Get a precision voltmeter (mine goes out to four decimal places), and measure each cell in the brick after you charge it. Connect one meter lead to one brick side plate and probe each of the 69 cells directly by the fuse wire on the other side with the remaining lead. Immediately compare that reading to the overall brick voltage (between the side plates). With good cells, all voltages will be the same, a bad cell will read lower, as it is absorbing the current thru the tiny fuse wire. You can expect to have numerous ones that are drawing it down, so start by removing the fuse at the lowest voltage cell and keep testing/cutting until you can keep a charge. I have seen some bad sheets that you have to cut the fuses on over 20 cells, and in that case you will need a new sheet.