Choosing and Using Nickel-Metal-Hydride (NiMH) Rechargeable Batteries
March 6, 2008
The number of portable gadgets that we use has grown substantially in the last few decades. Many of these, such as TV remote controls, some digital cameras, handheld GPS units, and flashlights, are designed to operate on either disposable alkaline batteries or rechargeable, and hence reusable, Nickel-Metal-Hydride (NiMH) batteries.
At the same time as our battery-powered device use has increased, rechargeable battery technology has improved dramatically. Unfortunately, information on the proper care and use of these batteries can be hard to find. This article attempts to provide this information and answer the questions I am often asked.
A Brief History
The earliest consumer rechargeable batteries were Nickel-Cadmium (NiCd), based on technology perfected in the 1950s. These typically had only about 10% to 20% of the capacity of disposable alkalines of the same size. Devices using these batteries would run for only 1/10th to 1/5th as long as they would on alkalines. Fortunately, when the batteries did run down, they could be recharged. Properly cared for, even these early rechargeable batteries would last for several hundred recharges.
A serious concern with NiCd batteries is that cadmium is a very toxic metal, making battery disposal problematic. Many NiCd batteries have made their way to landfills, contaminating our environment. Now, recycling of NiCd batteries is mandated by law in most western countries.
The mid 1990s saw the introduction of Nickel-Metal-Hydride (NiMH) batteries. These had very similar properties to NiCd, but with higher capacity, and more importantly, no super-toxic components. The only drawback of NiMH compared to NiCd (at that time) was a lower maximum current. High-current applications like cordless power tools and electric powered model aircraft and cars continued to use NiCd batteries for some time. As of 2008, some power tools still use NiCd batteries, although model aircraft have switched almost entirely to NiMH and Lithium-Polymer batteries.
Characteristics of NiMH Batteries
There are four important characteristics of any battery: voltage, maximum current, capacity, and self-discharge rate.
Voltage is measured in Volts (V). Single-cell disposable alkaline batteries such as the common AAA, AA, C, and D sizes have a nominal voltage of 1.5V. The same sizes of NiCd and NiMH batteries provide only 1.2V. This lower voltage might seem to be a problem, but in most cases it isn’t.
Although a fresh disposable battery provides about 1.5V, the voltage gradually drops as the battery is used up. Most electronic devices will continue to work even when the battery reaches 1.1V.
A NiMH battery starts out at about 1.2V, but as it discharges, the voltage remains relatively constant, dropping only to about 1.1V just before the battery is fully depleted. Despite starting at a lower voltage, the NiMH battery still provides a usable voltage during its entire discharge.
Voltage is not the only important attribute of a battery. A battery produces a voltage, but the equipment it is powering requires that voltage to be delivered at some current, measured in Amperes (A). If you think of voltage as water pressure, then current is the rate of flow. All the pressure in the world won’t do you any good if the tap is closed. Batteries have a limit as to how much current they can produce, much like your tap can’t provide as much water as the fire hydrant on the corner, even though the pressure might be the same.
In this area, NiMH batteries have a big advantage over alkalines, being able to deliver significantly more current. This makes them well suited to high-current devices like digital cameras. As mentioned above, early NiMH batteries could not supply as much current as NiCd batteries, but this is no longer the case, with NiMH having surpassed NiCd in this area.
If voltage is water pressure and current is flow rate, capacity is the total amount of water available. A higher capacity battery holds more than a lower capacity battery. At a given current (flow rate), the higher capacity battery will provide its voltage (pressure) for a longer time.
Typical early AA sized NiCd batteries had a capacity of about 0.25 Ampere-hours (Ah). This meant that it could provide 0.25A of current for one hour. It also meant that it could provide 0.125A for two hours, 0.5A for half an hour, or any combination of Amperes and hours that multiplied out to 0.25.
As of 2008, a good AA NiMH battery has a capacity of about 2.5Ah. In theory, it can deliver 2.5A for one hour, although 2.5A is more than some batteries’ maximum current. However, such a battery could, for example, deliver 0.5A for 5 hours (since 0.5 × 5 = 2.5). This capacity is about the same as that of a good modern disposable alkaline battery, although the alkaline battery will usually have a much lower maximum current than the NiMH battery.
