Warning: Attempt at own risk. A lack of knowledge or improper equipment can result in the loss of equipment, electrical shock and fire. Only attempt if you fully understand what you are doing and are willing to accept the risks. I am not responsible for accidents that may occur when using these instructions.
Reason for this how-to
At this point almost every RC'er either owns or is looking to own lipos, they are just way too good not to, but with ownership comes responsibility. Everyone should know that lipos need handled with care and this includes taking care to dispose of lipos correctly. This how-to will hopefully shed some light on how to safely dispose of your unwanted lipos.
The basic concept of lipo disposal
The most important part of lipo disposal is to make the lipo safe, safe to be crushed in a garbage truck or safe when tossed in the recycle bin. We do not want to pass off a potentially dangerous lipo to people that do not understand what it is or how to handle it. As such I believe we have a responsibility to make them safe before they leave our care.
But how do we make them safe? That is actually quite a simple answer, we fully discharge them to 0V (that is zero volts). Lipos are only dangerous when they contain a charge. Remove that charge and they are basically inert. Once discharged fully, you could drive a nail through it, cut in half with a saw or drive over it with a tank, and nothing will happen except the lipo will be physically damaged. There is no risk of fire from the lipo.
How to get to 0V
How you get your packs 0V is up to you and there are lots of methods. Many will use some sort of light bulb setup but many electrical loads will work including things like 12V fans or even a 12V thermoelectric cooler. There are a few that use other methods but I do not recommend them.
There are many people who say that the safest way to discharge a lipo is to submerge it in salt water. The theory behind this is simple, salt water conducts electricity and water is a safe place to have a lipo in case of fire. The problems are that it can take a LONG time to discharge using this method and the salt water is corrosive, so there is a chance that the pack will not get fully discharged do to corrosion halting current flow. As such I do not recommend the salt water method.
Similarly some people like to physically destroy lipos in order to "discharge" them. Some take them to the shooting range and riddle them with bullets, while other stay at home and use a hammer and nail. This can be an exciting but unpredictable way to end the life of a lipo and although I have done this many times, it is not for most hobbyists.I will assume that most hobbyists appreciate a more controlled method, and as such I do not recommend the physical damage method.
Choosing the discharger
There are many uses for some sort of discharge device in RC. Yes you can use most chargers to discharge but they have very limited internal discharge capabilities and most will not fully discharge a pack. Instead why not buy/make your own discharger for just this sort of use. Check out my how-to "My light bulb discharger build(s)" to see what I have done.
How quickly you discharge the pack is up to you. Some people like to connect the pack to a very low wattage bulb, bury it in a bucket of sand and leave it for a week to discharge. Others, like myself, would like to complete the discharge more quickly and aim for an hour or two. The slower the discharge, the less heat generated and the safer it is likely to be but the longer you will have to watch it, so it is a trade-off.
You can estimate the time required by doing some basic math. Of course the actual time will depend on the charge in the pack and health of the pack but at least you can get an rough idea. The math is actually very simple and here is the basic equation used.
Ah / A = h
To start lets say you have a 2200mAh 3s pack and you are going to use a single 12V 20W automotive bulb to discharge it. First find the current draw.
Amps = Watts / Volts = 20W / 11.1V = 1.8A
Note: The nominal voltage of the pack is used here because that is approximately what the average voltage will be under load.
Now to find how long it will take to discharge.
Hours = Ah / A = 2.2Ah / 1.8A = 1.2h
The math says it will take 1.2 hours to discharge that pack with that light bulb, but remember that the state of the pack will play a big part in the actual time it takes. Now for another example but this time lets do a long discharge, say a 5000mAh 3s pack being discharged by a 12V .5W bulb.
Amps = .5W / 11.1V = .05A
Hours = 5Ah / .05A = 100h
This combination can require somewhere on the order of 100 hours to discharge the pack. Hopefully that will give you the basics needed to predict the time needed to discharge a pack with a load.
