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Memory Effect - What it is and what you can do about it
What's the scoop on the memory effect in rechargeable batteries? Is it true? Is it worth buying rechargeable batteries?
 
Almost anyone who uses rechargeable batteries has heard of the memory effect problem. For those who have not heard of this problem it is commonly believed that when rechargeable batteries are not fully discharged between charge cycles that they remember the shortened cycle and are thus reduced in capacity (length of use per charge). This problem was very common with rechargeable batteries several years ago. With improvements in batteries and charging technology this 'memory effect' is becoming a thing of the past.

Little known facts:
* 'Memory Effect' is the common term used to replace the more accurate term 'Voltage Depression.'
* Voltage Depression is more a problem with incorrect charging than a battery problem.
* Voltage Depression does not necessarily permanently damage a battery. It can most likely be corrected by fully charging and discharging the battery.
* Voltage Depression ('Memory Effect') is often incorrectly used to explain low battery capacity that should be attributed to other problems, such as inadequate charging, overcharge, or exposure to high temperatures.
* Voltage Depression can be affected by the discharge rate of a battery. Generally speaking, the depth of discharge will be less on discharges at the higher rates. This increases the capacity loss as less of the active material in the battery is cycled.
* Voltage Depression occurs primarily in NiCad batteries. NiMH batteries are almost never affected and Li-Ion batteries are minutely affected.

What else happens to NiCad cells where memory effect is blamed?

*Overcharging cells that are near the end of their overall life and their voltage is naturally low. No amount of charging will fix this and the battery needs to be replaced.
*Diminished capacity due to long storage or non-use.  This can sometimes be remedied by a number of deep discharges.  This has been shown to recover 70% or more of the cell's original capacity.  If the cell has been sitting for too long and the energy has been depleted it may not be possible to recover any capacity.
 
So how can you maximize the use of your rechargeable batteries? Here are a few steps to take to get the most use out of your batteries:

1. Invest in a good charger. NiMH batteries should not be charged in a NiCad charger, unless the charger is specifically made for both chemistries. There are cheap chargers and there are expensive chargers. Make sure the charger you get has good reviews and is well made. Chargers with micro-controller chips are usually the best choice.
2. When charging your batteries occasionally discharge them fully before recharging them. This is especially helpful to NiCad batteries. Be careful not to discharge too deeply. (Less than 1v per cell for NiCad and NiMH. E.g., a 3.6v pack to no less than 3v.) Discharging to absolute zero will make your battery useless. It's best if you have a charger with a conditioner that will cycle the battery for you.
3. Be sure to store your batteries properly. Do not leave your batteries in a hot car, or in humid conditions. The best storage conditions are a cool, dry place. The refrigerator is fine if you stick in a packet of silica gel with your batteries in a sealed bag to keep them dry. It is a good idea to charge your NiCad or NiMH batteries fully before use if they have been in storage.
4. Most cordless phones use NiCad batteries. To maximize your cordless phone battery life, make sure to leave your phone off the base every once in a while until it is dead. Then leave it on the base until it is fully charged. You should leave your phone on the base for at least 24 hours to charge it fully.

There is no need to avoid rechargeable batteries. They can save you significant amounts of money over time. Don't be scared off by the 'memory effect'. It is easily manageable if it ever occurs.

See all of our Battery Articles.