On Monday, May 6, 2013 5:14:28 PM UTC-5, Jim wrote:
Since you have all the batteries in series with the load the voltage drop across the bad battery it appears like the bad battery is being reverse-biased by the good ones. You would see the same kind of result if you replaced the bad battery with a resistor and measured the voltage across it. The bad battery obviously has what is called "High internal resistance". -PC

Yep. And the guy at All Pack gave the battery the highly scientific "shake test". Apparently, the innerds dried up and came apart. Sounded like shaking a box of rocks. 5 years of service from the battery, and $23 to replace it in the pack. Will be good to be able to secure the battery in the intended box again rather than having a smaller one out on the shelf behind me.