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Information Article About NiMH (Nickel Metal Hydride) Rechargeable Batteries

MEGAbatteries.com is pleased to provide our visitors with this information page dedicated to the subject of NiMH rechargeable batteries. They are one of the most popular and widely used form of rechargeable battery chemistry on the market today.

If this is your first time here, we thank you for stopped by and welcome you. We trust you’ll gain at least a little bit of knowledge about NIMH technology that perhaps you didn’t know before.

If you’re a returning visitor, thanks for coming back! Please let us know how we can help you in your battery or charger questions or requirements.

In many consumer electronic devices today, a popular rechargeable chemistry used is called NiMH. That’s the abbreviation for Nickel Metal Hydride. This type of battery provides a terrific alternate option to another type of battery also commonly used today, namely, 1.5 Volt Alkaline, (non-rechargeable) batteries. An NiMH Battery typically contains two or three times the power (this is also called capacity) of say, a NiCD (Nickel Cadmium) battery, (which is also rechargeable). NiMH, because of its’ greater capacity, is very popular among people who use high-drain products (high-drain means the product consumes a lot of power). Some of these products are: mp3 players, digital cameras, some wireless power tools, and GPS units just to name a few.

To understand what the capacity of a battery means and therefore the larger capacity that NiMH batteries have versus Alkaline and NiCD batteries, a good analogy to illustrate the point with that most people can relate to would be a bathtub filled with water. Everyone’s seen a filled bathtub.

Think of the water in the tub as power or energy. A smaller tub is limited to the amount of water it can hold versus a larger bathtub which logically, can hold more. The larger the tub, the more capacity it has to hold water. The more capacity (or energy) stored in a battery, the longer the run time of that cycle, the battery will deliver. Now you might be wondering, what is “run-time”? Run-time simply means the length of time the product you have the battery in, will run on that battery during 1 charge cycle. Simply put, a charge cycle is the “life-span” of a fully charged battery – the length of time it can deliver power until it runs out.  Capacity is measured in mAh. Milleampere hours is the long form of that abbreviation.

Here’s an example: a rating of 8000 mAh (D battery) provides longer power than one rated at 2200 mAh (AA size).
We’d like to address what seems to be a myth about the use of NiMH rechargeables in outdoor solar lights. They don’t work in them! In solar lighting, it’s always recommended to use Nickel Cadmium batteries because they charge at a slower rate. However, having said that, there exists a newer technology in NiMH batteries that provides a slower rate of discharge. As such, they are called Low Discharge (also known as Low Self Discharge, or LSD). This type of battery costs more than regular discharge NiMH because of this particular benefit. Originally, low discharge NiMH was made by one of the leaders in the battery industry, namely Sanyo, but soon after it was released to market, other manufacturers got on the bandwagon with their own versions of NiMH rechargeable batteries.
Something else to know about NiMH that you might not be aware of. There was a time, not too long ago that cell phones and even portable computers (laptops, notebooks) were being powered by NiMH batteries. Advances have been made in the power for these devices and now, it is Lithium chemisty that powers them. Lithium batteries have now taken the place of NiMH in these two product industries.

As with Alkaline batteries and NiCD batteries, the sizes that you will find in NiMH chemistry are: AA, AAA, C, D and 9 Volts. They are the most common size batteries in use today in homes, offices and public addresses such as Libraries, Hotels, Restaurants and Industry etc...
A term that is very important to know about, and understand, is a term called Memory Effect. You might have heard about it, but never really stopped to inquire what it means. If you use rechargeable batteries, it’s important for you to know about it. Memory Effect is a consequence.

It doesn’t exist in NiMH rechargeable batteries though, and that’s a good thing.

Here’s what it is:

Every battery has a total power level. An AMOUNT of battery power stored within it.

Fair enough? Well, rechargeable batteries, need to be recharged when they have become fully drained of power. Actually that’s not entirely accurate. A battery that is missing only a little bit of power can still be charged to be “topped off” of whatever power is missing. However, and here’s where memory effect comes in. In a battery that does suffer from memory effect, if it is recharged before it has become fully dead, only what was missing will be replenished and this new amount will become the battery’s NEW total level of power. This effect happens in NICD batteries. One must wait until the battery is totally depleted of power before recharging to maintain the total, original volume of power capacity. In NiMH batteries, it doesn’t matter how much power has been used – the battery will NOT remember the NEW level if capacity if it is only topped off.

