Just found that the PS4 controller batteries are compatible with the MKR.
My son was stripping one down to swap out a joystick from a dead one.
Turns out the JST connector fits directly into the MKR.
WARNING don't connect it right away as the power leads need to be swapped !!!
To do this you just need a needle to slightly pry up the retaining plastic on the connectors and slip out the lead. swap them over and hey presto.
Capacity is 3.65 v @ 1000 mHa so well within the specs for the MKR.
Currently I have this charging and the charge light is steady on.
EDIT
The battery charged and the charge light went out and a simple blink sketch will run on the PS4 battery...Nice little reclaim tech