A swollen LiPo battery after a crash is genuinely dangerous because the internal chemistry has been compromised and the electrolyte is producing gas. I've dealt with this more times than I care to count in racing, and the most important thing is treating it with respect because these batteries can catch fire or even explode if mishandled.
First, never attempt to charge or use a puffed battery again. Move it to a fireproof location immediately. I keep an ammo can with sand in my garage specifically for this purpose. Place the damaged battery in there while you prepare for discharge.
The safest home discharge method is the saltwater bath technique. Mix a concentrated saltwater solution using roughly one cup of table salt per gallon of water in a plastic bucket. The bucket should be large enough that the battery will be completely submerged with at least two inches of water above it. Place this bucket outside on concrete or dirt, away from anything flammable. Your garage, wooden deck, or anywhere near your house is not appropriate.
Before submerging, check the battery voltage with a multimeter if you can do so safely. If it's reading above 3 volts per cell, you'll need to be patient. Carefully place the swollen battery into the saltwater solution. The salt creates a conductive path that slowly drains the cells. You should see small bubbles forming, which is normal.
Leave the battery submerged for a minimum of two weeks. I typically go three weeks to be absolutely certain. Check it periodically from a safe distance. After this period, measure the voltage again. Each cell should read below 1 volt, ideally close to zero.
Once fully discharged, the battery is much safer to handle but still needs proper disposal. Do not throw it in regular trash. Take it to a battery recycling center, hazardous waste facility, or some hobby shops that accept dead LiPos. I've found that calling ahead confirms they'll accept it.
Throughout this entire process, never leave the discharging battery unattended for the first 24 hours. Have a fire extinguisher rated for electrical fires nearby. Some pilots use commercial LiPo discharge bags, but saltwater is more thorough for damaged cells. The goal is making the battery inert before disposal, and patience is your best safety tool here.
