The Mysterious Memory Effect of Rechargeable Batteries

Computerworld - Memory effect refers to a temporary or permanent loss of capacity in a battery when it's discharged. Experts don't totally agree on why this happens or how to keep it from lessening the capacity of batteries.

Analysts at Frost & Sullivan Inc. in Houston say the phenomenon occurs most often in nickel-cadmium (Ni-Cad) cells, when they're discharged to a certain point and recharged a successive number of times. They say you can avoid memory effect by discharging the cell completely during each use and before recharging it.

But what memory effect is has been a point of confusion, some experts say. "Memory effect has become a catch-all phrase for any bad experience associated with battery products," says Paul True, president of NRG Research Inc. in Grants Pass, Ore., which makes Ni-Cad cells.

True says memory effect usually refers to voltage depression, which is an abnormal dip in discharge voltage. It slowly progresses from the end of a discharge cycle toward the beginning over the life of a cell. If the equipment being powered has a preset shut-off point, it may mistake the voltage dip for an empty cell and shut down prematurely.

Voltage depression is primarily caused by overcharging a Ni-Cad cell, True says—when a charging device fails to sense when a charge is complete, coupled with a demand for shorter charge times. But True says he believes that discharging the cell fully and recharging it can easily erase the voltage depression, echoing the advice given by Frost & Sullivan. The discharge needs to be down to less than 1 volt per cell, he says.

The life expectancy of most batteries subjected to a full discharge is shorter than that for cells subjected to shallow charge and discharge cycles, True says. That presents a dilemma for users: either discharge fully and shorten battery life or discharge less than fully and face memory effect. Some manufacturers now offer overcharge-tolerant cells and better charging systems to help resolve the issue.

True says he believes memory effect isn't really a problem for today's Ni-Cads in cell phones, since their chargers are more precise in preventing an overcharge. The memory effect was a bigger concern several years ago, when Ni-Cads were introduced for video equipment batteries as an alternative to lead-acid batteries. For professional video equipment, when cells hold a large charge and can be charged at fast rates, voltage depression can be a concern and more precise charging devices are necessary, True says.

"But for the most part, memory effect doesn't impact the average cell phone or appliance user any more," he notes.