Note that most batteries have their capacities advertised in milliAmpere-hours (mAh). One Ah is equal to 1000mAh. So for example, a 2500mAh battery is the same as a 2.5Ah battery.
Compared to disposable alkaline batteries, tradtional NiMH batteries have one serious disadvantage: self-discharge. A quality alkaline battery will have a shelf life of about three to five years. You can leave it in a drawer until you need it, open the package, and expect it to still have virtually all the capacity it ever had.
Until recently, most NiMH batteries had a very high self-discharge rate. They would lose capacity even when not in use. A good NiMH battery had a self discharge rate of about 1% per day. After each day of sitting idle, it would have only 99% of the capacity that it had the day before. After about a week, it would be at 93%; after a month, 73%; after three months, 40%.
Recent improvements in NiMH technology have produced low-self-discharge batteries which can retain up to 85% of their charge after sitting idle for a whole year. For a comparison of low self-discharge batteries, please see my article, Pre-Charged (Low Self-Discharge) Rechargeable Battery Roundup.
Choosing a NiMH Battery
There are several factors to consider when choosing a NiMH battery. From most important to least, these are: size, desired usage, capacity, and brand/price.
Size – AAA, AA, C, or D?
Obviously you need to buy the right size battery for the device you plan to use them in. Fortunately, NiMH batteries come in the same sizes as disposable batteries, so just buy the same size of NiMH. By far the most common size is AA. AAA is also common, and C and D sizes are available in some brands.
In order to reduce manufacturing costs, some manufacturers’ D sized batteries have the same capacity as their C size, since they simply enclose the innards of a C battery in a D size shell. Other manufacturers have gone even further, producing only AAA and AA sizes and selling separate C-sized and D-sized shells into which you can insert an AA battery.
If you will be using your batteries immediately in a high-drain device such as a digital camera in a day-long photo session, freshly charged conventional NiMH batteries are suitable. If on the other hand you will put them in a device with very low drain and/or one that is expected to run for a long time such as a TV remote or a wall clock, low-self-discharge batteries are a better choice. In fact, even a high-drain device that is used intermittently, such as a point-and-shoot digital camera that you keep in your pocket or purse, is better off with low-self-discharge NiMH technology.
I’ve personally stopped buying traditional NiMH entirely, using low-self-discharge batteries everywhere. The disadvantage of their slightly lower freshly-charged capacity disappears after a week or two of sitting idle. It’s good to know that my digital camera will be able to take 200 pictures today, or next month. If I used higher capacity traditional NiMH batteries, it might be able to take 250 pictures today, but only a handful if I let it sit for a few weeks.
The capacity of rechargeable batteries has increased steadily over the years, although it appears to have levelled off at around 2700mAh for the highest capacity AAs. Low-self-discharge AA batteries have about 2000mAh capacity although this may improve in the future. Often, batteries sold as part of a bundle with a charger will have lower capacity than the state-of-the-art. These lower capacity batteries are effectively obsolete, meaning that the charger manufacturer probably paid next to nothing for them, and can afford to use them to entice you to buy their charger.
Price and Brand
One thing I’ve found with NiMH batteries is that you generally get what you pay for. Buy cheap batteries and you’ll likely be disappointed. A cheap battery may have the same advertised capacity as a better brand and it may even have the same actual capacity at first. However, I’ve found that the cheaper batteries quickly deteriorate, and after a few recharges, will no longer hold as much charge as when they were new.
Which is the best brand? My personal favourite is Sanyo. Before the switch from NiMH to Lithium-Polymer in the electric model airplane hobby, Sanyo was king. Their batteries took the abuse that we dished out (charging in under 20 minutes and then running at such high currents that we depleted them in 5 or 6 minutes) and kept on working for years. My experience with Sanyo’s consumer AA batteries has been the same; they hold up after years of use and hundreds of recharges.
Another good brand (common in Europe) is Varta. I have an eleven year old Varta battery pack that, when freshly charged, still has 80% of the capacity that it did when it was new.
Brands to stay away from are those you’ve never heard of. I personally also have not had much luck with rechargeables from the two big brands of disposable batteries. Think about it: if your primary source of income was the repeated sale of high quality disposable batteries, would you jeopardize that business by selling a rechargeable battery that only needs to be replaced every five or ten years?
How to Charge NiMH Batteries
The best rechargeable NiMH battery you can buy will not last long if you don’t take good care of it. This primarily means charging it correctly. There are two main classes of chargers, “dumb” and “smart”.
