The “memory effect” describes a phenomenon where a rechargeable battery appears to lose its maximum energy capacity if it is repeatedly recharged after only being partially discharged.
Understanding Memory Effect
The memory effect primarily impacts Nickel-Cadmium (NiCd) cells, and to a lesser extent, Nickel-Metal Hydride (NiMH) batteries. When consistently recharged before being fully depleted, these batteries can develop a “memory” of that shallower discharge point. For instance, if a battery is always recharged when it still has 20% of its capacity remaining, it might eventually behave as if 20% is its new “empty” state.
This apparent capacity reduction results in shorter operating times, as the battery delivers power for less duration than its rated capacity suggests, even when fully charged. The name “memory effect” comes from the battery appearing to recall its previous discharge level, effectively limiting its usable energy range. The impact on NiMH batteries is often referred to as “voltage depression” rather than a true memory effect, as it is generally less severe and more easily reversible.
Behind the Battery Behavior
The memory effect in NiCd batteries is caused by the formation of specific crystals within the battery’s internal structure. When a NiCd battery undergoes repeated partial discharge cycles, cadmium in the negative electrode can transform into larger cadmium hydroxide crystals. These larger crystals are less efficient at converting back into cadmium during the charging process.
This crystallization hinders electrochemical reactions, preventing the battery from delivering its full stored energy. Larger crystals reduce the active surface area, impeding complete discharge. Consequently, the battery’s voltage drops prematurely, appearing to reach its end of discharge even with remaining energy.
Managing Battery Performance
To prevent or mitigate the memory effect in NiCd and NiMH batteries, perform periodic full discharge-recharge cycles. This process, often called “conditioning” or “exercising,” involves completely discharging the battery until it is nearly empty, then fully recharging it. This helps to break down the larger crystals that may have formed, restoring some of the battery’s lost capacity.
Avoiding repeated partial discharges is a practical strategy; instead of topping off a partially depleted battery, use it until it is significantly discharged before recharging. Using chargers specifically designed for NiCd or NiMH batteries, which may include a discharge function, can also help maintain performance. While these methods can reduce the impact of the memory effect, it is important to note that the effect is not always completely reversible, especially if the battery has been subjected to many cycles of partial discharge.
Memory Effect and Today’s Devices
The memory effect is largely a concern of the past for most modern electronic devices. Today’s smartphones, laptops, and tablets primarily use Lithium-ion (Li-ion) batteries. Li-ion chemistry, based on lithium ion movement, makes them immune to the memory effect.
Therefore, users of contemporary gadgets do not need to worry about fully discharging their devices before recharging. Charging a Li-ion battery at any point will not diminish its overall capacity or lifespan due to a “memory” phenomenon. This chemical difference means advice for older NiCd or NiMH batteries does not apply to today’s Li-ion technology.