NiMH Batteries and Chargers--A Primer


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I find it interesting that I don't see a whole lot of information in here about the fuel for our electronic callers. That is, the batteries. So here's a few things I have learned along the way with batteries, particularly NiMH rechargeable batteries.

First off is the rating system of batteries. For those not familiar with the power ratings of rechargeable batteries it's the mAh number. Typically it's in big numbers on the side of the battery as it is an advertising point. This is a measure of power a battery can provide before being depleted. To provide a simple example, a 2500 mAh could provide 2500 milliamps of power for one hour and a 1600 mAh rated battery could provide 1600 milliamps of power for one hour. In theory a flashlight that draws 1 amp per hour would last 1.6 hours on a 1600 mAh battery and 2.5 hours on the 2500 mAh battery. While the battery industry uses testing methods over multiple hours, this example serves simply to illustrate relative capacities.

So one would hope as not all batteries are created equal. Quit simply many batteries don't live up to their claims while others go beyond what they are rated at(see here for an example). So it isn't uncommon to see a quality 2100 mAh rated battery outperform a poor quality 2500 mAH rated battery. Also keep in mind that the ratings are based on room temperature use. Also, as a battery lives out its life, it will lose useful capacity.

Extreme hot or cold can drastically alter the performance of a battery. NiMH batteries can operate down to 0 degrees Celsius/32 degrees Fahrenheit. Below that temperature, you may want to consider looking at Lithium batteries that are capable of operating down to -40 degrees Celsius. Do not confuse these with Lithium-Ion (Li-Ion) batteries as they are not the same.

Charged NiMH rechargeable batteries have a very short shelf life. Most NiMH batteries lose about 1% of their charge per day just sitting idle. Many people have learned this the hard way. They put some fully charged batteries in a device and use it for a short while and put it away. A month or two later they return to that device only to find a low or dead battery. Keep your rechargeable batteries topped off by placing them on a charger once a month if they aren't being used. When batteries are new, realize that it may take a few charge/discharge cycles before they begin operating at their stated capacity.

There are a couple of new NiMH batteries on the scene that are self-discharge resistant. The Sanyo Eneloop is probably the most popular. Others include the Uniross Hybrio and the Rayovac Hybrid. These batteries are sold in a charged state and they can be used out of the package without charging. The Eneloop claims to retain 85% of it's charge after one year of storage at room temperature. Independent tests seem to confirm this. The Eneloop AA is rated at 2000 mAh, with Sanyo claiming a minimum 1900 mAh rating. It's an excellent choice for devices that don't see a lot of regular use.

Be aware that NiMH are typically rated at 1.2 volts, where a comparable sized alkaline battery is rated at 1.5 volts. However, where alkaline batteries tend to lose their output voltage at a linear rate, NiMH cells will maintain their output voltage for a much longer period before quickly dropping off. Both battery types will show a rapid initial drop, but this is more pronounced with the alkaline and the NiMH actually run longer with a higher output voltage. This is also why many devices with battery strength gauges show NiMH batteries as being fully charged after a long period of use and then quickly dying. Where NiMH batteries shine is their ability to deliver more power than alkaline batteries. Click here for an illustrative chart.

Proper care and feeding of NiMH batteries will help ensure they live a long and productive life. There are two big enemies of rechargeable batteries: Heat and over-discharge.

Over discharging a rechargeable battery can cause the polarity on the batteries to reverse, also known as cell reversal. Essentially this kills the battery and it should be disposed of. Most electronic devices will shutdown long before this happens, but other devices like flashlights or devices that always maintain a slight draw, if allowed to completely run down, can cause polarity reversal.

Heat is probably the biggest killer of batteries. Most heat exposure comes from the charging of the batteries. While the batteries will heat up during discharge (use) this is typically not as great as that from charging. The faster a charger charges a battery, the hotter that battery will get. The more heat the battery is exposed to, the shorter its useful life. It's a matter of convenience (faster charging time) versus cost (reduced lifespan of batteries). The most important thing to avoid is overheating the battery. If charging on a rapid charger where the batteries build up a fair amount of heat, directing a fan across them can help limit damage.