Now lets get to it, it's time for a real example of what to expect
The discharger used in this article is one I made a while back and offers 12V 5A, 12V 10A and 24V 10A loads. I had five 4s4p 8000mAh Thunder Power packs at work that needed discharged and disposed of, so they will be the subject in this example. First step is to measure the pack voltage.
This pack is right at 15.44V or 3.86V per cell. Since the voltage is higher than 12V, I will connect the pack to the 24V 10A load on my light bulb discharger. The math says that the initial current draw of just over 6A and then will drop as the pack voltage drops.
These packs happen to have Power Pole connectors on them and 4mm bullets will fit into them well enough for this use.
I placed the pack in a steel electonics cover that I use for charging lipos. If I was at home I would use a lipo sack. Some people will bury the pack in a bucket of sand. Whatever you do, make sure it is somewhere safe in case there is a problem with the pack. The pack was then connected to the discharger. I did this on my work bench in my office while working at my computer and I tried to check the pack every 10min, both to feel the pack to assess its "state of happiness" and to use a volt meter to check the progress.
The pack stayed relatively cool for most of the discharge but as the voltage dropped near the end, the last 10-20min, it began to build heat. When the lights finally dimmed all the way I removed the pack and checked the voltage and temp. The pack voltage slowly rebounded and the temp was at 120F. The pack had also swollen significantly. Again all of this happened in the last 10-20 minutes of the discharge. Concerning the voltage rebounding, this will always happen to some extent but once it will not light the bulbs in discharger, it can be considered fully discharged. To be safe make sure and measure the voltage afterwords to make sure it is well below 3V per cell. In the case above, the pack eventually rebounded to about 8V which is 2V per cell. Different packs will behave differently though.
The last step is to permanently connect the positive and negative pack leads together. I like to snip off the leads, to keep and reuse if needed, and solder the wires directly together. That way there is no way for the pack to hold voltage. When soldering the leads together, there will likely be a small spark from the residual energy in the pack. Once the pack leads are permanently connected, it can be considered safe.
Potential concerns and problems while discharging
The behavior of a discharging lipo is not always predictable. Some lipos will simply lay there and die, while others might put up a fight. Basically you are altering the internal conditions of the lipo by discharging it all the way, and that may very likely cause side effects like heat generation and swelling, and it could even get bad enough to result in a fire. There are just too many variables to take into account and so you have to be ready for anything.
An example of what can happen
When I discharged the 5 packs used as an example in this article, I had one unpredicted thing happen. The 3rd lipo I discharged puffed a set of cells as it neared the end of the discharge. They were hot and fully ballooned to the limits of the foil pouch each is encased in. I stopped the discharge immediately and placed the pack outside on the concrete to cool. The pack was almost discharged, so once was cool I continued the discharge. Once again the pack built a great deal of heat in the 2nd set cells. I ended up stopping once more to let it cool before getting it fully discharged. In the end it was not a big deal but it was a little worrisome to see one set of cells balloon like that.
As for what happened, that isn't too hard to figure out. This is an older pack and it has no balance connection. Over the years the 2nd cell became unbalanced, low actually. When I started the discharge all was well but as it neared the end, the 2nd cell was further discharged than the other three. This caused the resistance in that cell to rise before the others, causing it to build heat and swell. Because the pack lacked a balance connection, I had no way of knowing this was going happen. If the pack would have had a balance connection, I could have checked the cells and found out that one was low and at that point I would have chosen a slower, more gentle method of discharging it.
It is my hope that all RC hobbyists are responsible lipo users. This includes both using and maintaining lipos, as well as disposing of them properly when the time comes. All it takes is fully discharging a lipo to render it inert and safe, and then can be simply tossed in the trash. Being responsible for your lipos is not hard, it just takes some basic knowledge and a little bit of effort. So keep that in mind next time you have a lipo that needs retired.