When good batteries go bad.....

Well, today ended up interesting. Firstly, we have just had a lovely week away on a narrowboat relaxing and enjoying the UK's canals! Secondly it's my birthday today and thirdly I spent the afternoon going through my trucks and getting a team photo!

However, things went weird early afternoon. After mowing the lawn I decided, naturally, that it was time to get the XRay 1/18 MT out for a blat round the garden. I charged the battery, drove the truck for 10 mins or so with the odd gap here and there whilst I set up jumps etc. I then took the same battery and after running it down in the truck, finished it off in a discharger before putting it back onto charge. About ten minutes later my wife screamed out that I needed to come upstairs right away and I was greeted by the sound of the battery hissing then within seconds the thing begin to explode. 5 of the 6 cells blew up with one of them catching me on the chest as I tried to lob the pack out the window.
I will be adding this to the TC forum for thoughts/advice as to potential cause but thought also best to add it here.
1)Battery was FTX Voltz 1400 7.2 mini stick pack - used a few times before with no problem.
2)Charger Pro Peak Delta 3 AC (only two weeks old) - 5A charge
3)battery discharged fully before first charge today, got hot as expected before charger tripped over to trickle. Then run, discharged and set off again - EXPLOSION.

Pictures show aftermath and a bit of man flesh I'm afraid to show my war wound.