Without taking them apart, there is an easy way to tell which battery is in there....for some of them.
A round charging jack indicates on voltage of battery, a rectangular charging jack indicates the other type of battery. (I am so old, I forget which, I am sure someone will chime in.)
There are some PS2 engines with no charging jack....good luck with those. (see above comment)
It would probably be far worse, if you had PS1 engines.
ALSO.... that comment, "be advised, any MTH PS2 with the 5V system ( 9V type rechargeable battery) is potentially on borrowed time" has gotten me thinking about some things!
1) if an engine is opened up, and a "9v imposter" is in there (the 5 v system) then I am betting one does NOT want to insert a normal, 9v rechargeable battery in there!! I assume, that would fry it all!?
2) if one owns a 5 v system, and it eventually does fail, can someone comment on the logical options? I assume the guts could be stripped out and the engine used as a dummy. I guess, that some tech on here, or a LHS if there is one around, could "update" the engine with new boards - but for some cheaper PS2 F7 is it economically viable?
Perhaps someone more educated on this than me can start a thread on this. Happy trails, Greg