ok i can answer my own question now.
There are two versions of BMS boards made by TTi in the ryobi OP4026 40V packs.
model # | board # | date |
13086012 | 280146 | jan 6, 2011 |
13086045 | 280296 | dec 17, 2013 |
i had guessed just opposite for the two chips on the board: the square quad pack U3 is the battery monitoring chip by O2 Micro, OZ8940ATN; and the rectangular 28-pin SSOP designated U2 is a PIC16F1786-1/SS microcontroller.
i had found a 40V and a 18V pack in the recycle bin at HD. The 40V is completely dead, the cells have been totally drained. The other pack indicates 9V when reading from the outside terminals, but all the cells read 4.0V when opened up.
Depending upon the model, there are either one or two big FETs (IRF1404Z) on a heatsink on the bms board that switch the low side of the pack closed/open that functions like an ON/OFF switch. Something must be causing this to stick ON and draining the packs—there are a huge number of dead packs for sale on ebay, plus lots of posts from folks with problem packs. These cells should last 10 years—something is killing them early…
These packs, like nearly every other power tool brand, are built under patent 7554290, invented by Todd Johnson et al, and assigned to Milwaukee Electric Tool Corp. The BMS has no shutoff or low-power mode, and it gets it’s power from the pack—so it is constantly draining the pack. Too bad we have to pay these guys royalties for a POS system, there are better solutions out there e.g. Texas Instruments