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NiMH Battery Knowledge

Source:JYH Technology News Date:2019-09-20

 

Ni-MH batteries are synthesized from hydrogen ions and metal nickel. Their energy reserves are 30% more than Ni-Cd batteries. They are lighter than Ni-Cd batteries, have longer service life and are environmentally friendly. The disadvantage of nickel-hydrogen batteries is that they are much more expensive than nickel-cadmium batteries and have worse performance than lithium batteries.

 

Chemical Composition

 

The "metal" part of a nickel-hydrogen battery is actually a metal intermetallic compound. Many kinds of metal intermetallic compounds have been used in the manufacture of Ni-MH batteries. They are mainly divided into two categories. The most common type is AB5. A is a mixture of rare earth elements (or Ti); B is nickel (Ni), cobalt (Co), manganese (Mn), or aluminum (Al). In some high-capacity batteries, the "multi-component" electrodes are mainly composed of 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). All these compounds play the same role: reversibly forming metal hydrides. When the battery is charged, the hydrogen ion (H+) in the electrolyte of potassium hydroxide (KOH) will be released and absorbed by these compounds to avoid the formation of hydrogen (H2) to maintain the pressure and volume inside the battery. When the battery discharges, these hydrogen ions return to their original place through the opposite process.

 

Weight

 

In terms of the voltage of each cell, Ni-MH and Ni-Cd are 1.2V, while Li-ion batteries are 3.6V. The voltage of Li-ion batteries is three times that of the other batteries. The weight of lithium battery is almost equal to that of nickel-cadmium battery. NiMH batteries are heavier. As you can see, each battery has a different weight. However, due to the high voltage of 3.6V, the number of single batteries in combination can be reduced by one third at the same output voltage, and the weight and volume of the formed batteries can be reduced.

 

Memory Effect

 

Ni-MH batteries have the same memory effect as Ni-Cd batteries.Therefore, regular discharge management is also necessary. This periodic discharge management is dealt with in a fuzzy state, and even some discharges are carried out under incorrect knowledge.  This tedious discharge management is unavoidable when using Ni-MH batteries. Relatively speaking, lithium batteries are very convenient and simple to use because they have no memory effect at all. It does not need to pay attention to the residual voltage at all. It can be charged directly and the charging time can be shortened naturally.

 

Charge

 

When charging quickly, the overcharge of batteries can be avoided through the microcomputers in the charger. Nowadays, Ni MH battery contains a kind of catalyst, which can remove the danger caused by overcharge in time. 2H2 + O2 - Catalyst - > 2H 2, but this reaction is only effective within C/10 hours from the start of overcharge (C = the capacity indicated by the battery). When the charging process starts, the temperature of the battery rises significantly. Some fast chargers (less than 1 hour) contain fans to avoid overheating. Some manufacturers believe that using simple constant current (and low current) chargers, whether or not there is a timer, can safely charge nickel-hydrogen batteries, allowing long-term charging current of C/10h (nominal battery power divided by 10 hours). In fact, that's how some of the cheapest cordless Telephone base stations and the cheapest battery chargers work. Although this may be safe, it may have adverse effects on battery life.

 

For the long-term maintenance of Ni-MH batteries, using low-frequency pulse-high current charging mode is better than using trickle current charging mode to maintain the battery state.

 

Newly purchased or long-term unused nickel-hydrogen batteries require a period of "activation" to restore battery power. Therefore, some new Ni-MH batteries need several charging and discharging cycles to achieve their nominal power.

 

                                   

                                   

 

Discharge

 

Care must also be taken in the use of batteries. For several batteries in series (e.g. the usual arrangement of four AA batteries in digital cameras), it is necessary to avoid the battery completely depleting electricity and then "Reverse charging". This will cause irreparable damage to the battery.  Usually, these devices (such as digital cameras) can detect the discharge voltage of series batteries, and when it drops to a certain extent, it automatically shuts down to protect the batteries. A single battery will not be more dangerous than that. It will only discharge until the voltage is zero.  This will not cause damage to the battery. In fact, the battery is periodically discharged and then filled to maintain the capacity and quality of the battery.

 

NiMH batteries have high self-discharge effect, about 30% or more per month. This is higher than the self-discharge rate of 20% per month for nickel-cadmium batteries. The fuller the battery is, the higher the self-discharge rate will be; when the electric quantity drops to a certain extent, the self-discharge rate will decrease slightly. The temperature of battery storage has a great influence on the self-discharge rate. For this reason, the nickel-hydrogen batteries that are not used for a long time are better charged to 40% of the "half-full" state.

TypeInfo: Battery Knowledge

Keywords for the information:Ni-MH Battery