Many radio-control enthusiasts experience disappointment with the cycle life of their Lithium-based batteries in electric aircraft. Often this is because they're not entirely sure what's going on inside the cheap lipo RC batteries, and choose a capacity or voltage that's inappropriate for their application. Ultimately, this manifests itself in "swelling" or "ballooning" of a Lithium battery. This editorial attempts to explain what's actually going on when this happens.
Chemically, there can be three causes for the swelling of a LiPo battery, and one exacerbating condition that makes it worse across the board. These occur in hard-shell Lithium Ion batteries, too, but the hard shell can withstand several atmospheres of pressure before expanding.
Puffing can also occur if cells are overcharged or charged too quickly. When this happens, you end up with excess free lithium on the anode (metallic lithium plating), and free oxygen on the cathode. A free oxygen atom is small enough to freely move over the separator without carrying an electric charge, resulting in lithium oxide or lithium rust. You then have the problem where lithium oxide uses fewer oxygen atoms than existed in its ionized state and the result is more free oxygen. To emphasise this point, if you were to overcharge a LiPo battery underwater, it would still catch fire as it produces its own oxygen supply!
Over discharge lithium polymer batteries or discharge them too quickly, and the reverse to the above takes place. You end up with lithium oxide on the cathode, albeit at a lower rate because there’s simply less there. Basically, an abused battery quickly develops corrosion on both poles of the battery, inside the case. The more you abuse the battery, the worse the corrosion gets. With corrosion taking place, the resistance goes up and the battery then has to work even harder which causes more damage, so it’s a vicious circle! Regardless of quality, over-discharging lithium polymer batteries is the most common cause of puffing, so hopefully this emphasises the problem and reminds you just how damaging it is to flatten your batteries!
Here are some answer from experienced hobbist.
If a Lipo pack is drained below lipo 2s battery it can irreversably damage the pack. How have you been judging when your pack is depleted? Are you timing your runs after you first found out how long they can go from previous runs so you can creep up on a runtime that stops short of drawing them down below 3 volt per cell. Of coarse if any changes are made like prop size or trim adjustment even can change your safe runtime considerably.
Hobbyists using lipo for high power toys will have the best advice about how to handle them. Lipo-specific chargers will monitor each cell and balance their charge levels. Never exceed 4.2v in any cell, never drop below 3.2v in any cell under load (about 80% depleted by the mAh rating), and remember that voltage varies with temperature so don't leave them where it's warm after topping off the charge. They will puff gently as they age but beware any significant puffing. To decommission a lipo that you don't want (dead or not), use an insulated-handled steel awl or knife and puncture all cells, then leave the steaming mess in a bucket of water for a day or two outdoors. This renders the resulting lithium bag harmless. People have lost cars and houses to lipo fires. It is one suspect for flight MH370's disappearance. Getting them shipped many places requires surface transport now.
Chemically, at some point in a LiPo’s life, it will puff regardless, so we can only prolong the effects of puffing by treating our batteries with a level of respect they deserve. If you have a pack that has started to puff and you continue to use it in the same application, it will only get worse and should this occur the cells should be discarded fairly quickly! A little puffing is a sure sign that the pack is dying and must be used with caution. Once a battery has cooled down, the puffiness can reduce and most will generally keep using the battery, so if that’s you then keep on eye on it. Ideally, you should stop using a battery once it remains in a puffed state as you run the risk of fire, should the cell rupture, so be warned!
When to Retire Lithium Polymer Batteries?
There are so many variables that determine the lifespan of your batteries and if a pack is abused, it will only last a handful of charges, yet if treated properly, you’ll see 200+ cycles from your packs. This of course depends on your application. As a general rule, when your battery no longer holds more than 80% of its original capacity, it’s time to retire your pack and certainly if there is excessive puffing.
Unlike NiCd or NiMH batteries, Lithium Polymer batteries are environmentally friendly. For safety reasons, it’s best that lithium polymer batteries be fully discharged before their disposal and this should of course take place outdoors in the open for obvious reasons. A slow discharge to completely flat with a bulb or series of bulbs is now what I do. Once all cells are showing zero voltage, it is pretty safe to assume the pack is now inert. Unlike other batteries, inert LiPos can be simply thrown in your normal trash. If a battery is physically damaged then, discharging fully could be dangerous and greater care is required when disposing! Search the Internet and you’ll soon find various recommended solutions for disposal. YouTube is a great source of how NOT to do it and if you’ve not seen a fire, then please have a look, it’ll certainly make you think!
That’s about it for this instalment. Hopefully this feature will help you appreciate and get the very best from your batteries. This subject is an ever-evolving subject. I’m always open to feedback and would love to hear from other people and their real world experiences. For me its all about spreading the word with accuracy and integrity. Without our batteries we couldn’t do what we do!