Lipos get a bad rap for being dangerous and unpredictable. The truth is that they are only as dangerous as you let them be and they are predictable enough for safe use once you are properly educated on them. As such the biggest problem with lipos is not the batteries themselves but the users. The next time you read about a lipo burning down someones house or car, remember it was not the lipos fault, it was the user of that lipo. With a basic understanding of lipos and the use of good solid equipment and practices, lipos can be as safe as any other battery type.
Handling lipos safely
Lipos are fairly tough in certain ways and fairly fragile in others. Most of this is common sense but here are a few pointers.
Make sure you
Generally speaking lipos are safest when they are just sitting around doing nothing but you need to make an effort to keep both them safe from damage and you safe from them if something were go wrong, no matter how unlikely it might seem.
Chances are that if you are ever going to have a problem with a lipo, aside form damaging one while in the crash of course, it will be while charging. Modern chargers are designed to be safe but they are by no means perfect and there is always a chance, no matter how remote, that something can go wrong. With this in mind lipos should be placed in a location where if there is a problem, it will not grow into a much larger problem like catching your house or car on fire. This does not mean you need to build a bunker in your backyard to charge lipos in but some thought does need put into where lipos are charged.
The best place to charge your lipos is where there is no combustible material. With that consider all the places you have at your disposal. It could be your garage, your driveway, your bathtub, in your oven ... anywhere really, as long as it is not near anything flammable or combustible. Think of it this way, if you can pour a pint of gasoline on it and light it without catching anything else on fire, then that is a great place to charge a lipo.
Many people don't stop at just finding a good place to charge a lipo, they also charge it in something that will contain the lipo if there was a problem. This can be something as simple as a cash box, as complicated as a customized toolbox or as commercial as a lipo sack. I consider this an added safety step that is not required but is never a bad idea, especially if your location is not perfect. For example if you choose to charge on the kitchen counter, a lipo sack is cheap insurance in case there is a problem.
Last, but definitely not least, is the requirement that you be nearby whenever charging a lipo. I am not suggesting you pull up a chair and watch the charger for the duration but you should be in ear shot and preferably line of site of the charger the duration of the charge. If you need to leave the charger, simply stop the charge and place the battery somewhere safe while you are away. Then you can start the charge again when you return. If you log the mAh then just write down the amount at the time when you stop the charge and then add the final amount to it for a total.
Storing lipos is pretty straight forward. Keep them in a cool safe place where if there were a problem, it would be contained with little to no drama. Lots of people use an ammo can or a fireproof safe and keep them in their garage or shop. This will definitely work but there are many other solutions. Basically you are looking for several things, a usefully designed container, a place that stays cool and is away from flammable or highly combustible materials, and a plan.
First up is the container. There are a wide variety of containers that will work for this purpose. I would think any metal box that has useful dimensions and is relatively air tight would be the easiest and safest container to use. This could include tons of boxes including tool boxes, ammo cans, cash boxes, etc. The use of lipo sacks is a fine idea but I would not simply place your lipos in a sack and set it on the shelf. That would likely contain any heat/fire a lipo could produce but it won't do a very good job of protecting the lipos from getting damaged. As such I believe lipo sacks are fine for charging but storage still needs to be in a hard container. If one were overly paranoid, I suppose each lipo could be placed in its own lipo sack and then the sacks could be placed in a metal box. That would definitely cover all your bases but it would be difficult to use and very expensive. Whatever you decide to use, just make sure it is as useful as it is safe or you may not use it.
Next up is the location. The location needs to be out of the way and it needs to stay cool. A cellar would be ideal but a basement could also work depending on the type and use. Most people will use their garage or shop but that is not always the best place due to temperature fluctuations. Where I live, the southwest, garages and shops are like ovens in the summer and freezers during the winter. This huge swing in temperature can damage lipos. Another place that may work is the refrigerator. Lipos are non-toxic, so keeping them in a box in your fridge is very acceptable solution. Just make sure you let them warm up to room temperature before you use them, and that includes charging them.
Lastly is the plan. You should have a plan for what, where and how long you plan to store your lipos. For example if you plan to fly tomorrow, then you could simply take the packs, charged or not, put them in your lipo sack and leave them in your flight case. But if you want to store your packs for the whole winter, then you need to prepare the lipos, make sure you have a well thought out place for them for the 3-6 months of storage and that you check on them once in a while during that time. In the end just make sure you have at least a basic plan for your expensive lipos, both for their sake and yours.
This is something a lot of people struggle with. Of course care needs to be taken to keep the lipos safe from damage while you move them around and if there is a problem, the lipos need to be well contained. I believe those concerns are secondary to the fact that what ever you transport the lipos in needs to be well thought out and useful. This way it does not burden you while moving it or using it. In other words it needs to be both handy to use and fit in your car easily. I would assume that a great solution would be a metal toolbox. If done right it could have individual storage places for each lipo and also house your charging equipment. Plus if a lipo went off while inside, it would very likely contain the heat with little or no damage to anything around it. Whatever you decide to use make sure it is fairly air tight, has a handy internal setup and has a good latch so that if it is dropped, the contents don't come spilling out.
