It is often very handy to store batteries for later use. If you know which is the right battery for your device it is a good idea to have a few units of it in order to quickly replace it when you need to. But storing batteries has to be done the right way in order for it to be safe and efficient. Here are a few guidelines for storing your batteries:
- Use a dry and cool place for storage. Remove batteries from the device before storing.
- If the voltage of a Li-ion battery has stayed below 2.00/V/cell for more than a week it is time to discard.
- Charge lead acid before storing and monitor the voltage or specific gravity frequently; Boost if below 2.10V/cell or a specific gravity below 1.225.
- Nickel-based batteries can be stored for years even at a discharged state but must be primed before use.
- It is optimal to store Lithium-ion batteries at 40 % state of charge
Most batteries should be stored at a recommended temperature of 15 ℃ (59℉). Lead acid batteries should be kept at full charge but lithium based ones - at 40% state-of-charge (SoC). This is done in order to minimize age-related issues such as capacity loss. It allows self-discharge and keeps the battery in operating condition. The voltage of the battery is used as a rough fuel gauge indicator as it is difficult to find the exact 40% level. Lithium-ion (Li-ion) batteries have a state-of-charge of roughly 50% at 3.80V/cell and 40% at 3.75/cell. Before measuring the voltage it is important to allow 90 minutes after charge/discharge to get a balanced reading. Keeping the temperature and SoC right will slow the effect of aging of the batteries.
High temperatures and overcharging are harmful for the life expectancy of all battery types. Since Lithium-ion batteries are used for cell phones and laptops, they suffer a lot from these problems. Leaving your cell phone in the hot sun or leaving the laptop plugged in the grid even after the battery is charged are examples of bad maintenance. Always remember not to let the battery sit at maximum charge voltage for long periods.
Nickel based batteries generally store well. During storage a drop in capacity often occurs. This can be reversed to some degree through priming. It is necessary to prime if the voltage drops below 1V/cell.
Sealed lead acid aka SLA batteries can be stored for up to two years. It is important to apply a charge when the battery falls to 70% charge. This means around 2.07V/cell and a specific gravity of roughly 1.218. Low charge can induces sulfation, which lowers charge acceptance and makes charging take more time. It is basically a oxidation layer that deposits on the negative plate and inhibits current flow. Small SLA cells may be unable to charge due to sulfation. Often a higher than normal voltage can help. At first, the cell voltage under charge may go up to 5V and absorb only a small amount of current, but in time the current converts the sulfate crystals into active metal. Also the cell resistance drops and the charge voltage gradually normalizes. At a voltage of 2.10–2.40V the cell is capable of accepting normal charge. When charging an SLA with with a higher voltage, current limiting must be applied to protect the battery. Always choose the lowest setting and be sure to monitor the voltage and temperature Do not attempt to perform this service if the power supply does not allow setting current limiting.