I flew a Falcon 900ex with nicad batteries. I flying Orlando to UK. We had just joined the tracks east of Canada when we had a thermal runaway on one of the two batteries. Even after the battery was disconnected the temperature continued to rise. We diverted into St Johns. Three hours after landing the battery was still too hot to hold. It was bulging and distorted. If we had been further along the tracks I dont know what we would have done. Its alright having 3 engines but if you only have 2 batteries and one of those is a potential timebomb sat out of reach 60 feet behind you you are in queer street. The next aircraft we had was an exEasy with lead acid batteries. Felt much safer. Peter.