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NiMH Battery FAQ


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.  Some people argue that NiCd batteries deliver faster discharge rates than NiMH batteries.  While this may be true under certain circumstances, the difference is not relevant when considering power sources for electronic devices like digital cameras or portable music players. (If you are choosing a battery to drive a high torque power screwdriver, then NiCds can outperform NiMH).  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.

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Is it normal for my digital camera to use so many alkaline batteries?

Unfortunately, it is. Digital cameras use alkaline batteries at a shockingly fast rate.  (For an explanation why, see the following question). If you are using lots of alkaline batteries for your electronic devices you'll probably want to switch to rechargeable NiMH batteries ASAP. Not only will the NiMH batteries power a digital camera (or most other electronic devices) much longer than alkaline batteries will, but they are much less expensive to use.

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Why do my alkaline batteries run down so quickly when used in a digital camera (or other electronic device)?

Alkaline batteries were not designed to meet the very high power demands of today's electronic devices.  Alkaline batteries have a high rated capacity, but they can only deliver their full capacity if the power is used slowly. (See alkaline drain chart).   Electronic devices such as digital cameras place a high power drain on batteries, so it is much better to use rechargeable NiMH or NiCd batteries for these type of devices.   Lithium ion rechargeable camera batteries also work well in high drain applications like digital cameras but can be more expensive.

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Do Ultra or Advanced Formula alkaline batteries last any longer than standard alkaline batteries?

Yes, for high drain applications Duracell Ultra and Energizer Advanced Formula batteries do last longer than standard alkalines.  Unfortunately, they only last about 30% longer.  So instead of lasting for say 15-25 images they might last for 20-40.  That's a little better but it's still pretty poor, especially considering that these new style alkalines cost as much as $1.50 each. Rechargeable NIMH or NiCds are the better choice for high drain applications. They last much longer on a single charge and don't have to be discarded after a single use.

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What about rechargeable or renewable alkaline batteries?

Rechargeable alkalines work well for some uses but, they are not a good alternative for use in digital cameras.  They typically have an even lower capacity than standard alkaline batteries.  This means that if a standard alkaline only lasts for a few exposures, a rechargeable alkaline will last for even fewer!


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

Yes, for most high drain electronic applications NiMH batteries are ideal substitutes and you needn't worry about the apparent voltage differences.  Even though alkaline batteries are rated at a nominal 1.5 volts, they only deliver 1.5 volts when they are fully charged.  As they begin to discharge the voltage of alkaline batteries continuously drops.  In fact, over the course of their discharge, alkaline batteries actually average about 1.2 volts.  That's very close to the 1.2 volts of a NiMH battery. 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.

There are a couple of cases where their actual voltage difference may be important to you.  In the case of a device like a radio, where a higher voltage can mean a stronger signal, a fresh alkaline battery may be more desirable - but more expensive -  than a rechargeable NiMH battery.  This is also true for a flashlight, which will be brighter with the initial higher voltage of alkaline cells.  This minor difference may not be important to you and is probably offset by the much lower cost of operating NiMH batteries.  And keep in mind that the alkaline battery only has a higher voltage when it is fully charged.  Once it gets to 50% capacity or less, it will be delivering a lower voltage than a NiMH battery.

The one time when the voltage difference of the two is important would be in the case of a device that checks the voltage of a battery to estimate the amount of charge left on the battery.  Because the voltage of an alkaline battery drops at a very predictable rate it's possible to estimate the amount of capacity left in an alkaline battery based solely on its voltage.  (1.5 volts - fully charged, 1.25 volts - 50% charged, 1.0 volts - almost fully discharged). But a NiMH (or NiCd) battery stays at about 1.2 volts until it is nearly completely discharged.  This makes it almost impossible to know the amount of capacity left based on its voltage alone.  When a device that's using NiMH batteries indicates the battery is low, it's time to change the batteries now!

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Do NiCd batteries really have a memory effect?

Answering this question is a sure way to start a flame war on the internet.  The simple answer is: Technically speaking NiCd batteries do not have a memory effect.  However, 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, even if it is not technically correct to call it that.  There is a lot of disagreement in the battery industry over what actually causes voltage depression.  The phenomenon itself is very real.  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 (down to about 1.0 V per cell) after charging it.  If a full discharge followed by a charge cycle is done several times, a battery suffering from voltage depletion (voltage depression, memory effect, or whatever you would like to call it) 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.  Overcharging is usually caused by  poorly designed first generation battery chargers.  These chargers continue to deliver current to the batteries even after the batteries are full charged.  "5- hour" and "8-hour" timer type chargers can damage NiCd or NiMH batteries if they are frequently used to charge batteries that are only partially discharged. 

The other common cause of damage to NiCd and NiMH batteries is leaving them in a device like a flashlight left "ON" after the battery has run down. Electronic devices normally switch themselves off once the battery is discharged.  But other 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 (the plus end actually becomes minus and vice versa).  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.

The big difference between voltage depletion, the so called "memory effect" and damage caused by overcharging or improper storage, is that reduced capacity due to overcharging is not reversible. 

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Do NiMH batteries have memory effect?

Technically, NiMH batteries do not have a "memory effect", but strictly speaking neither do NiCds.  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.

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How many times can a rechargeable battery be recharged?

I normally answer that question by simply saying "hundreds". The reason I can't be more precise is because this is a more complex question to answer than it might seem.  The number of times a battery can be recharged depends on how the battery is used.  An analogy that is sometimes used is to compare a rechargeable battery  to a loaf of bread.  Suppose someone asked, how many slices can be cut from a loaf of unsliced bread?  The answer, of course, depends on how thick or thin the bread is sliced. If the slices are very thin it can be cut into more slices.  The same is true for recharging a battery. Every time a rechargeable battery goes through a charge and discharge cycle it loses a tiny bit of capacity.  

If the battery is completely discharged before it is recharged, that takes a bigger "slice" of the battery's capacity, if it is only partially discharged a  before recharging, it uses up a smaller "slice". A NiMH battery can be charged and discharged hundreds of times, but whether that means 200 times or 800 times has a lot to do with how big of a "slice" you take each time. 


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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%. (see Battery Myths)

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.

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Does rapid charging reduce the life of batteries?

No.  So 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 specifically designed NiMH battery charger.  Chargers designed to charge NiCd batteries can overcharge NiMH batteries.  Even a standard or slow NiCd charger can damage NiMH batteries.


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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.  If you think of a battery as a small fuel storage tank, which in a sense it is, mAh a measure of how much "fuel" the battery holds.  (This is roughly comparable to using gallons to measure how much fuel a gas tank can hold.  The more gallons of capacity,  the more fuel the tank can hold.) With a battery the higher the mAh rating the more electrical energy it can store.

While it is useful to think of mAh as being the rough equivalent to gallons, the analogy is not a perfect one.  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. 


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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. (Staying with the fuel tank analogy from above, it would also take longer to fill a 20 gallon gas tank than a 12 gallon tank). 

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What is the shelf life of a NiMH battery? 

You probably mean to ask: What is 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.  NiMH batteries stored at a lower temperature self discharge at a lower rate (Self discharge chart available soon).

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