The battery memory effect is the tendency of rechargeable batteries to "remember" a certain percentage of their available capacity after they are repeatedly partially discharged. In other words, plugging in your cell phone every night at 10 p.m. when it is only 47 percent discharged may eventually strip away the remaining 53 percent of its battery's memory. But chances are, if a battery has a short lifespan, it isn't the memory effect; something else is responsible.
Actual battery memory loss is quite difficult to reproduce. Various scientific studies have concluded that if a battery is not consistently discharged to the same percentage (plus or minus three points) and fully recharged without being overcharged, true memory loss is practically impossible. Some batteries, even when subjected to the most rigorous charge-discharge cycles, persistently retained their full memory. So if a scientific expert has difficulty reproducing the memory effect, what is the most likely reason your battery loses some of its capacity over time?
Nickel-based batteries are vulnerable to a variety of problems. Consistent overcharge can create minute crystals which steal valuable charge capacity. Also, complete charge-discharge cycles may lead to the destructive reverse charging of cells. Another phenomenon is called "voltage depression," which appears to be quite similar to the memory effect. Operating in high temperatures may also lessen battery life. If a battery does suffer from the memory effect, it may be fixed by discharging each battery cell to 1.0 volts and then fully recharging. Batteries can also be shocked back into fuller capacity by a high-voltage charge.
But the most realistic reason why a battery is no longer operating at full capacity is simple: like all electrical devices, batteries don't last forever, and their charge capacity (Ni-Cad: 1000 cycles) depletes as time goes by.