Quick answer
You press the release, pull, and the pack won't budge. Nine times out of ten it's one of three things: you're not fully depressing the latch, dirt has packed into the slide rails, or the battery has swollen and physically grown inside the dovetail. The first two are a two-minute fix. The third one means the pack is done, and how you get it off matters for your safety.
Free it without breaking anything
- Press the release all the way, then rock side to side. Most "stuck" packs are user error: the release button isn't pushed in far enough, or it's pushed and the pack is pulled straight back when it needs a small wiggle. Hold the button fully in and rock the battery left-right along its slide direction while you pull. That breaks whatever is binding in the track.
- Brace and tap it loose, but only if the pack looks normal. Do not tap a pack that is visibly swollen or leaking; stop and treat it as a damaged lithium battery (see below). Still stuck? Set a wood block or a folded rag under the foot of the battery to protect it. Hold the release down, grip the tool in one hand and the pack in the other, and give it a firm tap against your palm or the block in the direction the battery is meant to slide off. Short, controlled taps, not a hammer.
- Clean the rails. Dust, sawdust, and dried grime in the dovetail will glue a pack in place. Blow the rails and the latch out with compressed air or an electronics cleaner like CRC QD. Skip WD-40 and oil. They attract more grit and can creep onto the terminals. Check your tool maker's guidance before using any lubricant on the rails.
When the pack has swollen
If the battery feels tight only near the cells, looks bulged, or the case seams have spread, you've got a swollen lithium pack. That's a failing battery, not a latch problem. Heat, age, and a deep over-discharge cause the cells to gas and expand, and once a pack grows even a millimeter it can wedge itself into the tool's dovetail.
Get it off gently using the rock-and-tap method above, and never pry it with a screwdriver or puncture the case. A damaged lithium cell can vent or catch fire. Once it's free, stop using it. Don't charge it again. Do not put it in a store drop-off bin or the trash: Call2Recycle (The Battery Network) says swollen or damaged packs are not accepted in its collection boxes. Set the pack on a non-flammable surface such as sand or cat litter, away from anything flammable, and contact the battery maker or your local household hazardous waste facility about how to ship or recycle it. A swollen pack is a replace, not a repair.
When it's the latch, not the battery
Plastic release tabs wear out, and the small spring behind the button can break or pop loose. If the button feels mushy, doesn't spring back, or stays depressed, the latch hardware has failed. Sometimes the broken piece is on the tool, sometimes it's the catch molded into the battery.
To tell which, try the same pack on a second tool in the same platform and a second pack on the stuck tool. If every pack jams on one tool, the tool's latch is the problem. If one pack jams on every tool, that pack's catch is worn. Tool-side latch failures are often a cheap part to replace, and if the tool is newer it may fall under warranty. A worn battery catch usually means retiring that pack.
A note on platforms
This is a mechanical jam, not an electrical fault, so it has nothing to do with charge level or the charger. Worth remembering, though: batteries only seat cleanly on their own platform. A Milwaukee M12 pack won't lock into an M18 tool, Ryobi 18V ONE+ and 40V don't share a foot, and Makita 18V LXT and 40V XGT use different mounts. If a pack feels like it's fighting the rails from the start, make sure it's even the right platform before you force it.
Bottom line
Press the latch fully and wiggle, clean the rails, and brace-and-tap for a stubborn one. If the case is bulged, treat it as a damaged lithium pack, get it off without puncturing it, and dispose of it through a household hazardous waste facility. And if the release button itself is broken, sort out whether it's the tool or the battery before you spend money on either.