Supercapacitor charging voltage?

In summary, it is safe to charge a supercapacitor from a higher voltage source as long as the voltage across the capacitor does not exceed its rated voltage. The source of the current does not matter as long as the dielectric of the capacitor does not experience more than the rated voltage. Overvoltage can cause the electrolyte to boil and lead to the capacitor swelling and exploding, so it is important to have a reliable way to switch off the charging before the voltage limit is reached. Failure modes of large supercapacitors include being wedged between metal plates with high current, mechanical damage, and repeated exposure to high currents. It is important to be cautious and have proper equipment when charging supercapacitors.
  • #1
Artlav
162
1
Is it safe to charge a supercapacitor from a source with more than it's rated voltage, provided that the voltage across the cap never exceeds the rating?

For example, the cap is rated at 2.7V, and i charge it from two AA batteries giving just over 3.2V in open circuit.
Once the battery, which gives vastly less current than the cap can draw, is connected to the empty cap, the voltage across it stays near zero, and is slowly climbing.

What i can't figure out - is the "never-to-exceed" limit of 2.7V apply to the voltage across the cap, or the voltage of the source that charges it?

In the example above, the first is true, but the second is not.
How does it actually work?
 
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  • #2
Artlav said:
Is it safe to charge a supercapacitor from a source with more than it's rated voltage, provided that the voltage across the cap never exceeds the rating?

For example, the cap is rated at 2.7V, and i charge it from two AA batteries giving just over 3.2V in open circuit.
Once the battery, which gives vastly less current than the cap can draw, is connected to the empty cap, the voltage across it stays near zero, and is slowly climbing.

What i can't figure out - is the "never-to-exceed" limit of 2.7V apply to the voltage across the cap, or the voltage of the source that charges it?

In the example above, the first is true, but the second is not.
How does it actually work?

If your cap can draw enough current for long enough, it will burn out the battery since it will act like a short circuit across the battery.

If the battery can supply enough current and not burn out, then it will charge the cap to 3.2 volts and if the cap can't stand that voltage, it will arc over and burn out.

Best you learn more about this stuff before hooking anything up.
 
  • #3
phinds said:
If the battery can supply enough current and not burn out, then it will charge the cap to 3.2 volts and if the cap can't stand that voltage, it will arc over and burn out.
So the important part is the voltage across the capacitor?

If i can provide a reliable way of switching off the charging part before it goes over the voltage limit, would it be safe to use a source with higher than rated voltage capability?
 
  • #4
Artlav said:
So the important part is the voltage across the capacitor?

If i can provide a reliable way of switching off the charging part before it goes over the voltage limit, would it be safe to use a source with higher than rated voltage capability?

Yes, that would be OK.

As long as the dielectric of the capacitor does not have to experience more than its rated voltage, it doesn't matter where the current comes from.

In this case, most of the voltage is being dropped across the internal resistance of the battery.
 
  • #5
And it well may require hundreds of batteries in parallel where now is one to charge the cap.
 
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  • #6
You can charge that super cap up to 3 V but the lifetime will rapidly decrease as you overvoltage the electrolyte (whatever it does to it). Excessive overvoltage won't actually make the cap arc but will boil the electrolyte instead, causing the cap to swell and eventually explode. Don't do that :). DC ESR of ultracaps varies with model and size but it will be in the range of mOhms -> an Ucap can sink or source thousands of amps (for a second or so).

Source: work for ucap manufacturer
 
  • #7
SunnyBoyNY said:
You can charge that super cap up to 3 V but the lifetime will rapidly decrease as you overvoltage the electrolyte (whatever it does to it). Excessive overvoltage won't actually make the cap arc but will boil the electrolyte instead, causing the cap to swell and eventually explode. Don't do that :).
That overvolting them is Bad Thing i know, just had a practical question of common-sense-applicability in this case.
Got a variable power supply to charge them, so it no longer apply directly.

So, in case you want to charge one with no special equipment and is careful, connecting a power supply with higher voltage and disconnecting it before the voltage across the cap would cross the limit should be safe?

SunnyBoyNY said:
Source: work for ucap manufacturer
Nice, can you provide some concrete info or documents on the failure modes of big supercapacitors?
I.e. what would happen if one got wedged between metal plates thick enough to conduct over 9000 amps the spec lists as the "maximum possible current"?
What would happen if it is mechanically damaged?
What would happen it is repeatedly subjected to 500-1000 amps while rated for 200 A continuous and 2000 in peaks?

Etc.
Realistic worst-case scenarios to be on guard against?

Yuri B. said:
And it well may require hundreds of batteries in parallel where now is one to charge the cap.
Not exactly - one cheapest salt AA battery gets it up to 1 volt, then 2 AA NiMh batteries (2.4V) get it up to 2.2V (mind the heating).
Haven't actually tried two regular AA batteries.
 
  • #8
Artlav said:
That overvolting them is Bad Thing i know, just had a practical question of common-sense-applicability in this case.
Got a variable power supply to charge them, so it no longer apply directly.

So, in case you want to charge one with no special equipment and is careful, connecting a power supply with higher voltage and disconnecting it before the voltage across the cap would cross the limit should be safe?

