That sucks.....
I grid charge/balance my pack 3 times a year for preventative maintenance to avoid being in a situation like that. It seems with all the cells in series they charge and drain at different rates and when they get soo far off it causes problems.
Supplying the constant charge at a low enough amperage it prevents over heating of charged cells, yet allows enough to charge the weaker cells. THis is why you need to watch the charger and observe the voltage. When the voltage stops moving at 15 minute checks over an hour you are pretty sure its "balanced"
While you are at it, what is the starter battery doing? Is it fairly new? DOes your radio forget the presets? Many insight owners notice a drop in mpg when the starter battery starts to go. Then one dayyour key fob doesnt work,you cant start your car, but the alarm goes off and all the idiot lights blink on the dash.