In general, lithium-ion (Li-ion) batteries should not be stored for long periods of time, neither charged nor fully charged. The best storage method, as determined by extensive experimentation, is to store them at a low temperature, not lower than 0 °C, between 40 and 50% of their capacity. Storage at a temperature between 5°C and 15°C is optimal. The problem with a refrigerator is that it's a humid environment, which could cause parts of the battery to oxidize.
Therefore, before you put the battery in the refrigerator, be sure to put it in a Ziploc bag. If you want to take extra precautions, I also suggest that you place a desiccant in the Ziploc bag, along with the battery. Getting one shouldn't be too difficult: almost everything you buy today will come packed with one. However, if you can't find one, buy one on Amazon.
For example, I have 3 Canon LP-E6s and, if they are fully charged and stored even for a month (sometimes I forget to turn and use 2 batteries all the time), it will lose most of their charge. The following guide is based on batteries that are kept at the right temperature and humidity and in the correct state of charge. Most manufacturers recommend that, when stored for the long term, lithium-ion batteries be charged or discharged to approximately 40% of their capacity.
Lithium batteriesmust be kept in a state of charge (SoC) of between 40 and 50% to be ready for immediate use (that is, approximately 3.8 volts per cell), although tests have shown that if this type of battery is kept fully charged, the recoverable capacity decreases over time.
Not only do lithium-ion batteries last longer when stored partially charged, but they are also less volatile during transport in the event of an anomaly. Batteries are often exposed to unfavorable temperatures, and leaving a mobile phone or camera on the dashboard of a car or in the blazing sun is an example of this. Table 2 illustrates the remaining capacities of lithium and nickel-based batteries after one year of storage at different temperatures. This white paper is designed to give value to the reader through greater knowledge about lithium battery storage.
Lithium-ion batteries that have been under stress can operate normally, but are more sensitive to mechanical abuse. I always fully charge my batteries before storing them so that they don't run out completely when I decide to use them one day. If you store it in the fridge, you can charge it to 100% and use it immediately when you need it, while if you charge it to 40% and then need the battery without having time to charge it, you are semi-screwed. Batteries lose energy over time, so if stored uncharged, there is a chance that they won't accept a charge again.
If you follow the instructions above, your lithium-ion battery will be in good condition the next time you need it.