Overnight “Dumb” Chargers (14 to 16 Hours)
A “dumb” charger charges the battery very slowly, typically taking 14 to 16 hours to fully charge a dead battery. When the battery is full, the charger keeps charging it anyway. The excess charge is turned into a small amount of heat, which won’t harm the battery as long as it doesn’t go on for too long. The process is somewhat like filling a bathtub with a very slow stream of water, counting on the excess water to evaporate faster than it accumulates on the floor after the tub overflows.
Dumb chargers were the norm in the NiCd days, and many a NiCd battery was ruined by leaving it connected to the charger all the time. A prime example of this is the popular rechargeable handheld vacuum cleaner, most of which don’t last more than a year or so before the battery refuses to remain charged.
NiMH batteries are more prone than NiCd to damage caused by being left connected to a dumb charger, so such chargers are starting to disappear from common use.
If you do use a dumb charger you should remove the batteries from the charger when the charging is complete. The problem is knowing when this has happened. Most dumb chargers are designed to charge at a rate that takes 14 to 16 hours for a full charge. However, this is only the case if the batteries were fully depleted before beginning the charge. Partially depleted batteries will reach full charge sooner. Furthermore, if you use the charger with batteries of a higher capacity than it was designed for, a full charge will take longer than 14 to 16 hours. In short, properly charging with a dumb charger is a guessing game.
Fast “Smart” Chargers (2 to 5 Hours)
The reason dumb chargers were so popular is that they are inexpensive to make. It needs no brains to determine when to stop. Although overuse of such a charger damages the battery, this damage appears as a gradual reduction in capacity rather than a catastrophic failure.
With NiMH batteries’ lower tolerance to continued overcharging, so-called “smart” chargers have become more common. In addition to not overcharging, these chargers charge much faster, typically in one to five hours depending on the charger. The reason there are no dumb fast chargers is that overcharging at these higher rates can result in a battery overheating, popping its seals, and possibly starting a fire.
A good fast charger uses one of two methods to determine that the charge is complete: −ΔV or ΔT (negative delta-voltage, or delta-temperature). The first of these detects the voltage drop that a NiMH battery exhibits if you attempt to keep charging it when it won’t take any more. The second method detects the temperature increase once the excess charging current starts getting turned into heat. Some chargers use both methods, with −ΔV as the primary method and ΔT as a safety backup.
A good fast charger is much better for the battery than blindly slow charging it. However, a bad fast charger (one that doesn’t turn off very soon after charging is completed) can damage batteries too.
If you travel a lot, there are now several USB-powered AA NiMH smart chargers on the market, including one by Sanyo (reviewed here), and one by Eveready. I’ve also published a do-it-yourself USB-powered charger design.
Note: Be careful when purchasing a USB-powered NiMH charger. The term “USB Battery Charger” has taken on two separate meanings: a USB-powered NiMH charger like we are discussing here, or a device meant for powering other USB-powered devices (like MP3 players). Vendors, even in traditional stores, often don’t know the difference.
Super-fast Chargers (15 Minutes to 1 Hour)
Charging rechargeable batteries in well under an hour is not new. We’ve been doing this for years in the electric model airplane and car hobby, primarily with NiCd batteries. Recently, several 15 minute chargers have appeared for AA consumer NiMH batteries. At first glance this might seem like a great idea, but it isn’t.
Due to the internal resistance of any battery, the charging process produces heat. The amount of heat produced is proportional to the square of the charging rate. In other words, if you charge four times as fast (for example, 15 minutes instead of 1 hour), you will produce sixteen times as much heat!
The reason we got away with it for model batteries is two-fold:
- Because of the high currents used in electric powered models (typically anywhere from 10 to 40A), we used NiCd batteries with extremely low internal resistance (the same ones used in power tools). This translates to proportionally lower heat production during charging.
- The chemical reaction involved in NiCd charging is endothermic, meaning that it causes the batteries to cool. Up to a point, this cooling is more than enough to absorb the heat produced by the internal resistance. The NiMH charging reaction doesn’t have this handy property.
A good quality consumer AA NiMH battery has a much higher internal resistance than the larger Sub-C sized batteries used in electric models. At the same time, it has less surface area with which to dissipate heat. Charging it at the very high currents required for a 15 minute charge will produce immense amounts of heat, which, after a very small number of recharges, this will cause the battery to deteriorate.
