Batteries and supercaps are based on completely different energy storage methods. Looking at them more closely, it becomes clear why they cannot be simply interchanged:
Battery: electrochemical storage
A battery is in practice a voltage source. The voltage remains very stable during discharge over a wide range, and only drops sharply at the end of the discharge process. (see graphic 1)
During charging, the electrical energy is converted into chemical energy, and is stored as such. On discharge, it is released again as electrical energy. The stored energy is calculated by the formula: energy (Ws) = (capacity (Ah) / 3600s) x nominal voltage (V). Depending on the battery technology, this principle attains an efficiency of approximately 50 to 90 percent.
Batteries offer the advantage over capacitors of much higher energy content. Their disadvantages are sensitivity to high current peaks, which permanently damage the battery, and an operating temperature range limited to approximately 0 to 45°C. Values above or below those limits will shorten the life of the battery owing to its chemical composition.
Capacitor: electrostatic storage
As capacitors store the energy in electrostatic form, and the voltage drop as the current is drawn is virtually linear (see graphic 1), they are classed as a current-based power source.
They attain an efficiency of around 98 percent, and operate without damage within a temperature range from -40°C to + 65°C; at low temperatures their capacity even increases a little more. Thanks to the ESR in the milli-ohm range, current peaks of several hundred to a thousand amperes are possible. Their Achilles heel is their much lower energy content compared to batteries. The energy content is calculated by the formula: Ws = 0.5 x capacity (C) x voltage swing (∆V²)
Dimensioning of the capacitor
To migrate power supply from batteries to EDLCs, the dimensioning of the energy storage device has to be fundamentally redesigned owing to the different technologies and characteristics. Orienting solely to the battery rating is not normally useful, as they are often over-dimensioned in order to withstand the necessary current and power output peaks. Such over-dimensioning is not necessary for EDLCs because of their peak current withstand capability.
Anyone looking to switch to capacitors should answer questions such as: How much power is needed for the desired function - that is to say, what operating and buffer times are desirable or necessary at what current, potential voltage swing, and power output?
A capacitor can be utilized optimally if 100 percent charging voltage is halved. That corresponds to 75 percent energy efficiency. With smaller voltage swings, increasing capacity is required for the same power output, or a DC/DC converter is additionally needed to provide the required voltage range for the application.
Setting a purely energy-related appraisal against the volume of EDLCs and their component costs, it is often shown that a purely capacitor-based solution is not useful. The answer to the question "battery or capacitor?" is then "a combination of the two".
With a hybrid solution of this kind, the battery capacity can be better utilized, thereby extending the operating time per charge. At the same time, thanks to the lower current load the battery life is substantially extended - according to initial empirical data by as much as 100 percent. This can be implemented with various topologies, from a simple parallel configuration to actively controlled and logically linked systems.
Example application: cordless screwdriver
The practical example of a 14.4 V cordless screwdriver illustrates the performance of solutions purely featuring EDLCs, purely featuring lithium-ion batteries, and hybrid solutions (combinations of lithium-ion battery and EDLC) in terms of charging time, power and operating time.
At present, cordless screwdrivers are mostly fitted with lithium-ion batteries. They are lighter, and have much higher capacity than their predecessors, NiCd and NiMh batteries.
In the practical test, with five series-configured 350F EDLC cells and a charging voltage of 13.8V, approximately 40 wood screws (4.5 x 40mm) could be screwed into a wood board before the tool had to be recharged. At 20A charging current, the capacitor was fully recharged in about 35 seconds. This requires no charging electronics, just a charging end voltage limiter.
An advantage of this solution is that no low voltage cut-out is needed, as there is still sufficient torque available even as the rotation speed decreases. This is because EDLCs are able to deliver many times the current of batteries, and can also withstand very low voltages without damage. They permit over 100,000 recharging cycles.
Lithium-ion battery solution
With a lithium-ion battery with 1.5Ah nominal capacity, one charge was able to screw approximately 250 wood screws of the same size into the same wood board. It then took about an hour to fully recharge the battery.
It should be noted in these analyses that all higher-quality cordless screwdrivers have a low voltage cut-out to protect the battery. Because the depth of discharge (DOD) of a lithium-ion battery is approximately 70 percent, meaning that of the 1.5Ah only 1.05Ah is actually available. If the battery would be discharged beyond that, lasting damage would be caused. As the battery life progresses the usable nominal capacity decreases further. In practice, the battery can withstand between 150 and 200 charging cycles.
In the hybrid solution, the 1.5Ah lithium-ion battery was paired with 15 25F EDLCs. The cordless screwdriver was then able to screw approximately 300 of the wood screws into the board. The recharging time with this solution, too, was about one hour, but the battery life was doubled to as much as around 400 charging cycles, and the DOD improved, reaching 80 to 90 percent.
So the tests showed: The combination of lithium-ion battery and EDLC delivers the best performance with the same charging time as a purely battery-based solution, with the added bonus of double the battery life and a higher DOD, meaning a higher energy yield per discharge. So it remains to be seen when this technology will be introduced into battery-powered equipment such as power tools.
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