Sudden battery failure to accept charge

[james01]

My 2018 Carrera Crossfire has covered 5,000 miles. It has 2 batteries used in rotation, one to use & one to charge. All was well until suddenly, one of the batteries refuses to take a charge - the Green (=fully charged) light comes on almost as soon as the charger is connected even though the battery is down to 10%. The other battery is performing and charging normally, so the charger must be OK. The battery is 4 years old, but was showing no signs of deterioration. Is it common for batteries to fail without warning? The battery still has enough remaining charge to propel the bike, so it still works, just can't be charged. I'm tempted to open it up in case there's an obvious fault inside - any suggestions?

[yakdiver]

I have had two batterys fail after 5 years, it seems that is about the life of a bike battery these days.
Yesterday the charger failed after 5 years service.....£80 replacment.....who said cycling cheap

[hemo]

One likely issue is a cell group is out of balance and below the lvc, once a group falls below the lvc the battery bms detects a fault and will not allow any charge. One can try opening the battery and and with a meter take 10 cell group readings via the bms sense wire connector, this would tell if a cell group is at fault. Often any cell group sub 3.1v or 3v often prevents the bms from switching to accept charge current.