When manufacturers claim their batteries can handle up to 1000 charge/discharge cycles, this is based on slow charging on chargers that run for hours. While the quick 15 minute chargers are convenient, they are very hard on the batteries. Expect a battery's useful life to be drastically reduced if it is frequently used on rapid chargers. A battery that otherwise has a useful life of 1000 cycles may not make it to 100 if constantly abused by a rapid charger.

When it comes to chargers, like many other things, you get what you pay for. Many of the chargers that come included with batteries, such as what Energizer offers, require batteries be charged in pairs. The charger simply will not charge a single battery. The problem with this design is that it can lead to under-charging one of the two batteries in the pair. So placing a 1600mAh rated battery alongside a 2500mAh isn't a good idea. But even placing a pair of 2500mAh cells together may caused one of the cells to get left with a less than full charge. The listed rating is a guide. Batteries can, and do, frequently vary +/- 50mAhs even amongst the same manufacturer and same lot. So you can easily have two identical 2500mAh cells, but one is a 2450mAh and the other is a 2550mAh.

You want a charger that charges each battery independently of the other batteries. These chargers are microprocessor controlled and feature more advanced charging capabilities, including thermal protection, and are able to recognize when each battery is fully charged. Also, because each battery is charged independently of the others, you can mix and match batteries on the charger. Many of these chargers also include the ability to "cycle" a battery. Cycling is a series of charge/discharge cycles that can restore poorly treated batteries to health, or in the case of new batteries, quickly bring them to optimal working conditions. Since new batteries typically do not hold a full charge until having been charged and discharged a few times, the charger can take care of this process automatically.

These "advanced" chargers typically start at the $40 range. The ability to charge more batteries at once and fancy displays will drive the price north. Also, there are chargers, such as the La Crosse BC-900 or Maha MH-C9000 that can analyze batteries and display their mAh rating, allowing you to build a closely matched set of batteries.

I use the Maha/Powerex MH-C801D charger. It accommodates 8 AA or AAA batteries, charging each independently. I've also started to switch over to the Eneloop batteries for many devices, particularly my daughter's toys. Since they hold their charge for a long time, the toys that get neglected for a while aren't dead or dying when she picks them up again.

For more information, a good resource is Steve's Digicams and for those that really want to see what's beneath the hood of NiMH technology, Energizer has published an in-depth NiMH application manual. A comparison of battery performance can be found here.
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Alot of good information here! /ubbthreads/images/graemlins/ooo.gif Thanks Bronco! /ubbthreads/images/graemlins/grinning-smiley-003.gif Maybe one of the moderators here can pin this for reference to the top? MH
Not to be picky about your excellent post, Glenn, but consumer battery ratings are never derived from a 1 hour discharge rate (or C1). Most often it's from a 20 hour or 10 hour (C20 or C10) discharge rate (meaning a resistive load equalling 1/20th or 1/10th of the total capacity), these being the ideal conditions under which to extract the highest number.

Manufacturer's would do well to standardize on testing and prominently explain all this on their product or packaging.

I would also offer that although you're correct that a 15-minute fast charger may indeed lead to premature failure of a certain percentage of NiMH cells, I find the economics and convenience factor quite worth the risk.

(Suppose I should qualify this by saying it's no big deal with loose AA cells to lose one now and then, though it would be a much more expensive proposition were I having to toss whole purpose-built welded packs as often, with perhaps 7 out of of 8 cells still being perfectly good.)

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Thanks. I was trying to keep things simple in explanation for most folks. I'll go back and clean it up, however.

The 15 minute chargers are convenient, but many folks may not be aware of the risks with them. Like anything else, as long as people know the risks beforehand they can make an informed decision.

I'm sure there are plenty of folks like me that will actually lose batteries by misplacing them long before they reach the end of their useful life, even with a 15 minute charger.
Thanks for the info we all need to know but can't stand to look for. Researching info like you've done would put me to sleep in a 4 lane freeway so its nice to have the meat and potato's version.
BroncoGlen, Great post. I've been using rechargeable batteries in my trail camera. Recently built an e-caller, using the same batteries. I was out tonight and went through two sets of two AAA's in my MP3 player in less thantwo hours. Of course we had single digit temps.
Thought I would chime in with some personal experience regarding chargers. Many of us have a wide selection of batteries and packs that we need to maintain, and have a lot of money invested in them. I found a charger, built for the RC industry, that will charge any type of battery pack I own, plus lead-acid. It also cycles packs and reports the capacity. I've used it for a couple of years and it's top notch.

