Tips on Storing Batteries

  • When storing, remove the battery from the equipment and place in a dry and cool place.
  • Avoid freezing. Batteries freeze more easily if kept in discharged state.
  • Charge lead acid before storing and monitor the voltage or specific gravity frequently; apply a charge if below 2.07V/cell or if  SG is below 1.225 (most starter batteries).
  • Nickel-based batteries can be stored for 3–5years, even at zero voltage; prime before use.
  • Lithium-ion must be stored in a charged state, ideally at 40 percent. This prevents the battery from dropping below 2.50V/cell, triggering sleep mode.
  • Discard Li-ion if kept below 2.00/V/cell for more than a week. Also, discard if the voltage does not recover normally after storage.

Nickel-based batteries can be stored in a fully discharged state with no apparent side effect, and Li-ion cannot dip below 2V/cell for any length of time. Copper shunts form inside the cells that can lead to elevated self-discharge or a partial electrical shortIf recharged, the cells might become unstable, causing excessive heat or showing other anomalies. Li-ion batteries that have been under stress may function normally but are more sensitive to mechanical abuse. The liability for a failed battery goes to the manufacturer when a fault could have been caused by improper use and handling.

Finding the exact 40–50 percent SoC level to store Li-ion is not all that important. At 40 percent charge, most Li-ion has an OCV of 3.82V/cell measured at room temperature. To get the correct reading after a charge or discharge, rest the battery for 90 minutes before taking the reading. If this is not practical, overshoot the discharge voltage by 50mV or go 50mV higher on charge. This means discharging to 3.77V/cell or charging to 3.87V/cell at a C-rate of 1C or less. The rubber band effect will settle the voltage at roughly 3.82V. Figure 1 shows the typical discharge voltage of a Li-ion battery.

You can store a sealed lead acid battery for up to 2 years. Since all batteries gradually self-discharge over time, it is important to check the voltage and/or specific gravity, and then apply a charge when the battery falls to 70 percent state-of-charge, which reflects 2.07V/cell open circuit or 12.42V for a 12V pack. (The specific gravity at 70 percent charge is roughly 1.218.) Lead-acid batteries may have different readings, and it is best to check the manufacturer’s instruction manual. Some battery manufacturer may further let a lead acid to drop to 60 percent before recharge. Low charge induces sulfation, an oxidation layer on the negative plate that inhibits current flow. Topping charge and/or cycling may restore some of the capacity losses in the early stages of sulfation.