Check your batteries!

[Radagast]

Or go with protected 18650 Panasonics. Must be over a hundred million of them in laptops. About as safe as Li is going to be.

[AirRifleSport]

the cells in question were protected... On one occasion, I forgot to switch the device off and the battery cut off at about 10.6V (can't remember exactly). The same would stop charging at 12.1v, I always check the voltages before and after charging.

[Radagast]

I'll cease trying to teach you to suck eggs.

[AirRifleSport]

No worries mate! I wasn't trying to be funny about it.
On the photos that I posted it shows the protection PCB on each of the cells.

I think the problem is to do with the fact that the charger does not balance each cell but to do that the battery's cell need to have an extra wire each (there is a thread on this forum about it) and then using an IMAX charger to balance charge them.

[Radagast]

Yep. I've had a few blue box units. Each had protection circuits. Dead after one charge is the norm. Which is why I'm inclined to 18650s and swapping them out as needed.

[AirRifleSport]

Yes, that has crossed my mind...
will consider it.