To be more correct, they are called Rose Technology. Slightly funny how the name comes about as it has nothing to do with the flower, but how the company name spells in Chinese - '弱水‘, which actually means a mythical river.
MX500 has been unofficially reproduced by a Chinese company under a new name, partnering with the original OEM (Foster Japan) for Sennheiser no less. I compared the new model to the original MX500 and they are just about identical. Not meant to rain in your parade, but there is not much reason get NOS MX500 anymore when there are new one out there.
Just checked, it is a 11.1V battery, which means it is a triple stacked Li-ion (3.7V x 3). So the low end should be (3.2V x 3) = 9.6V. As long as it is around or above that number, the battery might be fine. But if it is well below that number, then it is likely a goner and required a replacement. Before measurement, do give it a recharge for a few hours so you will know for sure it is the battery not taking any charge. There should be 4 wires coming out of the battery, 2...
Scored a pair of used ER4 on eBay for less than $100 and that's a steal IMO. The cable and filter are both goner, but I really only need the working earpieces to rebuild the ER4 back, since I have all the spare parts. Good days!