The charger is a Delta-Q IC650 with a lithium-ion profile. This company is strictly an OEM offering no consumer support and will respond only to manufacturers incorporating their products. I have "dealt" with Delta-Q chargers for 16 years and know this the hard way, given the hulks of four worn-out Delta-Q chargers littering my garage, for a low-speed vehicle.
Needless to say the next time one dies - and it will - it'll be replaced with a marine-market multi-battery charger like a Guest. Unfortunately that's not an option for our mowers.
In both of your cases, does the LED bar display flash on the mower display panel after plugging in everything? If not, it is not accepting the charge. Also, be doubly-certain the parking brake is engaged during charging.
In
@IBM1401 's case (BTW, I cut my programming teeth on an IBM 1401!), did you leave the charger connected and powered all winter? The '8' on startup is nothing more than a LED check, and the flashing lightning bolt is during boot-up; both are normal "health checks". What is important is the green LED on the charger display after boot-up is done, if it pulses slowly it indicates charging in progress. Conversely, no pulsing = no charging.
Is it possible to get a loaner charger to diagnose? I suspect not, Cub Cadet is
very stingy with spares for the 42E, as discovered in diagnosing an overheating deck motor.