NiMH Battery knowledge

- Mar 09, 2017-

NiMH battery is made up nickel hydrogen ions and synthesis, power reserve 30% more than the nickel-cadmium battery and lighter than the nickel-cadmium battery, longer cycle life, and non-environmental pollution. The disadvantage is the price of nickel-metal hydride battery is more expensive than the nickel-cadmium battery.
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Nickel-metal hydride battery in the "metal" part is actually the intermetallic compound. Many types of intermetallic compounds have been used in the manufacture of nickel-metal hydride batteries, which are mainly divided into two categories. The most common are AB5 a class, A is a mixture of rare earth elements (or) plus titanium (Ti); B is a nickel (Ni), cobalt (Co), manganese (Mn), (or) as well as aluminum (Al). And some high-capacity battery electrode "containing a variety of ingredients" is constituted mainly by the AB2, where A is titanium (Ti) or vanadium (V), B is zirconium (Zr), or nickel (Ni), plus some chromium (Cr), cobalt (Co), iron (Fe) and (or) manganese (Mn). They are the same role played by all of these compounds, irreversible formation of a metal hydride. When charging the battery, potassium hydroxide (KOH) electrolyte hydrogen ions (H+) are released; it will absorb these compounds, to avoid the formation of hydrogen gas (H2), to maintain the pressure and volume inside the battery. When the battery is discharged, these hydrogen ions through the reverse process will return to the original place.

In each single battery voltage, the nickel-metal hydride and nickel cadmium are both 1.2V, while the Lithium battery is 3.6V. Lithium battery voltage is three times more than the other two. And for same battery, the weight of Lithium batteries is almost equal to nickel-cadmium battery, while nickel-metal hydride battery is heavier. We can see that battery itself has different weight , but because Lithium battery’s voltage is up to 3.6V when making a battery pack to output the same voltage, single cell number can be reduced by one-third, this can reduce the battery weight and volume.

Memory Effect
Nickel-metal hydride batteries and nickel-cadmium batteries have the same memory effect. Therefore, regular discharge management is necessary. This periodic discharge management belongs to the fuzzy state, even some discharge methods are not in the right way (each discharge or after discharging a few times is different due to different companies). This cumbersome discharge management can not be avoided when using NiMH batteries. In contrast, Lithium battery because of no memory effect, it is very convenient and simple to use. It is complete without regarding how much residual voltage the battery still has, can be charged directly, and the charging time can be reduced naturally.

When quick charging, it could avoid overcharging the battery by a microcomputer inside the charger. Now NiMH battery contains a catalyst that could relieve the danger because of over charging. 2H2 + O2 - Catalyst –> 2H2O, but the reaction is only valid to count from the time beginning of overcharging to C / 10 hours (C = battery capacity indicated). When the program of charging starting, the battery temperature will rise very significantly, some quick charger (less than 1 hour) battery contains a fan to prevent overheating.
Some manufacturers believe that it’s very safe when using some simple and constant current (and low current) chargers to charge NiMH batteries, despite no timer, the allowable charging time is C / 10h (standard capacity of the battery divided by 10 hours). In fact, some low-cost wireless phone base stations and the cheapest battery charger is such work. Although this may be safe, the battery life may have adverse effects. According to "NiMH battery charging guide" of Panasonic (link at the bottom of the page), it maybe damage the battery if we trickle charge (with a very small current charging time) a battery for a long time. In order to prevent damaging the battery, trickle charging current should be limited from 0.033×C per hour to 0.05×C per hour, the maximum charging time is 20 hours.
For long-term maintenance of NiMH batteries, low-frequency pulse to high-current charging way is better than trickle charging way to keep status.
It needs some “active” time to recover its electric energy when you buy a new battery or a battery you haven’t used it for a long time. Thus, some of the new NiMH batteries need to go through a few charge - discharge cycles to reach the nominal power.

We should be very careful to use the battery. For a few batteries in series, (Such as the arrangement way about 4 pieces battery of camera), it should avoid deplete electric energy, then reverse the charge. This will damage the battery irreparably. However, these devices (such as camera mentioned previously) could detect the discharging voltage of the battery in series. When it drops to a certain extent, it will automatically shut down to protect the battery. A single battery hasn’t the risk mentioned above. It will discharge until voltage to zero, this won’t damage the battery. In fact, it’s good for maintaining the capacity and quality to periodically discharge completely and charge completely.
NiMH batteries have a higher self-discharge effect, about 30% or more per month, which is higher than NiCd batteries that are 20% a month. The more battery charging is, the higher rate of self-discharge it can become. The rate of self-discharge will drop a little when electric energy drops to a certain extent. Storage temperature has a great effect on the rate of self-discharge. For this reason, it is good to charge NiMH battery to 40% “half-full” status if the battery won’t be used for a long time.
Low self-discharge NiMH battery was launched to the market in 2005; the manufacturer claims that it still remains 70-85% of the total NiMh battery electric energy in 20℃ room after 1 year. The battery could be charged by general NiMH battery charger. Some low self-discharge NiMH battery has better discharge characteristics than the alkaline battery and Li-ion battery during low-temperature environment. 