NiMH battery users therefore, most often choose them for that reason. They can charge and recharge them without concern for how much power was remaining in them at the time of recharge. A person will pay more for this type of battery over a battery that can sustain memory effect but to everyone who buys them, it’s well worth the price point difference for this luxury.

AA, AAA, C, and D NiMH batteries deliver 1.2 volts of power each. The 9 Volt batteries, deliver 9 Volts.

Here we provide a breakdown of some of the different milliampere hour ratings and/or other features/benefits of differently sized NIMH rechargeable batteries.
We find mAh ratings between 700 and 1200 in AAA size. Low discharge AAA NiMH batteries can hold their charge for up to two years. This feature comes in handy for institutions who not only buy in bulk quantity but schools in particular experience months at a time when they are closed and their inventory of batteries remain sitting on the shelf not being used. For any application where the battery will be idle for extended periods of time, low discharge batteries are recommended and appreciated. They discharge at a much slower rate than non low discharge batteries and as a result, still maintain most of their power over longer periods of time.
We see mAh ratings for AA size NiMH batteries in the range of 2200 and 2900. This size NiMH also comes in a low discharge format, once again, retaining their capacity (power storage) for up to two years.

One can get C size NiMH rechargeables in capacities of the 3500 to 6000 milleampere range. Just like AA and AAA sizes, a low discharge C size is also available and extensively used in schools and other places where a slow discharge rate over time, is economically preferred.

D size NiMH batteries range in capacities from 8000 mAh to 12000 mAh, and just lile AA, AAA, and C, D’s  are made in low self discharge versions as well.
To be technically accurate, we must say that 9 Volt NiMH batteries technically deliver only 8.4 volts of energy. Most common mAh ratings of 9 volt NiMH’s are: 175 mAh, 250 mAh, 300 mAh and 325 mAh. And yes, you guessed it! 9 Volt NiMH batteries also come in a low self discharge version!
And finally, here we take a little bit more indepth look at low self discharge NiMH rechargeable batteries.

As we mentioned, they’re manufactured to keep their charge for up to two years. For this reason alone, aside from some other benefits they possess, they’re fast evolving as the popular choice among everyday users. In very short order, they’ll begin to outsell standard NiMH batteries and when this switchover happens, their cost will be reduced relatively. Currently at the time this article is published, they are still more expensive than regular NiMH. We told you that low discharge batteries can retain their charge for about 2 years but we haven’t mentioned how long standard ones keep their charge for. Typically, (all things being equal like temperature for example), the time it takes for standard NiMH batteries to fully discharge on their own while not in use,  is between 30 and 40 days. There is some published literature about this that differs slightly from these numbers, but on average, that’s a fair statistic. When we speak of low discharge batteries holding their charge for up to 2  years though, note that they will keep approximately 85% of their charge that long, not their entire power.


Just what are NiMH battery packs? To the battery user who does not ever buy them, it’s not  hard to understand that they wouldn’t be aware of them. Here, we explain. Battery packs are clusters of batteries (2 or more individual cells) encased together to form a pack. The total voltage of all the individual cells combined, make up the actual total power of the pack, since the cells work together to actually form one “new” battery.

To illustrate this point on voltage, consider that 4.8 volt PACKS, 6 Volt PACKS and 7.2 volt PACKS, are actually made up of individual 1.2 volt EACH cells inside it.

You’ll need a battery pack charger, as opposed to a battery charger, to charge battery packs. The bays (slots/compartments) where the packs lay, are differently designed than the slots single cells go into, inside a charger. Note that when you charge your NiMH battery packs, it’s always recommended to use a Smart Charger (discussed below), rather than a Timer Controlled Charger (also discussed below), to ensure a full and long-lasting charge.


