The deal with any of the battery stuff here, 3-shifts/7-days a week/365 a year, regardless of mfg'r, is they just don't hold up. Can't tell you how many Milwaukee 1/4"/3/8" cordless ratchets have been replaced because of bearings/planetary gears, etc). A fair amount is user error no doubt, trying to do too much with 'em, using 'em as combination wrenches, torque wrenches, etc. A lot of the standard batt's have gone by the wayside...the XC series extended batt's hold up a heap better. As far as the Milwaukee RA die grinders go, they're kinda new to the scene here within the last few months, but many have gone tits up. Wrong size/abrasive grit Roloc's for the job, wrong RPM, forcing/pressing on 'em too much with cut-off wheels instead of letting the grinder do its' job, etc, etc. But ya can't fix stupid!
Bottom line, ain't nothing battery gonna survive in this day to day environment. Why the gubment even opted for this avenue to work side-by-side with pneumatic tools is mindboggling. And if you think Uncle Sugar is into buying repair/rebuild kits for the failed ones or batt's...you're simply outta your skull. And warranty...better read the fine print...this ain't no homeowner/occasional use/at best small-time, self-business world here. Let the powers that be figure it out as they buy dozens and dozens of new replacements each month to go across the aircraft squadrons.
Are most Milwaukee, Makita, DeWalt, Porter Cable, Rigid, etc worth the money for us bozo's in our h0mes, shops...absolutely.
Just don't bring a battery knife to a pneumatic gunfight world!