Re: FRC batteries wont charge
 

Also check your cable polarities! Apparently some APP battery connectors were wired with reverse colors this year. Your charger may be smartly preventing charging

Re: FRC batteries wont charge
 

Step 1: See if the batteries can be recovered to some level of usefulness. Similar to suggestions above, I would try an old-fashioned current charger (not a "smart charger", as sometimes they are too smart to work). The old charger may bring the batteries up to a level where the smart charger can work properly and finish the job.

Step 2: TEST the batteries to see if they are good enough for competition, or just practice, or should be recycled. Our team uses a Battery Beak. If the battery is good enough to keep, label it so everyone knows whether to use it in competition or not.

Step 3: Track of all of your batteries, retest periodically, record results. Retest means they need to be charged from time to time, whether they've been used or not. Re-evaluate batteries after testing, and disposition them accordingly. It's miserable to perform poorly in a match because the battery isn't as good as you assumed it was (we learned the hard way:( ).

Re: FRC batteries wont charge
 

Modern chargers look for a back EMF to ensure the charging clips are not meandering about energized. More of a safety issue really. Safety note: when paralleling cells with significant terminal voltage differences, a LARGE amount of current will flow for a short time, and sparks will issue when the last connection is made. If you should pursue this action, make the last connection on the good battery. Batteries evolve hydrogen when discharged, some residual gasses may be present. (do this outside, with a face shield) Hope this helps.