Companies and individuals may store batteries outside the device as backup power when there is a power failure, or when storing extra battery packs. Another reason to store external batteries is when the device won't be used often as the device may drain the battery even when turned off due to internal monitoring systems.
Storage conditions for external batteries will be based on the battery's chemistry, such as lithium-ion, nickel-based, lead acid, or alkaline. Understanding the battery's chemistry and how it responds to the environment and self-discharge rates ensures that you can safely store the battery without damage and place it into immediate use later.
Considerations for External Battery Storage
There are a number of factors to take into consideration when storing external batteries: the room conditions, the self-discharge rate of the battery, the capacity, and the state of charge (SoC). When having the ideal conditions for the batteries, knowing how long it will take for the battery to complete discharge, and if the battery will become damaged if reaching a low state of charge, can help batteries last for many years.
Room Conditions
It's always ideal to store batteries in a cool and dry place no matter the chemistry. The recommended room temperature for most batteries is around 59°F. If you are storing a battery in a place that has fluctuating temperatures due to not having any HVAC systems in place, such as an outdoor garage or warehouse, you may have times when the temperatures go higher in the summer and are very cold in the winter.
Lithium-ion battery cells stored in climatically controlled warehouse for peak battery performance.
The extreme high-temperature limit for most batteries will range from 112°F to 122°F. If undergoing extremely cold temperatures, some battery chemistries withstand the cold better than others.
- Lead acid: Can withstand low temperatures yet should not be allowed to freeze. The water in the electrolyte will expand when frozen and damage the battery.
- Alkaline: Alkaline can handle temperatures as low as -42°F, yet the minimum should be about -15°F to avoid damage. Before placing the batteries into use, they should be warmed up slowly.
- Lithium-ion: Lithium-ion batteries should never be stored below 32°F. Recent research has discovered that storing these batteries below this temperature may cause the parts to separate and crack.
- Nickle-based: Like alkaline batteries, nickel-based chemistries can also handle lower temperatures without experiencing significant damage. Minimum storage temperatures range around -4°F.
Self-Discharge Rates and SoCs
When it comes to self-discharge rates of external batteries in storage, keep in mind that higher room temperatures cause faster discharging. For this reason, keeping the room cold and dry can minimize fast discharging. All batteries go through self-discharge, yet the rates will vary based on chemistry.
Lead acid batteries go through a discharge rate of 5% every month and can last for up to 2 years in storage. The batteries should be stored while fully charged and undergo charging when the capacity falls below 70% state of charge.
Lithium-ion batteries can be partially discharged when placed into storage yet should never have a voltage drop below 2.0v or it will experience permanent damage. A charge should be given to maintain it above this voltage but not exceed 4.1v. This chemistry will go through a 1.5% to 2% discharge rate every month when in long-term storage. The batteries can last about 2 ~ 3 years in a stored state.
Nickel-based batteries experience the highest self-discharge rates when compared to other battery chemistries. They may lose from 10% up to 25% per month. These batteries can be stored fully charged or discharged. They will also need to be recharged if they are stored for very long periods. For nickel-cadmium batteries, they should undergo 2-3 deep discharge cycles to reach their full capacity. The batteries can be left in storage for 3 years to 5 years.
Alkaline batteries can be stored for up to 10 years while going through moderate capacity loss. The batteries will go through a 2% to 3% discharge rate per month. Never freeze the batteries in an attempt to prolong the battery.
Capacity Loss
Battery chemistries will underdog capacity loss went they undergo self-discharging. They also go through capacity loss that is not recoverable. This capacity loss will be based on how long they are in storage and the temperature in the stored room. If the battery is stored at an even rate of 32°F and 77°F for one year, there may be a significant change in the recoverable capacity depending on the battery's chemistry.
- Lithium-ion batteries at 100% charge: 94% capacity at 32°F and 80% at 77°F
- Lithium-ion batteries at 40% charge: 98% capacity at 32°F and 96% at 77°F
- Lead acid batteries at full charge: 97% capacity at 32°F and 90% at 77°F
- Nickel-based (at any charge): 99% capacity at 32°F and 97% at 77°F
As the temperatures rise, the capacity loss increases, and recoverable rates are lower for certain batteries. At 104°F, lead acid batteries and lithium-ion (100% charge) batteries can lose up to 30% of their recoverable capacity. Nickel-based batteries have the least amount of capacity loss at this temperature.
Summary
Preventing damage to stored batteries involves creating a monitoring schedule to check the state of charge of the batteries and providing a charge to specific battery chemistries. Any damage from the environment, such as humidity, moisture, or mishandling, could cause a higher capacity loss or permanent damage. If the stored external batteries are not charging correctly after taking them out of storage, it is better to discard the batteries and replace them for the specific device.
