I have found a solar powered garden light that has a 18650 lipo inside.
I am thinking I can add a second 18650 in parallel and connect them to the battery terminal on the dash.
This should give me plenty of mAh and a really cheap charging solution complete with outdoor housing.
I am making an assumption that the charging circuit in the light will never provide more than 4.2V because that would kill the lipo. So I think that it would be ok to connect the Dash to the lipo rather than the light output which is switched based on light levels (solar panel voltage)
The second assumption is that the Dash internal charging circuit will have some under voltage protection for the lipo.
In my mind, this should all work.
Have I missed something obvious?