Triton computer charger

There is also a thermal probe that plugs in to the side of the charger to monitor the temperature of LiPo or Li-Ion batteries during charge, as these can represent a fire hazard. Since it uses an 11-15V DC input, you can use this charger from a vehicle battery to charge a pack at night or while you're in the field. It's expensive, but quite sophisticated, and I've had a great experience using it.

To power it, I used a $35 PC power supply +12 output, rated at 14A. I jury-rigged the wiring on the PC supply so that it runs without a motherboard connected, using the following instructions:

PC power supply

You will also need to short the green wire to any one of the black (ground) wires for the supply to start. (The guy didn't mention that little gem of knowledge in his instructions). /ubbthreads/images/graemlins/wink.gif

Thanks for the great write-up, it's very well written and informative. /ubbthreads/images/graemlins/grinning-smiley-003.gif
Great write-up! Let's get this one pinned up to the top. I see a lot of complaints about e-callers that are due to lack of battery education.
I am glad I read this. I can say though that I am very happy that I bought the rechargable battery for the FX3 I bought it lasts a long time compared to regualr batteries.
I got some of the eneloop batteries in AA and AAA and they WERE fully charged when they arrived. Good tip on those quality batteries. I also got one of the La Crosse BC-900 chargers including 4 AA and 4 AAA batteries from Amazon and the whole deal only cost about $37. That is a real deal. I sure like the computer controlled charger. Way to go. Whish I had your information years ago. Thanks BroncoGlenn

Good Hunting... from Varmint Al

Great post and some great information B.G., much appreciated.......... I've copied and pasted this today from our other forum over at HuntingPA that I posted last week:

"I spoke to a major battery rep. yesterday in regard to getting updated information on maintenance for our lead acid batteries for the "off-season" (If you're not using your light to hunt yotes at night) This is the deal:

First, there are only three recognized major battery manufacturers left in the U.S. They are Exide, Deka, and Johnson Controls, and they private label for many, many companies, such as Cabelas, etc.

Lead acid batteries, no matter who the manufacturer is, are best served by maintaining a storage charge on them when not in use. Batteries will discharge after a time while sitting there, and need "topping off" every month if using a standard low amperage charger....

Most knowledgable retailers, however, have DISCONTINUED selling the devices known as 'Battery Tenders". Evidently there is some type of flaw that will burn out the batteries if left hooked up over long periods of time when used. A newer, smarter charger is now on the market, known as a "Battery Minder", which is made/provided by a company out of Englewood, N.J. known as VDC Electronics. They may be labeling them for retailers under different brands, but they must be "battery minders", NOT tenders. These battery minders are much smarter, will work with ANY battery, and will not burn out a battery due to overcharging. I am personally getting rid of my "battery tender" that I use for my Hog battery and picking up a "Battery Minder to keep my predator light batteries charged when not in use. Of course, you can always run the light a few times a month a month and then charge the battery according to specs, but for those of you that are going to put them away until fox starts in October here in Pa., the battery minder seems to be the way to go.......and these battery minders are for 12 Volt ONLY, although you can charge 2- 6 volt batteries by putting them in series. You can also charge up to 5- 12 volt batteries at a time with the battery minder.....all 5 hooked up in parallel, mind you......

Please be respectfully advised that I do not personally have anything to do with this company in the way of representation, stock, or any other financial involvement. I just figured you'd all want to know the latest.....I try to keep updated on this stuff because it's important....


Dittos on the thank-you BroncoGlenn. As someone who is just now jumping up to a Foxpro, this information derived from your experience has undoubtedly saved me quite a bit on money that I can put in the gas tank instead. Thanks again,

That was a very informational article!
After reading it a couple of times and doing a little bit of searching, I just ordered a charger and batteries.

Got everything from Thomas Distributing:
MAHA MH-C801D Charger
Powerx 2700mAH batteries - 8 pack.

Should be good to go for the "NEW" FX3 I just got last week.
(a mistake in labeling at a local retailer netted me a savings of $118 - out the door was $399).
The NiMh batteries will go longer for you than the Ni-Cd. The MAHA MH-C801D Charger that I mention above will not charge Ni-Cd's.

This charger will charge the Ni-Cd's.
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