christmas lights for layout, solar power supply quit?

Is that a single green cylindrical battery?  If it's 18mm x 65mm (slightly larger than a AA), then it's likely the popular, widely-used 18650 Lithium rechargeable type in which case the charged voltage would be closer to 3.8V.  A typical circuit that expects 3.8V (or so) would likely not work at 2.8V. 

Thanks Stan. It had even less voltage today so I put it on my bench for some charging. It was going back upwards to 2.43 volts when I last checked. I think from your description, it maybe the battery is so low that the circuit won't operate. I'll see if it will go up to where it should be.

Stan, I put 3+- volts directly to the battery terminals with the solar covered and the lights came right on. The solar only puts out 2.7 volts. The battery is labeled 3.7 volts 270 mah. I'm not sure if that will ever top the battery off fully? So it may not be wise to waste the money on a new battery?

I found some old phone batteries that have similar specs. I may play with or

I will replace the power supply with a standard one.

Just playing around with these. It's pouring rain outside.

Right.  If the solar charging circuit is only presenting 2.7V to the 3.7V Lithium battery, it will never charge.  I may be missing something, but if this is for a train layout you have gobs of power "lying around" so seems your idea to simply replace the power supply is the way to go.  That is, if the battery supplies 3.7V to the lighting circuit then one of those inexpensive eBay DC-output stepdown regulator boards should do the trick.

Yeap, Just what I was thinking of from your last ideas for me.

It seems everyone is adding lights to things. The LEDs are great. I just have to get a handle on where to focus my work. I am going in too many directions. I want to light up my big bridge at night. I haven't even built it yet!

I think it's good to plan ahead so I know to add the conduit for the lights when I build.