Batteries that have tabs on the ends of them, are used for building Battery Packs (discussed above). These are the voltages for battery packs: 2.4 volt, 3.6 volt, 4.8 volt, 6 volt, 7.2 volt, 8.4 volt 9.6 volt, 10.8 volt, 12 volt, etc... and in mAh ratings of as little as 120 mAh, and as high as 10,000 mAh. The tabs are necessary to “connect” the batteries together with. They are welded together.  They are the preferred choice for many who wish to rebuild their own battery packs for their devices that have dead battery packs and wish to save money by building them themselves rather than purchase new battery packs. In some retail stores, purchasing battery packs for some devices is almost as expensive as the original device cost in the first place. Rebuilding battery packs is a very economical approach to keeping a product powered.


These batteries are also great for use in building any size battery pack (2.4 volt, 3.6 volt, 4.8 volt, 6 volt, 7.2 volt, 8.4 volt, 9.6 volt, 10.8 volt, 12 volt ..etc) for those who are able to put on the tabs themselves. That’s what the flat top is. Flat top batteries are less expensive than those that contain the tabs and for anyone who is even a little bit handy or experienced in using a spot welder, the tabs can be applied rather easily.


Sub C size NiMH rechargeable batteries are the best choice for use in drills (cordless drills) and in building/rebuilding radio-controlled (RC) battery packs.


To charge NiMH batteries, there exist two different types of charging technology. 1 type is called the Timer-Controlled Charger and the other type is called the Smart/Intelligent Charger. They have different price points and the reasons are clear: Here, we show you the reasons...


What you need to know is this: They operate on a timer that is built in during manufacturing. What this timer does, is automatically shut off after a specific, pre-determined length of time. Shut off times that are available range from 4 hours as in the case of AAA 750 – 1000 mAh batteries, to 27 hours, as in the case of D batteries rated at between 5000 and 12000 mAh. But irrespective of which batteries timer-controlled chargers have in them, still, the unit will shut off when the timer has elapsed.  Timer-controlled chargers are inexpensive for bur that savings, this is the potential consequence: Since all batteries within it are charged for the exact length of time, regardless of how much each one needed to be recharged for, some batteries will become overcharged and some will remain undercharged. You see, a Timer-Controlled charger doesn’t have the ability to determine how much power is left in each battery. It merely treats them all the same. In reality, most batteries go into a charger at different stages of charge remaining. Yes, they’re cheaper to buy, but only really recommended for the occasional user who doesn’t mind the possibility of losing some batteries to ruid (overcharge ruins them) and undercharge means they are not fully charged. It’s a trade off.

FOR SMART CHARGERS (Also called Intelligent Chargers) :

You get the intelligence that the charger can evaluate and actually detect the charge remaining for each battery in it. If some is half full, it will be charged to full. If another is only 1/3 empty, it will be replaced a third’s worth. In the case with NIMH, as we mentioned earlier do not suffer memory effect, it’s an ideal choice to use an intelligent charger. Even if a battery has only ¼ the power left, topping it off does not produce a new memory for the battery. It can do this based on a completely different technology than Timer-Controlled chargers. Smart chargers actually treat each battery within it, individually and detect exactly the correct amount of recharging each one needs. Once a specific battery has been replenished, it receives what’s called a Trickle Charge just to keep it topped off until the battery is removed from the charger. Kind of keeping a cup of coffee on a hot plate just to keep it warm and not cool off. Does that makes sense?

In smart charging technology, there is no worry that batteries might become over or undercharged. It’s well worth the investment to protect the battery investment.

To the right of this information article, you’ll see our in-house inventory AA NiMH Rechargeable Batteries in 8 Packs for your convenience. Of course we supply all sizes of NIMH chemistry. AA, AAA, Sub-C, C, D, 9 Volt. Please browse around if you’re in the market for them but remember our mandate at MEGAbatteries.com:

“Nothing demonstrates good customer service better, than providing accurate product information to a prospective customer, before they buy. This shows we don’t just want to make the sale, we want the customer to get the CORRECT product for their needs. We want to make the RIGHT sale”.

So...........if you require any additional information on NiMH Rechargeable Batteries that haven’t been discussed in this information article, we invite you to contact any one of our battery experts. We’ll guide you correctly.

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