Different models (particularly different volume) of the battery, the higher capacity of the battery is, the longer using time it can achieve. Despite the size and weight factors, of course, the higher capacity is better. But the with the same cell type, nominal capacity (for example, 600mAh) is the same, but the actual measured initial capacity is different: for example, one of 660mAh, the other is 605mAh, then 660mAh is better than the 605mAh.

The actual situation is that electrode increases the initial high capacity material, while reducing the stabilizing material of the electrode, the result is after reusing dozens of times, the high-capacity battery's capacity rapid exhaustion, and the low capacity battery is still strong. Many domestic battery manufacturers often use this way to get the high-capacity battery. And after six months of standby time, standby time is really bad.

Those consumers’ AA NiMH batteries, generally is 1400mAh, but there is also a standard ultra-high capacity (1600mAh), reason is the same.

Improve the capacity will sacrifice the life cycle if manufacturers do not modify up on battery material, it is impossible to truly improve the battery capacity. Battery materials, specific surface area, is very important to study, the specific surface area of the battery material testing data only uses the BET method to detect if the result is true and reliable, there are many domestic types of equipment can only do direct contrast detection, and now the country will also be eliminated. Now, domestic and foreign unify multi-point BET method, domestic and foreign make standard based on BET method, please refer to China National Standard (GB / T 19587-2004) - BET gas adsorption principle solid material surface area measurement method. The specific surface area detection is actually relative to time-consuming work, because the adsorption capacity of different samples, test some samples may need to spend a whole day, if the test process is not fully automated, and that testers can not leave at all, and to be highly concentrated, observe the instrument panel, control knobs, a little inattentive will lead to the failure of the testing process, this will waste testers a lot of valuable time. F-Sorb 2400 tester is a truly detection instruments to achieve the specific surface area (both direct comparison method), the more important is that the surface area F-Sorb 2400 tester is so far the only fully automated intelligent machine in the surface area of the testing equipment, its test results are in high consistency with international, stability is also very good, while reducing human error and improving the accuracy of test results. 

Care and Use
(1) In general, for the first time to charge before using, the new nickel-metal hydride batteries contain only a small amount of electricity. However, if the battery exits factory relatively short time, power is adequate, we recommend use first and then recharged.

(2) Buy a new nickel-metal hydride rechargeable batteries generally have to go through the process of using 3-4 times, in order to play to the best performance, a lot of friends encountered a small problem for the first time charging, for example, after the first charge film is not as much as imagination. After 3-4 times charge, it solved.

(3) Although nickel-hydrogen battery memory effect is small, we still recommend you to use up each time after their charge, and charge full one time, not charge a few times and then discharge. This is "longevity" an important point.

(4) Battery charging, the charger should pay attention to the surrounding environment, to deliberate a fan blowing there is no need, but note that the charger does not put too much debris around. Ordinary users in the use of the battery, the battery is often not having a dedicated storage bag; after the user replaces the battery, the battery will be habitual hand away, regardless of the place where is clean, damp. The consequence is that the battery they get dirty easily, easy to contact with the metal such as keys and others, easy to damp, and these are the enemy of batteries. Recommendation: Users should set up a special point to place cell, and keep the battery clean. In order to avoid the loss of power, keeps the internal battery and battery contact points at both ends of the lid clean, if necessary, uses a soft, clean dry cloth graze.

(5) For a long time when not in use, remember to remove the battery from the battery compartment in a dry environment, recommended put batteries into the box, to avoid short circuit.

(6) Without long-term use, after the Ni-MH battery stores a few months, the battery naturally goes into a "sleep" state, the battery life is greatly reduced. If the Ni-MH battery has been placed for a long time, I recommend that you first use the slow charge to charge. Because it is the best conditions for storage, that Ni-MH battery is charged to save about 80% capacity. This is because the nickel-metal hydride batteries self-discharge is larger (about 10% -15% a month), if the battery is fully discharged and then be replaced, for a long time without use, the battery self-discharge phenomenon will cause over-discharge of the battery, damage the battery. Do not believe? What do you think about buying a new nickel-metal hydride rechargeable battery; all of them have electricity that is the truth. Recommendation: compared more to correct the wrong point of view, maintenance the battery in the right direction, otherwise it will backfire.

(7) NiMH discharge. Expert advice: try not to use nickel-metal hydride battery discharge; over-discharge can cause charging failure, more harmful than Ni-MH battery memory effect itself. (8) Using Multimeter self-test to test battery is full or not. General NiMH battery voltage is less than 1.2V before charging, after a full charge, normal voltage is about 1.4V. According to this, it is very easy to judge the state of the battery.

