NiMH batteries are similar to Ni-Cd batteries with hydrogen absorbing negative electrode. These batteries have higher capacity as compared to Ni-Cd batteries and are typically used for applications where high drain current is required, such as in a digital camera. High drain applications are those where a large amount of power is required over a short span of time. NiMH is able to do well in this area because of its lower internal resistance.
NiMH batteries have the highest self-discharge rate, typically 30 percent per month. However, if longer shelf life is required then Low Self Discharge (LSD) NiMH batteries are available whose self discharge rate is as low as 2 percent per month. LSD NiMH batteries come at the cost of lesser capacity for the same size as compared to a normal NiMH battery.
A NiMH battery has the same nominal voltage as NiCd battery i.e. 1.2V per cell. A NiMH battery does not suffer from the memory effect problem but has, in addition, Self Discharging issues:
• Exposure to high temperature: At higher temperatures, chemical reactions tend to accelerate the aging process within the battery. Also, at elevated temperature NiMH batteries tend to discharge faster.
• Cell Reversing: Same as discussed with Ni-Cd battery.
Lead Acid Cell:
As the name implies, this battery uses lead-derived materials for the electrodes and an acidic solution for the electrolyte. It uses Lead Dioxide and porous lead for the positive plate and negative plate of the cell respectively in an electrolyte of Sulphuric Acid (H2SO4). Use of heavy metal elements makes this battery toxic and hazardous if not properly disposed of.
Lead acid battery is one of the most popular and extensively used batteries in the industry because they are robust and low cost. They are available in various variants to suit the needs of many end-applications. The biggest market for the lead acid battery is the automobile industry where high current drive is required. In such applications, the battery is used for starting the engine and to provide charge when electricity generated by the alternator is not sufficient to meet load requirements. Because of the application requirements, batteries used in automobiles are not designed for full discharge.
For applications which require deep discharge, thick plates are used for the positive and negative electrodes. This increases the resistance of the plates which in turn reduces the peak current but makes them capable of withstanding frequent discharge.
Lead acid batteries can be shipped without electrolyte; this provides a distinct advantage of “infinite” shelf life. Each lead acid cell provides 2.1V and is stacked together to come in wide range of sizes and capacity. These batteries, however, suffer from the following problems:
• Gassing: When the battery is charged faster or more than what it can absorb, the excessive energy is turned into heat which causes the electrolyte to boil and evaporate. This causes the production of Hydrogen and Oxygen. Sealed batteries are designed to recombine them into water, thus prolonging the life of the battery, but in case of batteries with vents, loss of electrolyte may create explosive conditions and can permanently damage the battery. Therefore, these batteries, require regular maintenance of their water level.
• Sulfation: This is a product of deep discharge which causes the crystallization of lead sulphate on the battery electrodes. It hinders the recharging of battery and may permanently damage the battery by expanding further causing short circuit between the two electrodes. Sometimes this can be corrected by equalization where the battery is overcharged in a controlled environment to boil the electrolyte and gas to break the bonds formed because of sulfation.
• Corrosion: Corrosion of the external metal contacts occurs because of different materials being used for the contacts. Because of overcharging or spillage of electrolyte, sulphuric acidic fumes or electrolyte reacts with the metal and makes it corrode.