You will find the correct battery type at shaveroutlet.com. See link below. You will need two batteries.

http://shaveroutlet.com/shaver-replacement-parts/shaver-replacement-battery-aa-nimh-p-108.html

I suggest that you read the installation tip on the item page as well.

Also, does the green charging indicator light still illuminate when you plug the shaver in to charge? If it no longer lights, don't waste your time with the batteries because your have waited too long to change them and the circuit board is now damaged. Unfortunately, circuit boards for model 6550 are no longer available from Braun