Hmmm.........

Most of these replies came in after the horse had bolted.

The battery is LiOn, the 19V laptop charger was nobbled by its manufacturer to only put out put a decent amount of milliamps if it can €œsee€� a Dell laptop.

Nearest I could get was a wall wart marked 12V which put out 16 and a bit volts both on and off load (battery has just 2 connectors). (The wall wart is a big beast and I suspect has a transformer, rectifier and a smoothing cap inside, the original was tiny).

I charged it for its 4 hours maximum charge, checking regularly that neither the wall wart nor battery were getting hot.

At the end of that, the battery output was nearly 17V (up from 15 and a bit..

The user reports that the hedge trimmer is working fine.

I suppose it might shorten the battery life, but hopefully nothing will go bang.