Q:

Some say that if you run the battery on a cordless power tool completely dead before charging, the battery will last longer. Others say the opposite is true. What is the truth about batteries?

George Davis, via email, None

A:

Senior editor Justin Fink replies: Memory effect is the popular term for a battery’s lack of run-time or life cycle. The truth is that battery memory is largely a myth.

The term memory effect more accurately refers to cyclic memory. The concern was that if a battery is habitually recharged before being entirely drained, it can “forget” that it can discharge completely. This myth spread like wildfire, and people began to “condition” their batteries by deliberately draining them before recharging them, placing them in the freezer between uses, or monitoring charge times to avoid the risk of “overcharging.”

Cyclic memory isn’t a concern with Li-ion tools, and even if you have NiCd or NiMH batteries, memory effect is still only a slim threat.

Cyclic memory is mostly a problem in situations where a battery is drained at the same rate to the exact same point over and over. Usually, batteries in power tools are used to drill large-diameter holes in the morning and a box worth of drywall screws in the afternoon. In other words, these tools aren’t performing and discharging at the constant rate necessary to create a problem.

What most people blame on memory is probably either age or that the battery cells have been drained to the point of damage. It’s tempting to force every last electron out of a battery to finish sinking a screw, but doing so can damage cells. Several Li-ion batteries are now electronically protected to prevent this deep discharge; this quality is responsible for the odd experience of driving a screw one second and having no power the next.

In the end, the best way to prolong battery life is to put the battery on the charger at the first sign of a power decline.

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