The main advantage of super-fast charging is that you can quickly have a set of batteries ready for use. With the introduction of low-self-discharge batteries, this isn’t really necessary any more since you can store the batteries in their fully charged state, ready to go.
What About Trickle Charging?
Other than using low-self-discharge batteries, one way to ensure you always have a set of NiMH batteries ready for use is to attach them to an appropriate trickle charger. A trickle charger is similar to a slow dumb charger, except that it’s even slower. Typically, the charge rate is only slightly more than the self-discharge rate of the battery. The trickle charger produces only enough current to keep the battery from self-discharging. Think of filling a bathtub with an eyedropper, just fast enough to make up for the water that’s evaporating.
Some battery experts feel that trickle charging is detrimental to the long term health of the battery, much like continuous overcharging on a dumb charger is. An alternative approach is to use a dumb charger connected to a timer set to provide power to the charger for only half an hour per day.
I’ve personally used this technique with the 1.6Ah NiMH batteries in my radio-control transmitter and so far the battery is still going strong after about five years of this treatment. Once these batteries start to deteriorate, I will replace them with low-self-discharge ones and just recharge them after I’ve used them.
Sophisticated Battery Managers
If you want to monitor the health of your batteries, a good investment is a battery manager. Like a charger, a battery manager can recharge your NiMH batteries, but it can do more:
- Measure their capacity by discharging them at a known current while keeping track of the time this takes.
- Rejuvenate batteries that have suffered from voltage depression.
- Detect batteries that are having trouble accepting a charge.
- Measure internal resistance to help detect batteries that are going bad.
Traditional NiMH batteries, if stored for a long period of time, should be stored in a cool dry place. They should also be recharged every few months. Turning them over periodically also seems to help, since the electrolyte tends to pool in one side of the battery.
The newer low-self-discharge batteries like Sanyo’s Eneloop are designed to hold their charge in storage. Sanyo recommends storing them at low temperatures, so the fridge might be a good place. It’s probably also worthwhile to recharge them once a year or so.
I keep my spare Eneloops in a drawer in the house, and try to use the least recently recharged ones first. When one becomes depleted, I grab the next one in line. I recharge the depleted one and put it at the end of the queue.
The “Memory Effect” Myth
As NiCd and NiMH batteries age, users often observe that they last less and less long between recharges. This is usually attributed to a phenomenon called “memory effect”, caused by repeatedly using only part of the battery’s capacity before recharging it. The battery appears as if it “remembers” that only part of its capacity was used before it was recharged, and thus refuses to deliver more than that.
True memory effect only occurs in cases where the charge and discharge cycles are exactly the same every time. One of the few places where this occurs is in Earth-oribiting satellites which charge their batteries using solar power for some period of time, and then operate from their batteries while the satellite is passing around the night side of the Earth. These cycles are exactly the same length every time. After a while, NiCd batteries will suffer from memory effect. It is extremely unlikely that this effect is ever seen in consumer batteries.
There is another phenomenon known as voltage depression, which is caused by excessive over charging on a dumb overnight charger, and possibly by excessive trickle charging. This phenomenon manifests itself as a lower than normal voltage without a reduction in capacity. The reason this looks like memory effect is that the lower voltage makes the equipment in which the battery is being used “think” that the battery is almost depleted before it actually is.
NiMH batteries are often advertised as being immune from memory effect. This is probably true, but misleading because they are still prone to voltage depression. In fact, they are more easily damaged by overcharging than are NiCd batteries.
The solution to this problem is to not overcharge the batteries. If using a dumb charger, remove the batteries from the charger after 14 to 16 hours, or sooner if they weren’t fully discharged yet. Better yet, use a smart charger that stops automatically when charging is complete.
Fortunately, voltage depression can be cured. Fully discharging and then recharging a battery two or three times usually brings the voltage back to where it should be. A battery manager is ideal for this.
If you've found this article useful, you may also be interested in:
- Tenergy Centura Low Self-Discharge Rechargeable 9V Battery
- Pre-Charged (Low Self-Discharge) Rechargeable Battery Comparison
- Sanyo’s USB Powered NiMH Charger
- Testing Sanyo’s Eneloop Low Self-Discharge Rechargeable Battery
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