Hi Nick, The problem isn't usually holding the charge but that the current technology has not brought the cost down for the size of capacitors necessary to be a reasonable power source. Take a simple AA NiMH battery with 1.2 Volts and a power rating of 1800 mAh which fully charged has about 2.16 Watt hours of power. An equivalent super capacitor would have to have over 5000 Farads You can check on the cost for a 5000 Farad capacitor. The problem is complicated also by the larger size of the capacitor compared to the battery. Another problem is that unlike the battery which starts at 1.2 volts and holds that voltage for a reasonable period of time the capacitor will immediately loose voltage level. The only remedy is to make the capacitor larger and charge it to a higher voltage from which we can regulate to the 1.2 volt level. Now depending on what you want to use the capacitor to power it may not be totally impractical. A low current LED can be powered for a fairly long period by a super cap. Some low current circuits likewise can use a super capacitor for power for reasonable periods of time.
Depending on the materials that are used to make the capacitor there is a small internal resistance that will over time discharge a capacitor. This internal resistance will limit how long your capacitor will store a charge. For the practical applications for capacitors this just one more physical characteristic that has to be accounted for in a circuit design.
The technology is getting better and cheaper so, who knows, someday we may be able to use capacitors instead of batteries.
Hi Nick, The problem isn't usually holding the charge but that the current technology has not brought the cost down for the size of capacitors necessary to be a reasonable power source. Take a simple AA NiMH battery with 1.2 Volts and a power rating of 1800 mAh which fully charged has about 2.16 Watt hours of power. An equivalent super capacitor would have to have over 5000 Farads You can check on the cost for a 5000 Farad capacitor. The problem is complicated also by the larger size of the capacitor compared to the battery. Another problem is that unlike the battery which starts at 1.2 volts and holds that voltage for a reasonable period of time the capacitor will immediately loose voltage level. The only remedy is to make the capacitor larger and charge it to a higher voltage from which we can regulate to the 1.2 volt level. Now depending on what you want to use the capacitor to power it may not be totally impractical. A low current LED can be powered for a fairly long period by a super cap. Some low current circuits likewise can use a super capacitor for power for reasonable periods of time.
Depending on the materials that are used to make the capacitor there is a small internal resistance that will over time discharge a capacitor. This internal resistance will limit how long your capacitor will store a charge. For the practical applications for capacitors this just one more physical characteristic that has to be accounted for in a circuit design.
The technology is getting better and cheaper so, who knows, someday we may be able to use capacitors instead of batteries.
