With new gadgets like laptops, iPods, smartphones and GPS navigators hitting the market every day, we are increasingly becoming reliant on a portable power supply such as compact rechargeable batteries, also called cylindrical cells. The common types of rechargeable batteries that dominate the market today are nickel-cadmium (NiCd), nickel-metal hydride (NiMH), lithium-ion (Li-ion), polymer (Li-poly) and lithium-iron phosphate (LiFePO4) batteries.
By Srabani Sen
Tuesday, January 03, 2012: These cells are available in various specifications, such as AAA, AA, A, C, D, and F, with standard heights. Variations in heights have codes such as 1/3 AA, 2/3 AA, 4/5 A, 5/4 A, etc. For 18 mm and 26 mm diameter cells, variations in height result in cells of sizes 18650, 18500 (where 18650 implies a height of 1865 mm and 18500 implies a height of 50 mm), etc. This convention is used for Li-ion and LiFePO4 cells. With each dimension (for example, AA), various capacities are available. For example, AA in NiMH is available from about 800 mm to 2300 in the flat top design. “Batteries are available as consumer cells (button tops, which are not recommended for making packs) and industrial cells (flat tops, recommended for pack making). Industrial cells have much higher cell-to-cell consistency than consumer cells. While consumer cells are sold in attractive blister packs, industrial cells are sold in boxes since the buyers are generally pack makers,” explains Tarun Desai, chief executive officer, Prime Products.
All these cells have specific advantages and drawbacks but lithium batteries, together with metal hydride, are the ones with a stronger potential. Metal hydride technology has been developed earlier, but lithium technology is catching up quickly with growing market demands.
Small rechargeable batteries and battery packs today are used in almost all industries including automation, medical electronics, weighing machines, lighting, handheld terminals, equipment, etc. “In modern day industries, small rechargeable batteries are in great demand due to space constraints in the battery cabinets and due to the portability of the equipment,” points out Manoj Pawar, sales manager, SBU manufacturing, Evolute.
LED lighting is also triggering a large demand for small battery based battery packs. LED lights are used in torches, exit lights, solar lights, etc. Other important market drivers are instrumentation and medical devices.
Rechargeable battery technology
Nickel-cadmium: NiCd batteries are adopted to power small appliances such as video cameras, cellphones and computers. But these run out of charge too quickly, even when not in use. However, NiCd is the most durable and reliable battery. It has capacities ranging from 600-1000 Amp hours (mAH), has a good temperature performance envelope, and endures heat and overcharge best. The performance of these batteries can improve dramatically by interspersing discharge pulses between charge pulses. This is known as ‘burp’ or ‘reverse load’ charging. This method of charging allows the battery to more efficiently degass while charging.
NiCd batteries should not be left in a charger for too long, and should not be subjected to shallow discharge (that is, using the battery for a short period of time, then recharging). This type of use may result in crystal formation inside the battery which will diminish performance. This is known as ‘memory effect’.
NiCd cells have about a five year life span and usually fail during internal shorts or excessively high resistance. These cells can suffer from a form of voltage sag, but this problem can be corrected with a couple of exercise cycles and a good top-off charge.
“The major advantage of NiCd batteries is seen in cyclic applications. They are capable of delivering high currents (up to 5 C). Hence, these batteries are used for motor loads and also in some medical equipment where high current discharge is required,” explains Manoj Pawar.
Lithium-ion: Lithium-ion is the latest technology for portable equipment and electronic devices, but currently, this battery is the most expensive. These batteries have the highest energy density among commercial batteries—twice that of NiCd. They also have a very low self discharge rate. These are disposable batteries that have lithium metal or lithium compounds as an anode. Depending on the design and chemical compounds used, lithium cells can produce voltages from 1.5 V to about 3.7 V. Disposal of lithium based batteries may cause some concern since moisture may seep into the cell after corrosion, which could lead to an explosion.
Nickel metal hydride: NiMH cells are more or less similar to NiCd cells. A NiMH battery uses a hydrogen absorbing alloy for the negative electrode instead of cadmium. It can have two to three times the capacity of an equivalent sized NiCd battery. The energy density of a NiMH cell is similar to that of a lithium-ion cell, but the rate of self-discharge is higher. This means that a stored NiMH battery will lose charge more quickly.
A typical AA NiMH battery has nominal charge capacities ranging from 1100 mAh to 3100 mAh at 1.2 V, measured at the rate that discharges the cell in five hours. The typical specific energy for NiMH AA cells is about 100 Wh/kg, and for other NiMH dry cells about 75 Wh/kg (270 kJ/kg), compared to 40–60 Wh/kg for NiCd, or 100-160 Wh/kg for Li-ion. NiMH batteries self discharge quickly if you leave a NiMH pack unused, it will lose its charge much faster than NiCd packs will.
Yet, NiMH cells are considered the best rechargeable batteries. They come fully charged, and can hold their charge for a longer period. As a result, they are suitable for low drain devices like remote controls and flashlights. But they are also ideal for use in high drain electronic devices like digital cameras.
“NiMH batteries are more compact than NiCd batteries and, hence, are used mainly where space is a major constraint and batteries are kept on standby mode for a long duration and a low discharge current as compared to the capacity required,” says Manoj Pawar.
Polymer batteries: These are metal free lightweight batteries made of cellulose fibre (paper) coated with thin layers of composite electrode polymer materials
instead of toxic metals. They are recyclable, mechanically sturdy, and less expensive to make than the highly poisonous and dangerously flammable batteries that currently dominate the gadget industry.
