I just found an unusual battery in a secondhand PS-2 engine. It's labeled as 9.6 volts instead of the usual 8.4, which would indicate that it has 8 cells instead of the usual 7. What makes it worth mentioning is that it has a label from a hobby shop calling it a battery for an MTH loco. The battery is a Powerex NIMH, 9.6v, 230 mAh.
The label/price tag reads:
Colonial Photo Hobby Inc.
Battery - 9v - Mth Loco
9v - Bplus
$14.95
I suppose BPLUS means the source was Batteries Plus.
The loco worked with the battery. I removed it and charged it up, and it did indeed deliver over 10 volts fully charged. I don't intend to use it in an MTH locomotive, but I am curious as to whether there would be any downside to doing so.
While I was playing around with the Powerex battery, I charged up some 8.4 volt rechargeables I had lying around for comparison. An old white MTH battery charged to about 9.7 volts and still tested well over 9 volts a few days later. The best one was a cheap Thunderbolt brand NIMH cell from Harbor Freight; that one was rated at 200mAh and held over 9.5 volts several days later. A Radio Shack 8.4v NIHM cell was only rated at 165mAh and only charged up to around 8 volts. A couple of days later it was down around 7. That one went into the trash. And before anyone can bring it up, yes, I know that you have to test under load to get a truly correct reading on battery charge. Unfortunately, my load tester does everything but 9-volt rechargeables. The comparison test I did gives at least a rough idea of comparative battery performance; I don't claim it is definitive.