the best 36 cells from 40 are between 2099mAh and 2257mAh averaging 2165mAh.
- discharge cart.jpg (109.55 KiB) Viewed 715 times
Maybe with a higher discharge the figures would look slightly better.
Had no tool to measure IR, but they seem normal, under ~10A load they sag 0.11-0.14v
After all I am very happy with the cells, I was aware of slightly weaker performance than a brand new a123 cell. At this price this is great deal.
My bike goes wild and the battery sag is very low, even in cold weather.
I am not using a BMS for the moment, but it is very advisable if I want to protect my investment.
As I have already reported, Philipp is indeed a very nice guy, responded to all of my questions, was helpful, and did everything as we have agreed.
Time for me to order some more, if he still has them...
I then proceeded to build the 18s2p pack.
- loose a123 cells with soldered tabs
- a123 pack build 001b.jpg (75.77 KiB) Viewed 727 times
- a123 pack build 002b.jpg (82.2 KiB) Viewed 727 times
Unfortunately when I was taping around with duct tape, I have felt an istant stinger. I thought for a moment that I was electrocuted by 60volts. I then quickly realized it was a short caused by my metallic watch that heated up in a moment. My luck was the cells were 100% depleted to 2.0v, but still had plenty of power. It is probable the short was caused between only a couple of cells, the hole pack would have welded my watch.
The injury was still pretty bad.
My KFA
- moments after the incident
- a123 pack build 005b.jpg (24.9 KiB) Viewed 727 times
- the next day, and later became even a little worse
- a123 pack build 010b.jpg (18.98 KiB) Viewed 727 times
Watch out for your watch while handling these goodies!
( if someone has a raw cell lying around... I'm taking it)
.
now I need to finish the other 3 packs, and wire a few goodies, motorola dc/dc (cellphone charger), main contactor, charge plugs, put the new front sprocket, and my bike will be ready again for some real testing. 
at the end of the test, all cells were at exactly 3.31v 
I like these cells