Conductive polymer batteries, whether made with normal paper or another fibre, can literally be made paper thin and paper flexible. But when these batteries are made with normal paper, they show a rapid degradation in their ability to hold a charge. Algae cellulose fibre paper, made from a stringy variety of algae, is a better choice.
What players offer
Kandhari Photo Electronics, a major battery supplier, offers a large variety of rechargeable batteries under the brand names Digicam (for cells) and Green Power (for battery packs). Its NiMH batteries include AA, AAA, button cells, A, sub C, C size and D size batteries. There are standard and cut/plus sizes in diameters like 1/3AA, 1/2AAA 2/3AAA 4/3A, in different capacities. “The NiMH battery we offer has new technology that reduces self discharge rates and makes it a ready-to-use battery. If this battery is not used for six months, it will retain up to 70 per cent of its charge. This feature is very useful for consumers because they typically use their cameras after three to four months, and the battery will still be charged. However, most standard NiMH batteries get self
discharged within one or two months—they have high self discharge rates,” says SS Kandhari, managing director, Kandhari Photo Electronics P Ltd.
In the Li-ion battery range, Kandhari offers cylindrical 14500 and 18650 sized batteries in various capacities. These are single celled with a high capacity 3.7 V 4400 mAh and a high 800-1000 charge/discharge cycle life. It also offers prismatic and Li-poly batteries.
“We will soon launch an emergency charger with a USB 5 V output power bank, which would recharge mobile phones on the move, digital cameras with Li-ion/Li-poly batteries, iPods, etc. It is a slim, pocket sized charger, which can be recharged with a travel charger or any laptop/computer USB socket,” says SS Kandhari.
Kandhari Photo Electronics assembles battery packs for various customised applications as per customer requirements. These battery packs are made by joining the cells together with spot welding machines— direct soldering is not recommended on cell surfaces as it affects its performance and life.
Kandhari batteries and battery packs have a 6-12 month warranty. They are tested for quality and capacity before they are dispatched. All quality testing machines, like capacity testers, lifecycle testers, IR testers, etc, are installed inhouse.
Prime Products also offers a wide range of rechargeable batteries like NiMH, NiCd, Li-ion, LiFePO4 and NiZn. It also assembles batteries to meet customer
specifications under the brand name Surya Powerpacks. “Our customers are from different industry segments, which include UPS systems/inverters, solar energy systems, medical electronics, weighing scales, security systems, instrumentation, etc. Some of our clients include product development engineers who appreciate our batteries for their design,” says Tarun Desai.
Prime Products has started stocking NiZn cells, which can replace primary cells in applications where the cells have to be replaced frequently. NiZn has voltage of 1.5 V to 1.6 V unlike NiMH and NiCd, which have voltages of 1.2 V. NiZn is more expensive than primary cells, however, it will be cost effective if one considers advantage of its cyclic life.
Prime Products offers these batteries with capacities from 300 mAH to 15 AH. These can be combined in series and in parallel to provide battery packs of different voltages and capacities to meet customer demands.
While Prime Products imports basic cells from reputed international manufacturers, it assembles battery packs from these cells. “Our batteries are technologically advanced. Most of the recent advances are related to energy densities. Now, cells of standard sizes are packed with more and more energy. AA cells in NiMH are available in up to 2300 mAH in industrial versions and up to 2800 mAH in consumer versions. These capacities were earlier not possible,” informs Tarun Desai.
Fusion Power Systems offers NiCd, NiMH and Li-ion batteries. “The Indian market for rechargeable batteries for electronic appliances is not large enough to set up a single line high volume production facility for these batteries. Hence, we import these batteries and market them under our brand name,” says Kapil Sood, managing director, Amptek. According to him, NiCd batteries are by far the best in terms of the maximum number of charge/discharge cycles. Next come the Li-ion and Li-polymer batteries, followed by NiMH. “Be sure to take the number of charge/discharge cycles into account when comparing battery costs because Li-ion battery packs can have up to twice as many charge/recharge cycles as compared to NiMH batteries. Furthermore, the Li-ion battery pack needs to be charged less often due to its higher capacity, and due to its much lower self discharge rate,” says Kapil Sood.
Evolute offers a wide range of battery packs in all the three chemistries—NiMH, NiCd and Li-ion. “Having our own battery pack manufacturing setup enables us to cater to our customers’ specific requirements with respect to voltage and the configuration of the battery packs,” says Manoj Pawar. “Our battery packs are assembled using NiMH and NiCd cells of 1.2 V each. Thus, the battery packs can be assembled with voltages in multiples of 1.2 V, for example, 3.6 V, 7.2 V, 9.6 V and so on,” he adds.
Evolute has sophisticated battery pack making procedures and world class machinery. This comprises internal impedance testers, microcontroller based spot
welding machines, etc. It adopts the procedure of matching the internal impedances of the cells before assembling the packs. This leads to the optimum performance of battery packs. “Our battery packs have reliable protective circuit, which is necessary for the safe functioning of the battery packs. We have ready stocks of cells in ample quantities that can be assembled into packs as per customers’ requirements. With this, we can offer quick deliveries and help our customers in improving their delivery schedules,” informs Manoj Pawar. “Our advanced manufacturing process helps us in cost and quality optimisation,” he concludes.
Some applications of small rechargeable cells
- Digital cameras: AA NiMH batteries/Li-ion (prismatic shape) batteries
- LED lighting applications: Li-ion, NiMH batteries
- Handheld terminals: Li-ion batteries
- UPS systems/backup energy storage: NiMH battery packs, Li-ion
- Video cameras, cellphones and computers, some medical equipment: NiCd
- Remote controls, flashlights: NiMH
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