I can't help you with starting it from a battery, but...

Troy Bilt makes several models with an electric start option available at Lowe's in our area. The model we bought was $174 with a curved head. You need to buy the start mechanism separate for $34.95. They also have another option for $9.95. It's an Allen Wrench with a drill bit on the other end. You do need a high speed drill to start it. My husband is older too & has had shoulder surgery. This he can do & it was a hit right off. It also started easily by pulling the cord. It's a 4 cycle.

Our Craftsman attachments (brush cutter, etc.) fit it, so that's a real bonus.

Here is a link that might be useful: Troy Bilt Weed Eater

They also have a straight shaft weed eater & several optional attachments, if you're talking about weed eaters. :)

Reading about starting mechanisms for a car I see my idea of using a battery is simple minded. Starting an engine is very complex. So forget that. I thought I had found a use for my old battery!

That spring loaded starting system will drive ya nuts when the machine gets a year on it.
Check on the ones that have their own battery electric motor system. I found one by Black and decker. Seems ok.Don't know how long the charge will last, as my son took it home!