Batteries in a Portable World 2^nd Ed.
       A Handbook on Rechargeable Batteries for Non-Engineers

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15.1 Storage

Batteries are a perishable product and start deteriorating right from the time they leave the manufacturing plant. For this reason, it is not advisable to stock up on batteries for future use. This is especially true with lithium-based batteries. The buyer should also be aware of the manufacturing date. Avoid acquiring old stock.

Keep batteries in a cool and dry storage area. Refrigerators are recommended, but freezers must be avoided because most battery chemistries are not suited for storage in sub-freezing temperatures. When refrigerated, the battery should be placed in a plastic bag to protect it against condensation.

The NiCd battery can be stored unattended for five years and longer. For best results, a NiCd should be fully charged, then discharged to zero volts. If this procedure is impractical, a discharge to 1V/cell is acceptable. A fully charged NiCd that is allowed to self-discharge during storage is subject to crystalline formation (memory).

Most batteries are shipped with a state-of-charge (SoC) of 40 percent. After six months storage or longer, a nickel-based battery needs to be primed before use. A slow charge, followed by one or several discharge/charge cycles, will do. Depending on the duration of storage and temperature, the battery may require two or more cycles to regain full performance. The warmer the storage temperature, the more cycles will be needed.

The Li-ion does not like prolonged storage. Irreversible capacity loss occurs after 6 to 12 months, especially if the battery is stored at full charge and at warm temperatures. It is often necessary to keep a battery fully charged as in the case of emergency response, public safety and defense. Running a laptop (or other portable device) continuously on an external power source with the battery engaged will have the same effect. Figure 15-1 illustrates the recoverable capacity after storage at different charge levels and temperatures.

The combination of a full charge condition and high temperature cannot always be avoided. Such is the case when keeping a spare battery in the car for a mobile phone. The NiMH and Li-ion chemistries are most severely affected by hot storage and operation. Among the Li-ion family, the cobalt has an advantage over the manganese (spinel) in terms of storage at elevated temperatures.

Temperature	40% charge level (recommended storage charge level)	100% charge level (typical user charge level)
0°C	98% after 1 year	94% after 1 year
25°C	96% after 1 year	80% after 1 year
40°C	85% after 1 year	65% after 1 year
60°C	75% after 1 year	60% after 3 months

Figure 15-1:  Non-recoverable capacity loss on Li-ion batteries after storage.
High charge levels and elevated temperatures hasten the capacity loss. Improvements in chemistry have increased the storage performance of some Li-ion batteries.

The recommended storage temperature of a lithium-based battery is 15°C (59°F) or less. A charge level of 40 percent allows for some self-discharge that naturally occurs; and 15°C is a practical and economical storage temperature that can be achieved without expensive climate control systems.

While most rechargeable batteries cannot be stored at freezing temperatures, some newer commercial Li-ion batteries can be kept at temperatures of -40°C without apparent side effects. Such temperature tolerances enable long and cost-effective storage in the arctic.

The SLA battery can be stored for up to two years but must be kept in a charged condition. A periodic topping charge, also referred to as ‘refreshing charge’, is required to prevent the open cell voltage from dropping below 2.10V. (Depending on the manufacturer, some lead acid batteries may be allowed to drop to lower voltage levels). When self-discharged below a critical voltage threshold, sulfation occurs on most lead acid batteries. Sulfation is an oxidation layer on the negative plate that alters the charge and discharge characteristics. Although cycling can often restore the capacity loss, the battery should be recharged before the open cell voltage drops below 2.10V.

The SLA cannot be stored below freezing temperatures. Once a pack has been frozen, it is permanently damaged and its service life is drastically reduced. A previously frozen battery will only be able to deliver a limited number of cycles.

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