I'm having a similar problem with my camshaft position sensor on my car which is basically a magnet inside a casing which picks up a signal as a lump of metal rotates through its 360 degree cycle just like a bicycle speedo set up with a magnet on a spoke passing the pick up on the fork leg. The faster you go, the quicker the magnet passes the sensor and is then interpreted by the software as more MPH. Only in my case the cam sensor is for fuel timings and other things. They generally work on the "Hall effect" principle.
Boring physics lecture over.......It may be your speedo sensor ( a magnet pick up) is either loose or damaged or has damaged wiring, possibly to earth/ground)
Line of least resistance / expense is to locate it and check its condition and if possible check the voltage values on the wires. If it's a 3 wire sensor one should be roughly the same as the battery, one should be zero / ground and the 3rd will pulse, probably from 0.5 to 5 volts when the gearbox is turning (bike on mainstand and run in 1st gear), but these numbers are generic and a guess on my part. Maybe your mech has damaged it electronically when fitting the battery.
Or just buy this...item 11. Look at the part number and cut and paste it in to Google for a price (�100) roughly, which is a scam as my cam sensor is the same thing, different shape for �8.
https://www.ajsutton.co.uk/genuinepa...trical-2?uID=0