FRC batteries wont charge

[Mr V]

If you've let them discharge to far then some chargers won't charge them as they don't see the extremely low voltage as a valid battery and won't turn on. If you can find an old school charger you may be able to get them to charge. If you don't have access to a old school charger you can hook the battery up in parallel and charge them both for a short time. That should trick the charger to turning on and get them to a high enough voltage to make them accept a charge. However you have comprimised the batteries capacity and it may not have the power and length of use you desire. They may be suitable for another year as a practice only battery though.