• Can NiMh batteries be substituted for alkaline batteries even though they are only 1.2 volts?
• Which are better, NiCd batteries or NiMH batteries?
• Do NiCd batteries really have a memory effect?
• Do NiMH batteries have memory effect?
• Does rapid charging reduce the life of batteries?
• What is the shelf life of a NiMH battery? 
• How many times can rechargeable batteries be recharged?
• Does putting batteries in the freezer or refrigerator make them last longer?
• What does mAh stand for?
• Can I substitute a higher mAh battery for a lower one?

Can NiMH batteries be substituted for alkaline batteries even though they are only 1.2 volts?

Yes you can. For most high drain electronic applications NiMH batteries are ideal substitutes and you don't have to worry about the apparent voltage differences.  Even though alkaline batteries are rated at a nominal 1.5 volts, they only deliver 1.2 volts when they are fully charged.  As they begin to discharge the voltage of alkaline batteries continuously drops. The main difference is that an alkaline battery starts at 1.5 volts and gradually drops to less than 1.0 volts.  NiMH batteries stay at about 1.2 volts for most of their discharge cycle.

                        
Which are better, NiCd batteries or NiMH batteries?

For most electronic devices it is better to use NiMH batteries than NiCd batteries.  NiCd batteries use Cadmium, a highly toxic heavy metal, that can damage the environment if not disposed of properly. (They should be recycled not discarded).  NiMH batteries usually have a higher capacity than NiCd batteries of the same size. NiMH batteries require more sophisticated chargers than those typically used for NiCd batteries.  But smart chargers designed especially for NiMH batteries are now readily available.

Do NiCd batteries really have a memory effect?

NiCd batteries do not have a memory effect, but they do suffer from a voltage depletion or voltage depression phenomenon that most people call the memory effect.  So practically speaking, NiCd batteries do suffer from a memory effect. If a NiCd battery is repeatedly charged after it has only been partially discharged it will develop a lower voltage and a lower capacity. Fortunately, this effect is reversible by conditioning NiCds. Conditioning is simply fully discharging the battery after charging it.  If a full discharge followed by a charge cycle is done several times, a battery suffering from voltage depletion should be restored back to its normal voltage and capacity.

If you use NiCd batteries you should be aware that most of the problems experienced by NiCd battery users are not due to a "memory effect" but are due to overcharging or improper storage. Most chargers continue to deliver current to the batteries even after the batteries are full charged. Make sure you try to use a charger that cuts off after the battery has been charged. 

The other common cause of damage to NiCd and NiMH batteries is leaving them in a device after the battery has run down. Electronic devices normally switch themselves off once the battery is discharged.  But some devices like flashlights, cassette players, and many toys, will continue to put a small load (drain) on a battery even after the battery is run down.  Eventually (after a few weeks) this drain on a discharged battery will cause the polarity of the battery to reverse. Once this happens the battery will not take a charge again. Battery makers recommend that rechargeable batteries be removed from any devices that will not be used for several weeks or longer.

Do NiMH batteries have memory effect?

NiMH batteries do not have a "memory effect" However NiMH batteries can experience voltage depletion, also called voltage depression, similar to that of NiCd batteries, but the effect is normally  less noticeable.  To completely eliminate the possibility of NiMH batteries suffering any voltage depletion effect  manufacturers recommend an OCCASIONAL, complete discharge of NiMH batteries followed by a full recharge. NiMH batteries can also be damaged by overcharge and improper storage (see the NiCd section immediately above this one). Most users of NiMH batteries don't have to be concerned with this voltage depletion effect.  But if you use a device say a flashlight, radio, or digital camera for only a short time every day and then charge the batteries every night, you will need to let the NiMH (or NiCd) batteries run down occasionally.

What is the shelf life of a NiMH battery? 

The shelf life of a NiMH battery is actually consider the self discharge rate of a NiMH battery.  The rate of self discharge for any battery depends on the temperature at which it's stored.  Stored at 70 degrees F (20 C) NiMH batteries will lose up to 40% of their charge within a month.  If they are stored at a higher temperature, they will self discharge at an even higher rate.  Stored at a lower temperature they self discharge at a lower rate.

How many times can a rechargeable battery be recharged?

Normally "hundreds". It just depend on the equipment it is being used in. If the battery is completely discharged before it is recharged, that takes a battery capacity down, if it is only partially discharged before recharging, it uses less of its capacity. A NiMh battery can be charged and discharged hundreds of times, but how long depends on how you charge the battery.
                        

Does putting batteries in the freezer or refrigerator make them last longer?

It depends on which type of batteries and at what temperature you normally store them. 

Alkaline batteries stored at "room temperature" will retain 90% of their power for years without refrigeration. Under normal circumstances, refrigerating or freezing alkaline batteries will extend their life by less than 5%. 
NiMh and Nicad batteries, start to lose power when stored for only a few days at room temperature. But they will retain a 90% charge for several months if you keep them in the freezer after they are fully charged. If you do decide to store your charged NiMH cells in the freezer or refrigerator, make sure you keep them in tightly sealed bags so they stay dry. And you should also let them return to room temperature before using them.

Does rapid charging reduce the life of batteries?

No.  As long as it is done using a properly designed, smart charger, most NiMh batteries can be recharged in about an hour without any damage or reduction in their life. However, NiMh batteries must be rapid charged with a charger specifically designed for charging NiMh batteries.  Chargers designed to charge NiCd batteries can overcharge NiMh batteries.  Even a standard or slow NiCd charger can damage NiMh batteries.

                         
What does mAh stand for?

mAh stands for milli Ampere hour or milli Amp hour.  It is a measure of a battery's energy storage capacity. With a battery the higher the mAh rating the more electrical energy it can store.

Different types of batteries use different methods to measure mAh so comparing the mAh rating of one type of battery, say an alkaline battery to another say an NiMH battery, is not always meaningful.  However, in general, the mAh rating of a battery is a quick  way to compare the relative energy storing capacity of one battery to another battery of the same type. 

                     
Can I substitute a higher mAh battery for a lower one?

If the batteries are of the same type, i.e. both are NiMH or both are Lithium-ion, then you can use a higher capacity (i.e. higher mAh) battery instead of the lower one.  The reverse is also true. Using a higher mAh rated battery will allow the device to run longer on a charge.  So a camera should take more pictures and a music player can play more songs.

Keep in mind that it will take a little longer to charge a higher capacity battery.