What happens to a resistor and capacitor in line with a battery supply?

[goofeedad]

My son and I were discussing some RC circuits (very simple I might add), when he asked what happens in the cicuit if you put a batttery in series with a capacitor and a resistor to ground, so the current and voltage are applied to all components. I think he wants to drain the cap and use it as a small batttery somehow. I tried my best to explain that a cap gives up all its energy at once but wasn't very sucessful.

Any help here would be appreciated.

 

[BrownOut]

The time a capacitor charges and discharges depends on the value of the cap, and the value of the series resistor. The charge time constant is = to the resistance times the capacitance. The cap will charge to 63% of the applied voltage in the first time constant, then 63% of the remaining voltage in the secone time constant, and so on... After 5 time constants, the cap is considered fully charged. Caps can be used as analogs to tiny batteries in circuits when a voltage is need to hold on a high impeadance node for a very short period of time, ie self-latching relays might use a large cap to hold the current on the coil during the switching time. For powering devices, it won't work however.

 

[Vizier87]

When I was a kid, I dreamed of combining thousands of those small capacitors in parallel into one board to make a DIY rechargeable battery.. I didn't do it though.. but I'm wondering how feasible is that? I mean in terms of output power. Probably that's your kid's dream?

I suppose a large capacitor drains its charges in a short time, so it can't provide stable output power, yeah?

 

[Hero999]

Capacitors are very bulky compared to batteries, a super capacitor that stores the same amount of energy as an AA battery will be huge.

It's also impossible to use all of the stored energy, for example if you have a capacitor charged to 5V and a boost converter that works down to 1V, there'll still be a lot of energy left in the capacitor, when the voltage as dropped below 1V.

 

[crutschow]

It's also impossible to use all of the stored energy, for example if you have a capacitor charged to 5V and a boost converter that works down to 1V, there'll still be a lot of energy left in the capacitor, when the voltage as dropped below 1V.

Since the energy stored in a capacitor is proportional to the square of the voltage, you will have 1/25th or 4% of the energy left on the cap at 1V as you had at 5V.

 

[Hero999]

That's true, I didn't think of that, I suppose the most effecient way of using a super capacitor is to uses as higher voltage as possible, but you still have to suffer the energy loss in the DC-DC converter used to power your circuit.

 

[crutschow]

At this point in their capability, it would seem super-caps are more useful for buffering a power supply or battery to help provide large transient currents rather then using them as the sole source of power.