Nice, can you provide some concrete info or documents on the failure modes of big supercapacitors?
I.e. what would happen if one got wedged between metal plates thick enough to conduct over 9000 amps the spec lists as the "maximum possible current"?
What would happen if it is mechanically damaged?
What would happen it is repeatedly subjected to 500-1000 amps while rated for 200 A continuous and 2000 in peaks?

Etc.
Realistic worst-case scenarios to be on guard against?

Not exactly - one cheapest salt AA battery gets it up to 1 volt, then 2 AA NiMh batteries (2.4V) get it up to 2.2V (mind the heating).
Haven't actually tried two regular AA batteries.

Essentially all power supplies are current limited. Thus you can use them to charge ucaps. Just be careful when it comes to large (10 kW) devices. Those tend to have huge output capacitance and direct connection of two large capacitors results in a something that could be mathematically described as Dirac pulse current. It's loud and dangerous.

Regarding the 9,000 A question - you could possibly *weld* the bar to the terminals. Nothing bad would happen but of the cell is really large (3000 F+), there could be enough energy dissipated that the cell would overheat and get damaged.

Max currents are typically specced for 1 s.
 
  • #9
It's as well to calculate the total energy stored in all these cases. Also it's available 'all in one go' so it's much more of a potential problem. No prizes for burning your eyebrows off.
 
  • #10
SunnyBoyNY said:
Max currents are typically specced for 1 s.
That's what it says in the specs, but what about the currents between the max. continuous and max. pulse ratings?

I.e. for BCAP3000 it says that at 130A continuous it will heat up 15C, and at 210A up 40C, both labelled as maximum continuous current.
Then, 2200A is named as maximum peak current for 1 sec, and short circuit current is up to 9300A.

As i understand it, the problem is the heating.
Then, as the current go up it would:
0-200A: dissipate enough heat to stay within operational temperature indefinitely
200A-2000A: won't be able to dissipate enough heat and eventually overheat with "bad consequences"
2000A+: heats up internally fast enough for immediate "very bad consequences" (rapid electrolyte boiling? There is something that looks suspiciously like a pressure release valve on it's side)

If so, the question is what exactly would these "bad consequences" be, and how long should it cool off to avoid them?
At 3000F one such cell can supply 2kA for about 3 seconds, comfortably above the rated 1 sec.
 
  • #11
Artlav said:
That's what it says in the specs, but what about the currents between the max. continuous and max. pulse ratings?

I.e. for BCAP3000 it says that at 130A continuous it will heat up 15C, and at 210A up 40C, both labelled as maximum continuous current.
Then, 2200A is named as maximum peak current for 1 sec, and short circuit current is up to 9300A.

As i understand it, the problem is the heating.
Then, as the current go up it would:
0-200A: dissipate enough heat to stay within operational temperature indefinitely
200A-2000A: won't be able to dissipate enough heat and eventually overheat with "bad consequences"
2000A+: heats up internally fast enough for immediate "very bad consequences" (rapid electrolyte boiling? There is something that looks suspiciously like a pressure release valve on it's side)

If so, the question is what exactly would these "bad consequences" be, and how long should it cool off to avoid them?
At 3000F one such cell can supply 2kA for about 3 seconds, comfortably above the rated 1 sec.

Good questions but way too specific for a particular cell. At this point you will need to conduct experiments or perform a thermal analysis on a cell to determine appropriate current duty ratio so that the temperature rise is reasonable.

Rated means guaranteed. At the end of cells life, the ESR can go up by 200 %. Hence 3x more heat.
 
  • #12
Ok then.
Thank you all for the info.
 

1. What is the ideal charging voltage for a supercapacitor?

The ideal charging voltage for a supercapacitor depends on its specific design and manufacturer specifications. Generally, the recommended voltage range is between 2.3V to 2.7V per cell. It is important to follow these guidelines to avoid damaging the supercapacitor.

2. Can a supercapacitor be charged with a constant voltage?

Yes, a supercapacitor can be charged with a constant voltage. However, it is important to note that the charging rate may not be constant, and the voltage may need to be adjusted as the supercapacitor reaches its maximum capacity. It is recommended to consult the manufacturer's guidelines for the specific supercapacitor being used.

3. What happens if a supercapacitor is overcharged?

If a supercapacitor is overcharged, it can lead to irreversible damage and reduce its lifespan. Overcharging can cause the supercapacitor to heat up, leak electrolyte, and potentially explode. It is important to monitor the charging process and ensure that the supercapacitor is not overcharged.

4. How long does it take to charge a supercapacitor?

The time it takes to charge a supercapacitor depends on its size, capacity, and the charging voltage and current. Typically, it can take a few seconds to a few minutes to fully charge a supercapacitor. However, the charging time can vary greatly depending on the specific application and usage.

5. Can a supercapacitor be charged with a standard battery charger?

No, a supercapacitor cannot be charged with a standard battery charger. This is because the charging process for supercapacitors is different from traditional batteries. Supercapacitors require a controlled and regulated charging process to prevent overcharging and damage. It is recommended to use a specialized supercapacitor charger for safe and efficient charging.

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