The original battery of Oral B Triumph 5000 is a 1.2V NiMH cell in the format 4 / 5A, ie with dimensions 43mm high by 17mm diameter.
Such a battery is the HR-4 / 5AU of FDK (formerly Sanyo), fits perfectly, works perfectly, took me in the internetwork about one-third of the proclaimed here price.
I'm definitely of the opinion that knowledge can be paid and should, but here the required 200% premium for the simple knowledge of a only halfway right battery size then I think it is inappropriate.