Haven't been here in a long time but pleased to tell you that after three years I ride my XB-600 every day and it still works great. I long ago did the shunt mod and the 60 volt mod and I am very satisfied.
I usually charge with the stock charger, 5th battery disconnected and hooked to its own charger, then once a week I "top off" each battery. I know it's not an ideal solution but it seems to work pretty well.
I wanted a one-charger solution for every day, though, so I ordered a 60 volt charger from Monster Scooter Parts (part no. Q50-5801) and modified it to work with the XB-600. I've been using this for a week and it seems to work great. No more cumbersome charging, just plug in the one plug.
This charger does NOT come with the correct cable. You have to modify it yourself. De-solder the existing cable and install a standard plug-in power cable, the kind that are used everywhere on musical instruments. Make sure you get the polarity right or kaboom! And there are some BIG capacitors in there. If you don't know how to do this safely, DON'T do it.
I accept no responsibility for the result of anyone following this advice but it seems to be working very well for me. I ordered a long cable for the replacement so I'm not constantly worried about damaging the charger. Plus it just seems a little more robust than the stock charger.
Have fun and good luck if you try it yourself!