Vintage Simmons Drums (UK) Users Group

I have a Simmons TMI unit whose memory no longer works. I opened the box to change the battery and discovered that the battery has three prongs and is 3.6V. But, I have no idea what the correct amps for the battery are. Does anyone know? Is there a place to buy a replacement battery since Emmerich, the original battery maker, went out of business in the middle of this past decade? I'd even settle for a schematic that shows the value. Any information at all. I'm stuck. Any ideas out there?

Thanks!
Erick