Ni-MH battery is synthesis of H+ and Ni, Its power reserve is more 30% and lighter than Ni-Cd battery, longer service life and environmental friendly. The drawbacks of Ni-MH battery are that the price is more expensive than Ni-Cd battery and the performance is also worse than Li-ion battery.
The "metal" part of Ni-MH battery is actually intermetallic compounds. Many types of intermetallic compounds have been used in the manufacture of Ni-MH battery,it is mainly divided into two categories. The most common is AB5 type, A is a mixture of REE, or together with Ti; B is Ni、Co、Mn and Al. But some electrode of high-capacity battery “containing multiple elements” is constituted mainly by AB2,this A is Ti or V, and B is Zr or Ni, coupled with some Cr、Co、Fe and Mn. All these compounds are played the same role:Reversible formed Metal hydride. When charging, the H + in KOH electrolyte will be released and absorbed by these compounds prevent from forming H2, to maintain internal pressure and volume of battery. When discharging, these H + will return to the original place through the reverse process.
To look at voltage of each cell, Ni-MH battery and Ni-Cd battery are all 1.2V, while the voltage of Li-ion battery is 3.6V and triplication the others. In addition, the weight of the same type between Li-ion battery and Ni-Cd battery is almost equal, but Ni-MH battery is heavier. So, own weight of each battery is different, but in the case of the same voltage output, Li-ion battery because of 3.6V high voltage makes the number to reduce one-third when combination of single cell, so that weight and volume of the battery after forming can be reduced.
Ni-MH battery and Ni-Cd battery all have memory effect. So regular discharge management is also necessary. This regular discharge management is being handled under fuzzy, even some are discharging under the incorrect knowledge (each discharge or discharge after use a few times are vary due to the company) This cumbersome discharge management can not be avoided from using Ni-MH battery. Relatively,Li-ion battery is very convenient in use because it have no memory effect. It does not have to care the number of residual voltage and can be directly charged, the charging time will naturally be reduced.
When fast charging, the overcharging can be avoided by microcomputer in charger. Current Ni-MH battery contains a catalyst which can timely lift the risk caused by over charging. 2H2 + O2 -- catalyst --> 2H2O, but the reaction is effective only in C/10 hours counting from the start time of overcharge(C = the label capacity of the battery). When the charging process begins, the temperature will rise very significantly, some rapid chargers contain fan to avoid overheating(less than 1 hour).
Some manufacturers consider: to use some simple constant current chargers (the current is smaller), no matter whether have timer, all can charge safely for Ni-MH battery and the allowed Charge current is C/10h for long time(The nominal capacity is divided by 10 hours). Actually, some low cost wireless phone base stations and the cheapest battery chargers work like this. While this may be safely, but the service life may have adverse effects. According to Panasonic's 《Ni-MH battery Charging Guide》(link at the bottom of the page), the battery may be caused damage to use the trickle charge; In order to prevent damage, Trickle charge current should be limited between 0.033 × C per hour to 0.05 × C per hour,and the max charging time is 20 hours.
For long-term maintenance of Ni-MH battery, to use the charging method of low frequency pulse - high current maintain better the status than the trickle charging.
The Ni-MH battery which is new or unused for a long time should take a period of "active" time to restore power. Therefore, some new Ni-MH battery can reach their nominal power after several charge - discharge cycles.
In the process of using the battery must be careful. For the series together battery (such as usually arranged of 4 grain AA cells in digital camera), it should be avoided that the battery power runs out completely, and then Reverse charging occurred. It would have irreparable damage to the battery. However, these devices (such as the aforementioned digital camera) can detect the discharge voltage of series battery, and it will automatically shut down to protect the battery when it dropped to a certain extent. Single cell have no more risk and would have been discharged until that the voltage is 0. It have not damage to the battery, actually it is good for maintaining the capacity and quality of battery to charge fully after discharging to the end periodically.
The Ni-MH battery is provided with high self-discharge effect and about 30% per month or more. The self-discharge rate is higher than the 20% of Ni-Cd battery. The more battery charge, the higher self-discharge rate; when the energy drops to a certain extent, the self-discharge rate will decline slightly. The Stored temperature of the battery has a very large impact on self-discharge rate. For this reason, the best state of unused Ni-MH battery for a long time is charged to 40%.
Low self-discharge Ni-MH battery launched to the market in 2005, manufacturers claim that it can save 70-85% capacity to store at temperature 20 ℃ after a year and would be charged by generally Ni-MH battery charger. The discharge characteristics of some low self-discharge Ni-MH batteries are better than Ni-Cd batteries and Li-ion battery at low temperatures.
The mainly electrolyte is KOH (Electrolyte 7moL/LKOH+15g/LLiOH)
When chargeAnode: Ni(OH)2 + OH- → NiOOH + H2O + e-
Negative electrode: M + H2O + e- → MH + OH-
The overall reaction: MH + NiOOH → M + Ni(OH)2
Anode: NiOOH + H2O + e- → Ni(OH)2 + OH-
Negative electrode: MH + OH- → M + H2O + e-
The overall reaction: MH + NiOOH → M + Ni(OH)2
Above this , M for the hydrogen storage alloy, MH for the adsorption of hydrogen atoms in hydrogen storage alloys. The most common hydrogen storage alloy is LaNi5.