I've tried looking everywhere including the service manual and I can't find out what type of battery it uses.

I had the battery that was in it but now it's gone and I don't know where it is. It was definitely a cylindrical battery, not a flat plate battery. The service manual shows a flat plate battery but I know that's not the case here.

Anyone know how I can find this out?

 

Most Honda OEM transmitters use the flat batteries. You didn't state the transmitter for which car listed your signature.

The different types of batteries and their OEM transmitters are listed in this Honda TSB:

tegger.com/hondafaq/misc/98-011.pdf

However, if it was a cylindrical battery, then it probably is an A23 (which is used by most aftermarket alarms and many garage door openers).

 

I think Redbull is right.

If the battery isn't the standard CR2032 coin (button/watch/wafer) type battery than it must be the stubby little A23 that looks like 1/2 of an AA.

 

I tried A23 and the thing was way too big.

I thought it was OEM keyless but it was not. Turned out to be some generic thing. I can't believe I didn't look up the model number earlier, just never noticed it.

I looked it up and it turned out the battery was GP27A.