Is there a Deep discharge protection connector on your powertool ????
The thirth small contact in the battery connector or only 2 contacts ?
If not then its an advise it must be modificated then you have only to break off a notch on the new modificated battery holder connector so it can be placed in the older not marketing technical modificated housing, and you have to change the switch and the battery connector , the differences are 091A. 643844-1
old batteryconnector (2 contacts) change into 091B. 644808-8
New modified batteryconnector (3 contacts with deep discharge protection it has a limit until a level)
Switch old model (2 contacts) replace for a
New modified switch (3 contacts )
What you need is even expensive by this model if you have a modified type with the 3 contacts to know what can be wrong else is remove the brushes charge you battery and measure it first if its full and take a multimeter measure tool between both brushholder measure and pull the trigger that must between give 16and 20 volt DC if the battery was good charged . next measure is ohms resist measure both brushes in the brushholders and measure for a few ohms from brushcontact to brushcontact throught the armature inside the machine.
if not a resistance to get measure through the armature without brushes if some object or parts was comming trough the ventilation of the machine between armature and brusholder then the brush beet not the collector anymore different things to look at .
Look also how the kind of wiring technik of the armature is different left is a old kind of armature that will last for 5 years or more its having thicker wiring and an other winding pattern instead of the modern kind of armatures from the last years from 2009 and later Johnson armatures by PANASONIC MILWAUKEE, MAKITA , this new armature has a lot of more power but its faster defect because of the to tight tolerance of heat dissipation and ventilation but thats most in time of inbetween 2 or 3 years worn by the great strength and light machine makes this rapidly abuse when it is going to slowly get gives a shrill sound then i know there is also not much power on , batteries are fast empty and no power left ( AND THE PROBLEM IS PEOPLE THINK THE BATTERIES ARE GETTING OLD no thats not the problem but the armature is it and as before its eating carbon brushes in between 1 month again so fast if its worked in daily use. ALWAYS REPLACE BRUSHES AND ARMATURE (thermical shortings not to see but to hear and measure) at the same time it isnt not so expensive as that you choose new batteries and the batteries work great on a other tool . if the machine has eating the brushes with the old armature left on picture nothing was happens and it last again in good condition but the power difference is great between the old and the new armatures of the brand Milwaukee 18V to compare this old was Milwaukee 18volt and right the same kind of new sensitive armature like Panasonic or Makita have used in their tools.