Lifejacket batteries

[blue up]

Would anyone be able to tell me if lifejacket batteries have any sort of industry standard in terms of battery voltage? I assume the salt water ones use the salt as the medium between copper and zinc terminals? Are they still a fixed voltage and can they be exchanged with sealed battery cells?
I've just been sorting used LJs for Cabin Crew training and found maybe a dozen different designs of battery, most of them in varying stages of 'self destruction'.
Thanks
Rob