I bought a couple of 4F 5,5V supercaps.
They had about 2-3V charge.
I wanted to charge them with a 50mA current limited 5V regulated power supply.
I was surprised to find them going immediately to 5V and have only ~20mA running through them.
Is that OK?
Only you know what capacitors you have. Read the data sheet for them!
Paul
RIN67630:
I bought a couple of 4F 5,5V supercaps.
They had about 2-3V charge.
I wanted to charge them with a 50mA current limited 5V regulated power supply.
I was surprised to find them going immediately to 5V and have only ~20mA running through them.
Is that OK?
T = CV/I = 4F(2.5V)/20mA = 500 seconds
So, it appears something is amiss.
- Could be your "~20mA" measurement -- where did you get that from?
- Are you sure the power supply was in constant current mode? This could happen if the supply allowed a lot more current to flow.
- Did you measure the voltage directly across the supercap?
- Also, if you have a scope, it would be more telling to see a trace, rather than point test with a multimeter.
- What happens if you discharge the cap through a resistor? Use T=RC to predict a discharge time and experiment.
- Then, with the cap fully discharged, how does it behave when you connect the current limited supply?
There is another possibility -- long shot, though: Maybe the supercap isn't "formed". If further experimentation results in a continued indication of improper behavior, try this:
Check the datasheet, and find the ABSOLUTE MAXIMUM voltage, if it's more than 5.5V then set your power supply to 5.5V. If 5.5V IS the absolute MAX, then set your power supply to 5V. Let it sit for 72 hours. This was a recommendation I got from an engineer at Maxwell.
Are you measuring your 5V while the cap is connected to the charging supply? If you are connecting a fixed voltage current limited 5V power supply directly to the capacitor you're bound to see 5V while it's charging.
You'll only know anything useful about the state of charge when you measure the capacitor on its own, i.e. not connected to anything else.
Steve
RIN67630:
I bought a couple of 4F 5,5V supercaps.
They had about 2-3V charge.
I wanted to charge them with a 50mA current limited 5V regulated power supply.
I was surprised to find them going immediately to 5V and have only ~20mA running through them.
Is that OK?
If you got a regulated 5V DC supply, and truly have a limit of 50 mA, then you got to ask yourself..... is it possible for the capacitor voltage (with 3V across it) to abruptly develop 5V? Answer.... nope.
Limiting the current in this case would result in loss of voltage regulation in the power supply.
slipstick:
Are you measuring your 5V while the cap is connected to the charging supply? If you are connecting a fixed voltage current limited 5V power supply directly to the capacitor you're bound to see 5V while it's charging.
Not necessarily so. With the power supply in current limit mode, the power supply will reduce it's voltage to limit the current. What should be seen, in this case, is the supply voltage gradually rising. It should take around 400 to 500 seconds. I checked the datasheet on a 4F 5.6V supercap and saw an ESR of 30Ω, so at 50mA, that's a voltage drop of 1.5V, so if the starting voltage on the cap is 2V, then the Power Supply would be at 2V + 1.5 = 3.5V
3V + 1.5V = 4.5V, so that would explain why the OP is seeing a higher voltage than one might expect. Perhaps the ESR on the OP's supercap is higher than 30Ω.
ReverseEMF:
T = CV/I = 4F(2.5V)/20mA = 500 seconds
So, it appears something is amiss.
- Could be your "~20mA" measurement -- where did you get that from?
- Are you sure the power supply was in constant current mode? This could happen if the supply allowed a lot more current to flow.
- Did you measure the voltage directly across the supercap?
- Also, if you have a scope, it would be more telling to see a trace, rather than point test with a multimeter.
- What happens if you discharge the cap through a resistor? Use T=RC to predict a discharge time and experiment.
- Then, with the cap fully discharged, how does it behave when you connect the current limited supply?
You cannot use linear equations to determine charge and discharge times of a capacitor. See Charging a Capacitor
If current is constant then the voltage increases on a straight line.
But the current isn't "constant". The current is 'limited' so at Vc = 0 the current is limited to 50mA and the power supply output voltage is that determined by the capacitor's ESR and the limited current value. As the capacitor charges the current limit will stay effective until such time as the capacitor voltage is high enough to permit charge current below the current limitation value. From then onwards the charge current will decrease exponentially towards zero as the capacitor's voltage rises exponentially to the output voltage of the power supply. The only portion of the charge that may be described as linear is that area within the "limited' current phase due to the ESR of the capacitor
MorganS:
If current is constant then the voltage increases on a straight line.
True, but because of ESR, there is a resistive component that would cause more of a logarithmic rise -- so it's a curve somewhere between linear and logarithmic.
jackrae:
You cannot use linear equations to determine charge and discharge times of a capacitor. See Charging a Capacitor
You can if the goal is a "ballpark" determination.