Starter?

Peugeot Partner Battery new last Feb
.
Alternate working
Battery test multimeter at 20v reading engine off 11.43v
Turns over half turn then nothing.
Tested charger by connecting the charger leads to multimeter leads
Switched on charger set to fast charge multimeter reading 19.43
Switched to trickle charge reading 14.35
It's been turning over slow for few months started but struggled . Soon as cold weather arrives forget it.
Does this test prove charger is working?
Or could the problem be with starter drawing current but not delivering power to turn over engine. Going to try fully charged battery to jump start connecting live to live, earth to the car engine. This apparently works cutting out current from good battery going through one on car.
Thanks for reading