(9) Chargers are divided into fast charge and slow charge. Slow charging current is small, usually around 200mA, such as our common charge current is about 160mA. Long charging time, charging an 1800mAh Ni-MH battery should up to 16 hours or so. Although time is a bit slow, but the battery will be filled very full, and will not hurt the battery. Usually in the fast-charge current 400mA above, the charging time is significantly reduced; you can get 3-4 hours, also win everyone's favorite. Many different types of fast charge have different prices. So we often have questions, with a fast charge, why the price differences are so big? Good charger fast charge is especially good with anti-overcharge protection feature, for example, we commonly need Panasonic BQ390 charger is particularly outstanding in this respect, excellent chip software design makes excellent performance when charging the battery, but also minimum the damage to the battery because of fast charge.

(10) There is a conflict, slow charge not hurt the battery but the charging time is too long; fast charge can save time, but there is damage to the battery, even the best Panasonic BQ390 charger only reduces harm, but not completely avoided. The method to solve this conflict is to buy a quick charge and a slow charge. Charge with fast charge first, for example, after 5-10 times, uses a slow recharge twice. This will restore optimum performance of the battery.

(11) The battery generally used as the battery pack, that is 4 or 6 series, this time, the balance of each cell is very important, otherwise because of a battery problem has affected the work of the entire battery pack. First, to ensure consistency of battery capacity, it is better to select the same brand of the same model at the same time. Then, to maintain a consistent internal battery power, simply check if the electric battery is either full or empty. If there is more cell battery pack consisting of a number of groups, you can try the "Featured" look. Specifically to say, the capacity, battery voltage and other parameters similar series put into a single set of batteries, due to inadequate conditions, in general, measure the voltage after the full charge and full discharge.

(12) Finally, talk about the charge and discharge. Upscale NI-MH battery charger uses battery voltage detection -DELTAV to determine whether the battery is fully charged. Somewhat it is similar to the voltage discharge curve and battery charging, and is relatively fast rising at the beginning, then slowly rising, until full, voltage begins to decline rapidly, but the decreasing is not great. Prior to commonly used nickel-cadmium batteries are also similar, but the dropping curve is greater than the NI-MH. And most chargers (cheaper kind) are often used in the market is the constant voltage charging, such as the old GP rechargeable 1.4V treasure is constant when it rushes to 1.4V voltage because there is no difference and charge ends. This often results the battery can not be filled, especially in some older batteries, due to the internal resistance increases, lower real voltage adds to the battery. And this charger current tends to be smaller, often will charge more than 10 hours. But with -DELTAV automatically cuts off the charger, it can accurately control the charging time, it can be more reliable in the use of high-current charge. Large current charging nickel-metal hydride battery does the damage but not everyone thought the stakes, when on the contrary, the use of DC current situation, but also requires a high-current charge. The first is a matter of time, do not talk about it. Then, the nickel-metal hydride battery has a characteristic, that is, the greater your current charge, it can release the greater current, and now DC is electric tiger, the current is not small, so relatively speaking the use of relatively large current charge is a wise choice, you can let the battery fully discharge. General AA charge current should not exceed 1.5C, C for the battery capacity that is for 1000MAH batteries do not exceed 1.5A charge current. I generally use 0.5C to charge (my charger adjustable current). Under normal circumstances, DC black can be taken to charge, NI-MH has a little memory effect. However, after a period of time, as well as to balance the battery and activate the battery, it should control well the battery discharge termination voltage. NI-MH battery termination voltage is 0.9V, try not to over-discharge, and it can stop discharge when each battery reaches 0.9V. The Ni-MH battery is not strong as nickel-cadmium batteries on overcharge and over discharge and is more sensitive to high temperatures. Discharge temperature; in general, do not let the battery temperature above 45 degrees. When the battery is fully charged, the battery will hot, a large current for full charge should be about 42 degrees, not more than 45 degrees, otherwise, life will soon be reduced, and the battery internal resistance will increase. Also, after charging the battery temperature is higher, it has to wait for the battery to cool before discharge. After a long time without using, it is best to re-activate the battery by charging and discharging a few times. In order to avoid short circuit, try not damage the packaging intact skin in daily use. Do not beat or impact the battery, do not fire and so on. 

Electrochemistry Theory
Mainly KOH as electrolyte (electrolyte 7moL / LKOH + 15g / LLiOH)

Positive: Ni(OH)2 + OH- → NiOOH + H2O + e-
Negative: M + H2O + e- → MH + OH-
Total reaction: M + Ni(OH)2 → MH + NiOOH

Positive: NiOOH + H2O + e- → Ni(OH)2 + OH-
Negative: MH + OH- → M + H2O + e-

Total reaction: MH + NiOOH → M + Ni(OH)2
Remark: M is a hydrogen storage alloy, MH adsorption of hydrogen-absorbing alloy hydrogen atoms. The most commonly used hydrogen storage alloys are LaNi5.

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