Best Practices for Charging Lithium-Ion Batteries

Q. Is it OK to leave a tool battery—specifically a lithium-ion battery, which I assume is what most tool batteries are these days—on its charger for extended periods of time, even after it is fully charged? (By extended, I mean for a day or a week or even longer.) Does it harm the battery to leave it in a cold truck or shop overnight, or longer?

A. Dave Veprek, vice president of product development at SBD Inc., which produces DeWalt tools, responds: It is OK to leave batteries in DeWalt chargers, because our lithium chargers stop charging once the battery reaches a fully charged state. To the best of my knowledge, this is true industry-wide with lithium-ion batteries and chargers, and not specific to our company.

Generally speaking, Li-ion batteries have a low self-discharge rate of 2% to 3% per month. In comparison, the self-discharge rate for a standard lead-acid battery (the kind that is used in cars) is between 4% and 6% per month; earlier-generation NiCad or NiMH tool batteries were plagued with much higher self-discharge rates—as much as 20% per month for NiCad batteries and 30% per month for NiMH batteries. As a result, the battery chargers used for most modern Li-ion tool batteries don’t have a “trickle charge” mode; they just shut off once the battery is charged.

Another advantage of Li-ion batteries over older NiCad or NiMH batteries is that they don’t have what is called a battery memory effect, in which the battery “remembers” where it was in the charge/discharge cycle when it was most recently recharged, and tends to return to that point during the next recharge. Because a Li-ion battery doesn’t have a memory, you don’t have to worry about damaging or shortening the life of a partially discharged battery by throwing it onto a charger at lunchtime. And you don’t have to worry about “killing” a DeWalt battery by fully discharging it because our system has a cut-off that stops the tool from damaging the battery. I assume that this is true of the Li-ion batteries of other tool manufacturers, as well.

Cold weather affects how quickly any battery will charge, because it increases the battery’s impedance, or internal resistance, so that the battery isn’t able to accept the same amount of charging current as it can when it is warmer.

This is especially true of Li-ion batteries that are made with conventional organic carbonate solvents, which are more susceptible to reduced ion mobility than aqueous solvents found in NiCad and lead-acid batteries. This can lead to lithium plating during charging and also sluggish reaction and reduced power output in cold weather. It is a little hard to generalize, though, since there are many types of battery chemistries, though one positive effect is that cold temperatures reduce the self-discharge rate of Li-ion batteries even further, making it actually advisable to store a Li-ion in the cold.

Still, for best performance, it is a good idea to warm up a cold battery before use. In general, our recommendation is to store batteries in a location that is between 32°F and 104°F, and not to store or use the tool and battery pack in locations where the temperature may reach or exceed 104°F (40°C). However, I advise to always check the instruction manual for your batteries.

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