Proper 18650 Battery Storage
Have you ever wondered if just putting your batteries in the fridge is the proper way to store them and extend their usability? We can answer that.
The recommend storage temperature for most batteries is indeed 15 degrees Celsius (hence your fridge). The lowest they should be stored is minus 40 to minus 50 Celsius. Lead acid should be kept at full charge during storage, but nickel and lithium based make-ups should be stored at around 40% state of charge (SoC) to minimize age-related capacity and yet allow the battery to self-discharge and stay in operating condition.
It is hard to that magic 40% SoC since the open circuit voltage (OCV) of batteries is difficult to estimate. The best way is to use voltage as a rough energy indicator. The SoC of lithium-ion is about 50% at 3.80V/cell and 40% at 3/75V/cell. The lithium-ion should be allowed to rest for about 90 minutes after charge or discharge before checking the voltage.
SoC on nickel-based batteries is the most difficult to determine. This is caused by a flat discharge curve, agitation after charge and discharge, and voltage change on temperature. There really is no practical estimation tool for nickel-based batteries. Since the storage charge level is not critical, just add some charge if the battery is empty (don’t if it is not), and then keep it in a cool dry storage place.
Storage always causes batteries to age. On the other hand, elevated temperatures speed up permanent battery capacity loss. Lithium-ion batteries in particular are often exposed to high temperatures; consider how many people leave their cell phones or laptop computers in hot places like the front seat of their car on a sunny day. Conversely, low temperatures and partial SoC slow the aging process, but cannot stop it.
Another factor that will shorten the life of your battery is overcharging it, or keeping it at the maximum charge voltage for extended periods. Batteries are like people: they must have the opportunity to relax after being charged, even if they are just allowed to stay on a float or trickle charge.
A sealed lead acid battery can be stored for up to two years. However, it is important that you monitor the voltage or gravity and charge up the battery if the battery falls to a 70% SoC or lower. Low charge causes an oxidation layer to form on the negative plate and inhibit the flow of current; this is called sulfation, which can prevent the charging of small sealed lead acid cells.
Here are some basic guidelines to follow when storing your batteries:
– Remove the batteries from the equipment and store them in a cool dry place.
– Lead acid must be charged before storing; monitor the voltage or gravity often and apply a boost if it falls below 2.10V/cell or an SG below 1/225.
– Avoid freezing, and remember that batteries in a discharged state freeze more easily than those that are not.
– Alkaline and primary lithium batteries can be stored for up to ten years with moderate loss of capacity.
– Nickel-based batteries can be stored for five or more years, even at zero voltage; then prime them before use.
– 18650 batteries must be stored in a charged state; 40% is ideal. This will ensure that the battery will not drop below 2.50V/cell with self-discharge and go dormant. Discard them if the voltage goes below 2.00V/cell for more than seven days.
Following these simple guidelines should help you extend the life of your batteries and get more use out of them. If you are using a laptop rental, ask the rental provider what battery care is taken; you do not want to rent a laptop that will not perform at its peak when you most need it.
Remember to always handle your batteries safely. Wear safety gloves when handling lead, cadmium or electrolytes. If a battery were to ever rupture while you are handling it, and you are unable to avoid exposure, flush with water immediately and call your doctor.