Dealing with retired or damaged lipos
At some point you will have a lipo that either becomes useless or becomes damaged. You need to understand how to handle a damaged lipo and how to dispose of it.
How do you know your lipo is bad?
This question can sometimes be very easy to answer and other times not. If your pack is folded in half from a crash then it is most definitely bad. But if it looks physically fine and just doesn't perform well, the question is not as simple to answer. First lets cover what makes a lipo bad in terms of physical conditions and then we will cover other reasons a lipo should be retired.
Anytime a lipo has been physically damaged there is a very good chance it damaged beyond use. If any cell in the pack is punctured, crushed or badly swollen, the pack should be considered damaged beyond use. If any of the wires become disconnected from the pack then most people should discontinue to use the pack. Often times these type of packs can be fixed but the average user will not have the skills needed for the repair and in that case the pack should be considered damaged beyond use.
Now what about packs that look fine but shows signs of other problems? This is my one-liner on the subject "If a battery is no longer able to perform the task it is intended for, it needs retired.". A lipo can fall into this category for many reasons including age, a bad cell or just worn out. It could also have been damaged by heat, over-discharge or being shorted out. All these things can leave you with a battery that looks fine but just doesn't perform as needed.
What is meant by "retired"? There may be several outcomes when a battery is no longer fit for use in a heli. It could be damaged badly enough to need disposed of, or it may still be very useful for other uses such as a rx battery, setup battery or a starter battery. These are often great uses for old batteries that are just tired and not damaged in any way.
Dealing/handling a damaged lipo
So what do you do with a lipo that has been damaged, either in a crash or somewhere else? Let me just go through a few scenarios to show what I would do and then you can take it from there.
What to expect when a lipo "goes off"
Ok you crashed your heli and the lipo is getting warmer and warmer, what could you expect if it "goes off"? Let me give you my experience with lipos, both what I have seen in real life and what I have seen online.
First lets talk about the lipos you have probably seen the YouTube videos, where they explode into 2ft fire balls. Those are deliberate failures done to get the most "bang for the buck" out of a lipo. In other words the pack was purposely overcharged until it catastrophically failed. The only way to have reaction like that is to overcharge a pack for a long time. I suspect it takes upwards of an hour of overcharging to get the pack to build up enough heat in order to fail like that. This type of failure was more frequent on early lipos and chargers. Usually someone would set the charger to the wrong cell count and then would leave it unattended only to later find their garage on fire. This kind of failure is very rare these days and is almost never representative of what a crashed lipo looks like when it goes off.
Now lets talk about usually happens when a lipos "goes off". This is the kind of failure you could expect from a physically damaged lipo, like one that was crashed and suffered a crushed end. First thing that will happen is the building up of heat. Lipos need heat to set them off and one way this happens is from an internal short caused by crushing a pack. This build-up of heat could be very rapid or fairly slow, depending on the damage and the state of charge, but if it gets hot enough it will it will go off. When this happens it will rapidly expand the foil covering on the cell that is failing until it pops ...
Ok let me stop for a second and explain another concept. When a lipo "goes off" it is rarely, if ever, the whole pack. It is almost always one cell at a time. This means a 3s lipo could "go off" 3 times before it's all done but each time it does, it is a less violent reaction.
... Once it pops smoke will start to poor out. You may or may not see fire. I have seen 4 lipos go off at my field and never once has there been flames. This does not mean there is no heat though, in fact the last time we had one go off we measured 750F smoke coming out of it, so it is indeed very hot. I have also never seen a lipo move around or spin when it goes off, they just sit there and spew smoke. I am not saying you will never see fire or lipos shoot across the ground as they go off, but I have not seen it yet. I actually don't know what is in the smoke but I would guess it is similar to burning plastic. I don't think it is horribly toxic but it's not fun to breath, so keep that in mind as everyone stands around watching your lipo turn into a pile of charred metal. The total reaction can last a few minutes per cell and the resulting pile should be a combination of ash covering many thin metal sheets like aluminum foil. Once everything cools down, I would cut the balance and main connectors off if they are in good shape and place the remains in the trash.
If a lipo goes off in your heli before you can get it out, expect it to do some pretty good damage. Most of the photos I have seen include the battery tray and surrounding frame/components to be burned/melted. Usually the rear of the main frame is fine and that includes the electronics back there like gyro and tail servo. Most as not total losses but are not fun for sure. I would just recommend you do your best to get to the heli quickly and get the lipo out asap, no matter what the condition of the lipo.
Discharging lipos for disposal
There are several methods of fully discharging a lipo for disposal, see "Safe lipo preparation for disposal" for ideas.