Supercapacitors anyone ?

[pootler]

HI - new here.

I am trying to use two ( button ) supercapacitors to provide short term ( about 7 secs ) power to usb peripherals in my car so as to prevent them from dying when the car is cranked.

I am using two  4 Farad  5.5 V supercapacitors in parallel, and these are connected across the input to a usb3  hub being fed from a 12V to 5V / 3A  voltage adapter.

I tried to find some info about what I would require to provide 15W for approx 7/8 secs, and I think this should be enough.

So - I am thinking in the right direction here?

When the car is cranked, it disconnects all power to accessory feeds which is running the usb hub.

When the power goes, the caps if  charged, should have enough to ''bridge'' the power gap, until  power  appears again when the engine starts?

However, when power is restored, will the caps not suddenly start draining as much current as possible from the adapter  in  order  to recharge ( i.e. the full 3A ) which will remove power from the usb hub and create another problem?

Do I need some kind of resistor before the caps to reduce current claim?

Sorry for the language - noob :-)

pootler

[Manul]

I would say it is probably better to put capacitors on the 12V side, input of regulator. That way it would still provide regulated 5V while the cap is discharging. But you still need some sort of inrush limitting.

Huge capacitors on the 5V rail would be harder to limit the inrush, might cause instability of converter, would not hold exactly 5V (voltage would drop) and also most converters will alow current to go backwards from output to input if input disappears, so your capacitors might quickly drain into 12V rail, if it is cut off, but has some load on it. That is my thoughts.

[pootler]

Thanks manul,

I guess I would need another 5.5V cap  for that, and then wire in series.

Hopefully no need for balancing with only three caps.

''Huge'' in capacity - but only an inch in diameter:)

Cheers

pootler

[Manul]

Hopefully no need for balancing with only three caps.

Hmm... not so sure about this, especially if the charge / discharge currents are somewhat high.

[JustMeHere]

When the car is cranked, it disconnects all power to accessory feeds which is running the usb hub.

Actually the starter basically shorts out the rest of the car.  A 100 A draw on 12 V means there is almost no resistance. 

Make sure you protect whatever you do from being "sucked up" by the starter....diode between your caps and the car.

You will also need to protect the nasty transients caused by the starter turning off. 

I'm fairly sure TI tells you the answer here: https://www.ti.com/lit/pdf/snva681

Don't have time to read it now.