In the last ten years laptop disk drive densities have increased by a factor of 400. CPU (central processing units) speeds have gone up by a factor of 40. Cell phone weight has dropped by a factor of 10. But during this heady period, battery power capacity has increased by a factor of only two.
Most consumers are oblivious to this disparity between systems and batteries. Portable system users expect their devices to run for many hours on a single charge. And, when it comes to charging they expect it to be quick without any gradual loss of performance.
To meet the demand for more power with less weight a number of strategies have been adopted. These include R & D into new battery chemistries and revisiting older chemistry.
NiCd batteries have been the norm in portable electronic devices, offering decent power and reliability. But they are heavy, require frequent recharging and have significant environmental issues concerning the use of cadmium as an electrode due to its toxicity and recycling difficulties. NiCd also suffers from premature loss of battery capacity known as memory effect, where cells in the battery form internal shorts that prevent charging and reduce total battery voltage.
First introduced in 1989, NiMH batteries replaced NiCd, boasting reduced size and fewer environmental fears. NiMH batteries have twice the capacity of NiCd but have slower discharge and recharge times and are more expensive. Both have a high self discharge rate, with NiMH the worst offender at around 30% loss per month.
Past and present
Using a liquid lithium electrolyte, the Li-ion battery technology exhibits a higher energy density than NiCd or NiMH, thus requiring fewer batteries and allowing devices to be made smaller and lighter.
However Li-ion costs more than NiCd or NiMH and there are serious safety concerns with liquid lithium, particularly its flammable nature. Liquid Li-ion cells are designed to vent if abused or overheated. But, if a cell fails to vent properly under extreme conditions it may explode, spilling liquid lithium, which could catch fire if the external conditions are hot enough. Liquid Li-ion technology still uses a weighty metal case. This reduces the practical energy density of the battery and the more metal cases used in combination, the heavier the overall package.
A new chemistry is tipped to replace NiCd or NiMH designs as the preferred power source for electronic devices. The rechargeable Lithium-ion polymer battery (LiPB), has arisen to meet the demand for increased power density, smaller size, design flexibility, and safety.
Discarding the conventional metal-can assembly, the LiPB uses a solid polymer electrolyte rather than a liquid electrolyte solution. The solid polymer is physically a solid but appears to ions as a liquid that they can pass through. With no liquid to escape, the solid electrolyte is simply sandwiched between electrodes formed out of thin sheets.
The cell is contained in laminated foil and sealed at the edges to form an entire battery. The resulting cell is thin and as flexible as a rubber mat. The basic internal structure of a solid polymer cell can be configured to virtually any size and the cells stacked to produce ultra thin battery packs with a broad array of voltages and capacities. Batteries as thin as a 0.4mm credit card with the same power capabilities as standard battery packs on mobile phones have appeared. This is possible because lithium is the lightest known metal on Earth and has the highest energy density of any solid.
LiPB potentially delivers the best power available for rechargeable batteries. LiPB and Li-ion batteries have similar characteristics with both offering voltage in the 3 -4V range as compared to 1.2 – 1.5V for NiCd and NiMH cells. Its recharge cycle life of 500, similar to Li-ion designs, is its Achilles heel. This is where NiCd and NiMH still excel with lifecycles of up to 2000 and 1000 respectively.
The solid polymer design requires no venting, eliminating the problem with liquid lithium. The materials used are benign and contain no toxic metals . Moreover, solid polymer batteries require no special handling and face no transport and disposal regulations, thus alleviating a traditional industry headache.
Although the manufacturability of LiPB technology has not been verified, volume production is expected to be possible, claims the its developer, the Ness Corporation.
The use of batteries is booming, and recent brownouts in California highlight the limits of the power grid. At around 8%, the internet alone is the largest consumer of electric power in California. This includes data centres and server farms. Since no power stations are being added in California, there will be more power generation at sites, decentralising the power grid. This will include banks of batteries for load levelling, starting, and uninterrupted power supplies.
Thus efficient use of available energy is paramount. Usually more efficient devices consuming power are used to husband batteries, but now at the supply end smart battery electronics have been developed. Smart battery chips contain battery behaviour models that predict performance for certain conditions.
For example a lap top computer may run 30-40% longer using smart battery management software.
Smart battery management can’t increase the total capacity of a battery, but it can make more of the capacity available to the system. Undermining much management software is the inability to know accurately how much potential energy is available in a battery.
The most common method is to place a small resistance in line with the power transistor and sample the voltage drop across the resistor to determine current flow. From current usage the remaining capacity can be computed. The voltage drop approach has some drawbacks.
First, it consumes power in the resistance, thus reducing the total available power. Second, it is a sampling technique where the voltage drop is sampled periodically and the average of the samples is used to compute power consumption. Also the sampling affects the accuracy of the measurement, especially if there are high current peaks between sampling periods. Thirdly, it’s hard to measure low current using this technique.
Xicor Corporation is working on a way to measure current directly and continuously by sensing it passing through the in-line power transistor rather than through a separate sensing resistor. This removes the sensing resistor, and eliminates power consumption.
Despite these new technologies, battery use is still application specific. For example says Gary Allen of Exide Technologies, ‘the big advantage of NiCd over lead acid for backup in railway signalling and telecom applications is in temperature stability.
Sealed lead acid batteries suffer as temperatures rise. Remote sites can be exposed to extreme temperatures. Not only is their life shortened but also their charging must be compensated.’
However, bespoke solutions can be developed to overcome such problems. DMS Technologies offers cheaper lead-based chemistry solutions with an operating range of -40 to +80°C for high-current remote applications.
‘Older’ chemistries such as lead acid (Pb/Ac) can also be reworked to compete with the latest rechargeable technology. Innergy Power’s ThinLine sealed lead batteries can compete in many levels with Li-ion, particularly with regard to cost. In addition, the planar design overcomes the geometric incongruity of packaging round cells into rectangular cases resulting in a 10-25% increase in volumetric efficiency. This is true to some degree for all rectangular packaged batteries.
The thin planar design is accomplished through two patented components. First, a patented pin and socket case design provides internal reinforcement which is welded through the palates and allows the use of lightweight, thin ABS containers. Secondly, the use of thin foil current collectors which are embossed to increase surface contact area, further reduce the weight and volume of the cell. The current collector is consolidated in a thin foil and is work hardened to refine its grain structure, its rate of anodic corrosion (oxidation) is reduced over conventional grid-plate designs. This reduces the mass of lead required in the current collector to achieve the desired service life.
To help choose the right battery for the job Saft offers a range of free selection software. Combatt uses a simple interview process to establish criteria for sophisticated battery comparisons, comparing automotive lead-acid batteries, various nickel-cadmium types, and everything in between.
Saft’s Life Cycle Cost (LCC) program performs a life cycle cost analysis for various battery types. The LCC looks at the entire cost of ownership of a particular battery. Other costs that are considered include transportation, installation, maintenance, testing and replacement, as well as administrative costs.