Caring for lithium-ion batteries
Although the new tech lithium-ion batteries offer a lot of benefits over the older-tech NiMH battery type, including a longer expected lifetime, correct care will still help extend the lifetime of your lithium-ion battery, and therefore of your appliance or tools.
Correct care is even more worthwhile for appliances where the batteries are not removable, or cannot be replaced separately to the appliance itself, as for many cordless items it is the battery, not the appliance itself, that determines the working life of the tool.
For the older NiMH batteries, a regular full discharge and full charge, were recommended. However for lithium-ion batteries, partial discharge is a better method to extend their lifetime, where you use for example 20% or 30% of the battery capacity, before recharging.
Keeping the battery always fully charged can shorten the battery life over time. A full or deep discharge can also reduce battery lifetime and cause damage. Unless your appliance has a built-in monitor that reduces or turns off charging when the battery is almost full, also called trickle charging, it's best to leave your battery on charge only as long as is required to reach below 100% charge. The occasional full discharge, followed by a full charge, is still recommended occasionally, every 30 or so charge cycles.
Room temperature is best. A range of 20 to 25 degrees for use, storage and charging is ideal. Like other battery types, extreme temperatures should be avoided. Avoid charging when the temperature is below 0 Celsius. Avoid running an appliance for longer than needed, as any battery will heat up with extended use and heat shortens a battery's lifetime.
Although lithium-ion batteries don't heat up as much as other types, they should still not be left on charge longer than required, or exposed to extreme temperatures (such as being left in full sun or in a closed car in summer). Heat can be the biggest avoidable factor in reducing battery lifetime.
One or two batteries
Batteries deteriorate over time, whether they are in use or not. A spare battery that's not in use won't last much longer than the one in use. It is normal for batteries age over time, even if they are lithium-ion.
If you know you'll be putting a battery in storage for an extended time, store it at about 40% charge. Storing a battery with no charge (or a charge level low enough that it will run-out while in storage), can cause a deep discharge, and as a result, the battery may be unable to recharge later when taken out of storage. Extended storage would be multiple months. Be mindful of temperature extremes when storing. The old advice of storing batteries in your fridge still applies (fridge, not freezer!).
How long should a lithium-ion battery last?
'Should' is the difficult part as the care for the battery has a big impact on its lifetime. With excellent care, a quality lithium-ion battery may last 300 to 500 charge cycles. Heavy use, heat and other factors can shorten this lifetime. Some batteries have built-in protection from full discharge and over-charging, which helps achieve the full lifetime.
A full charge and discharge shortens the number of charge cycles a lithium-ion battery will last. Smaller discharges extend the number of charge cycles a battery will last. Exposure to high temperatures (above 30 degrees) is generally considered an elevated temperature, and when combined with being fully charged, can have the biggest impact on shortening a battery's lifetime.
It's difficult to give a number on years for how long a lithium-ion battery will last, but 2 to 3 years use is considered 'average'. Batteries in storage, even if never used, will lose about 20% of their capacity per year. Stored in good conditions and never used, a lithium-ion battery may have a shelf life of 3 to 4 years.