Battery memory effect: chemistry and capacity
Battery memory effect has a specific meaning that differs from the broader experience of a battery seeming to lose charge. It describes nickel-cadmium rechargeable batteries gradually losing maximum energy capacity when repeatedly recharged after only partial discharge. The battery appears to retain the smaller capacity as a kind of memory. Lithium-based cells are not normally subject to this behavior, so fully draining a lithium phone battery is not required to prevent it. A changing charge indication can have other explanations, including voltage changes that make a battery control system perceive a decrease in capacity.
The term began with an aerospace nickel-cadmium application under tightly controlled conditions. Computer control repeatedly discharged the cells to 25% of available capacity, with a variation of about 1%, and then recharged them to 100% without overcharging. That long-running, repetitive sequence produced a loss of capacity beyond the discharge point. The conditions were unusually exact: true memory cannot occur if the batteries reach full overcharge, if discharge varies between cycles beyond plus or minus 3%, or if discharge reaches less than 1.0 volt per cell. The pattern depended on consistent repetition rather than simply a battery receiving an occasional partial charge.
Cell construction also matters. True memory effect is specific to sintered-plate nickel-cadmium cells and is particularly difficult to reproduce in cells with lower ampere-hour capacity. A test designed to induce it found none after more than 700 precisely controlled charge and discharge cycles using spirally wound one-ampere-hour cells. A follow-up used 20-ampere-hour aerospace-type cells with a similar cycle pattern, and the effect appeared after a few hundred cycles. Those contrasting results make the construction and capacity of the test cells part of the explanation, alongside the charging routine. Even deliberate attempts to create the behavior did not produce the same outcome in every design.
Voltage depression is a different phenomenon often given the memory effect label. Here, output voltage falls more quickly than normal during use, although total capacity remains almost the same. Electronic equipment that uses voltage to indicate battery charge can then show rapid depletion. Repeated overcharging can cause small solid electrolyte crystals to form on the plates. As these grow, they cover surface area used for ion exchange, increase resistance, and lower the voltage of individual cells. The whole battery can appear to discharge rapidly as affected cells lose voltage. Nickel-metal hydride batteries can experience this form of apparent capacity loss as well.
The crystalline growth can also follow a decrease in internal battery temperature. Electrolyte may become supersaturated and form crystals on the charged sheets, reducing the concentration of ions in the liquid. These structures are called whiskers, dendrites, or crystals when they grow to significant lengths. For this particular voltage-depression problem, deep charge and discharge cycles applied to individual cells can generate heat in the very narrow, high-resistance whiskers and dissolve them back into the electrolyte. The distinction between an individual cell and a multi-cell battery is essential here: cells in a pack may discharge at different times, exposing depleted cells to reverse current from those still carrying charge.
Repeated deep discharge can cause permanent capacity loss in some rechargeable batteries. A pack contains similar cells, but their capacities are not identical. During a deep discharge, the cell with the smallest capacity may reach an empty state while the other cells continue forcing current through it. That cell can then experience reverse charging. Fully discharging a pack in an effort to prevent memory effect may therefore damage its weakest cell. Each additional full discharge can deepen that damage, creating an imbalance that limits the pack's overall performance. The consequences can include reduced capacity, shorter run times, and potential overcharging or overheating of the other cells.
Other operating conditions can affect battery behavior without that tightly controlled nickel-cadmium pattern. High temperatures can reduce both charged voltage and the amount of charge cells accept. Operation below 32 degrees Fahrenheit, high discharge rates above 5C in batteries not designed for them, inadequate charging time, and a defective charger are also possible causes of poor performance. Certain battery types release gas and need their electrolytic fluid levels maintained or refilled to help prevent whisker formation. These mechanisms involve temperature, charging equipment, electrical demand, and electrolyte levels, giving the same broad symptom several distinct physical explanations.
Age introduces another process separate from memory effect. All rechargeable batteries have finite lifespans and slowly lose storage capacity through secondary chemical reactions, whether they are being used or left idle. Some cells can fail sooner than others, reducing the battery's voltage. Lithium-based batteries have relatively long idle lives, but their operational lives still span approximately 400 to 1200 complete charge and discharge cycles. Higher temperatures and higher states of charge shorten their lifetimes even when they are unused. Storage conditions therefore continue to affect the cells while a device is sitting aside, with chemical aging proceeding beyond the moments when charging or active use is visible.