From the RC LiPo battery page, we now know how a LiPo battery is made, how it works, the safety concerns, what to look for when purchasing one, how to charge and balance one, why over discharging is so harmful, internal resistance, what puffing is all about, and breaking in; what more can there be to cover?
RC LiPo Battery Storage!
How you store your LiPo batteries between uses will greatly affect their life span and even how safe (stable) they are.
As I mentioned on the LiPo page, a LiPo cell that drops below 3 volts under load (about 3.5V open circuit voltage) is almost always & irreversibly damaged.
It will have reduced capacity, puff up or not be able to accept a charge or deliver current after a charge due to cell oxidation and/or electrolyte gassing / decomposition.
If your RC LiPo batteries are stored for any period of time in a near discharged state, you risk irreversible LiPo damage as well. 3.6 volts is the number given by many LiPo manufacturers as the absolute minimum open circuit resting voltage; however, that is most certainly not a safe storage voltage.
As batteries sit, they will naturally self-discharge.
LiPo batteries are actually very good in this respect and self-discharge much slower than most other rechargeable battery types, but they still do lose voltage potential as they sit. If you leave them for a number of weeks or months in a near fully discharged state (3.6V per cell), chances are they may be irreversibly damaged.
You must store them charged, but not fully charged either; that will also degrade and oxidize the cell matrix or cause electrolyte gassing.
Fully charged LiPo batteries are not happy and must be used soon after they are fully charged!
Basically, the speed at which a LiPo pack ages during storage is based on both storage temperature and state of charge.
You are likely okay to store a fully charged RC LiPo battery at room temperature for up to 2-3 days without doing too much damage. I follow two days maximum myself, and try not to go over one if at all possible.
Never ever store a LiPo in a hot car or hot shed fully charged for an extended time, that will certainly cause damage (puffed and may even vent) as I explained on the LiPo battery page, but it's worth repeating.
For optimum battery life, store your RC LiPo batteries in a slightly cool room (20C / 68F) or a bit lower if possible, 20% to 70% relative humidity, at about a 40-60% charged state.
This equates to LiPo storage voltage of around 3.80 to 3.90 volts per cell (open circuit resting voltage) as shown on this RC Battery Meter (such a useful tool in this hobby).
I've seen a few LiPo battery manufacturers list storage state numbers as high as 30-80%; about 3.75V to 4.00V open circuit voltage. However, since most computerized RC chargers set the LiPo storage charge at 40% to 50% (3.80 to 3.85 volts per cell), and most good quality LiPo packs come from the factory around that voltage range as well, that's what I recommend and what I follow myself.
Personally, I like storage charging my LiPo packs to about 50% capacity; only because the full charge cycle when I'm rushing to go flying is slightly shorter in time then if they were at a 40% charged state. It's not much time saved mind you, but when you and your birds are chomping at the bit to get airborne, every single minute saved counts :)
As I mentioned at the top of this page, and on my RC LiPo Battery Page in the LiPo Safety section; I personally store all my LiPo's in a metal tool box/chest.
I also have a smoke detector in close proximity to my LiPo storage area and a fire extinguisher.
I also mention in that LiPo safety section other good low cost RC LiPo Battery storage ideas such as concrete blocks, repurposing old BBQ's, clay pots, low cost army surplus metal ammo boxes, or making your own LiPo storage containers out of cement board.
The options are only limited by your imagination and of course that your RC LiPo storage container will properly contain a flaming LiPo or a few LiPo's if they were ever to go all out napalm on you. In short, the container must be made from a material that doesn't burn while it contains rapidly oxidizing LiPo electrolyte goo!
It's very unlikely a LiPo will catch fire while in storage, but if you are the type of person where the only luck you have is bad, it's not 100% impossible.
Anecdotally, I've never had a LiPo erupt into napalm while in storage over the past 15 years. I have however had two separate occurrences over that decade+ where worn LiPo power leads accidently shorted together, but no fire; only a couple ballooned hot packs that vented and smoked. I'm of course very glad they were in a metal chest when it happened and not a wooden or plastic one.
You'll sleep better at night and while away from home knowing your LiPo's are in a fire safe container of some sort; at least I sure do.
If you don't want to build your own fire proof RC LiPo battery storage container or convince your better half you need a shiny new BBQ so you can use the old crappy one for your RC LiPo battery storage, there are many options you can purchase as well.
Of these, the compartmentalized square LiPo storage bag types in my opinion are much better than the thin LiPo storage bags (for larger LiPo packs anyway), because most box type bags are made from a thicker fire proof material.
Here is just one example.
Stay LiPo Safe!