[Al Skierkiewicz]

Jean,
These batteries are made up of 6, 2 volt cells in series. A bad cell will be indicated by a two volt drop in terminal voltage. If the cell is truly dead, the battery will read 10 volts or slightly higher when removed from the charger but not higher than 12 volts. Each cell is made up of several plates, interleaved positive and negative, with a glass mat separating the plates. Should the electrical connection to any of the plates be broken then the cell will have reduced capacity. If any of the plates should be touching then the cell will self discharge causing high internal heat. There are two tests which will determine which defect is causing the problem.
Charge the battery and remove from the charger. Attach a voltmeter and monitor the terminal voltage. A good battery will show slightly higher than 12 volts for several minutes and then fall to 12 volts. If it begins to fall below 12 volts in the first 30-60 minutes and then remains at 10 volts (or a multiple of 2 volts), you have a battery with damaged plates touching in an individual cell. One area of the case should be warmer than the rest of the case. If the battery remains at 12 volts, then you must load test it. We use the Mountain Radio CBA-II. With this computer based tester, you can draw a steady 7 amps and chart the terminal voltage and it will calculate the amp-hour rating. A cell with a few broken plates will be indicated by an drop in terminal voltage of 2 volts before the battery becomes discharged. The following picture compares three bad batteries with a good battery using the CBA. You will see in this graph the normal discharge curve in black, two highly reduced capacity cells in blue, a single reduced capacity cell in green and a internittant cell in red. The "green" battery was reported to be suspect during practice at home where the robot was run for 15 minutes at a time. The "red" and "blue" batteries were also discoveered during practice but the intermittant nature of the red battery confused everyone until this test. Without the constant discharge, all of the batteries showed as 12 volts at the OI, when the robot was not moving except the "red" which showed a varying voltage.
It is likely, all of you have batteries that have one or more cells that are diminished capacity without knowing it. This is particularly true of batteries that are from 2006 or earlier due to the number of charge/discharge cycles they have been put through.