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

 

[Bluejets]

Perhaps think about getting a more appropriate CCA battery.

 

[Frankchie]

Are you sure the accessaries are disconnected during cranking? Usually the battery voltage drops and it may just look like a disconnect.

If it's not disconnecting it sounds like your adapter is not doing a good job. While cranking most batteries should not drop below 9v. I would think that a good adapter would work even at 8v, maybe even less. Might be worth shopping for a better adapter. Many cheap adapters don't give you much specification information, but if you can find specs pick one with the lowest minimum input voltage required to maintain the 5v USB voltage. Hopefully 8v or lower.

The supercaps may work, but a lot depends on the actual current draw of the load. If the current draw is one amp or less it should work fine. It may work at higher current, but voltage may drop below 4 volts and success depends on your load device's tolerance of lower voltages.

Further, like you said a limiting resistor is probably required to reduce the current draw upon recharge. Off the top of my head it sounds like a simple steering diode and resistor combination might be possible.

If you can determine the current draw maybe myself (or others here) can determine if the super cap solution is viable.

Frank

 

[Harald Kapp]

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.

A capacitor is not suitable here - unless impractically large. 15 W from 5 V is equivalent to 3 A. Once you start drawing current from the capacitor (due to the primary voltage being disconnected, the voltage across the capacitor will fall with time (note that this behavior is quite different from a battery which can sustain the output voltage for some time and will drop only at the end of charge). As an approximation one can assume a linear discharge with current being constant (which it is not in reality). The equation needed then is:
t = (C × dV) / I or C = (I × t) / dV
with
C = capacity
I = current
dV = change in voltage
t = time
Given t = 7s, I = 3 A and assuming a max. allowable dV = 0.5 V we get C = 42 F -you'd need 11 of your 4F capacitors!.
A much better solution would be to buffer the 12 V input to the USB converter. You can then allow for much more voltage drop on the 12 V side before the 5 V output starts dropping. Also, if the regulator is of the step-down type the input current will be much lower than the output current.
Example: Assuming a step-down converter with 75 % efficiency, the input current from 12 V will be:
I_out = 3 A @ 5 V -> I_in @ 12 V = 1.25 A (as input power = output power) / 0,75 (efficiency) = 1.7 A
Let's assume the step-down regulator works well down to e.g. 7 V input voltage (depending on the construction of the regulator), we have dV = 12 V -7 V = 5 V
The above equation now can be solved for C = 2,4 F @ 12 V
You now have to find supercaps that support 12 V (better 16 V for some headroom). And you will probably need decoupling of the capacitor from the 12 V power via a diode and a current limiting resistor. Plus a way to bypass power from the 12 V source to the regulator wehn the 12 V are present, e.g. like so (schematic, no values):

 

[ratstar]

What I can see here->>
If you convert from dc to ac and back again, you can limit the current with a capacitor, (thats when the car isnt cranked - because then ud have too much power to the peripherals.)

 

[Harald Kapp]

You could use the DC to power a radio frequency transmitter, collect the RF energy with an antenna, rectify it and charge the capacitor from that reovered energy.
Why would you want to do that? A resistor is so much more simpel and much less expensive.

Just because you can do something a certain way doesn't mean it is meaningful to do it that way if another, more simple method exists.

 

[ratstar]

If the peripherals have their own tolerances and regulation maybe u dont need what I said anyway. I do things from scratch, its a bit different working with the industry standard I know. Its why I have a hard time chiming in with something useful. =)

 

[pootler]

You could use the DC to power a radio frequency transmitter, collect the RF energy with an antenna, rectify it and charge the capacitor from that reovered energy.
Why would you want to do that? A resistor is so much more simpel and much less expensive.

Just because you can do something a certain way doesn't mean it is meaningful to do it that way if another, more simple method exists.

 

[pootler]

Ok,

Thanks for all the advice.
I have now hopefully got a few 'meaningful' figures.

I connected my multimeter in series on the output of the convertor - i.e, the input to the hub.
To my surprise ( and embarrassment) the current draw for the hub maxes at .45A !, when all devices are actively being used by my windows tablet.
The tablet has its own supply btw.
When I cranked the engine the reading on the meter stayed the same for about 2 secs, but then dropped to .4A
By then, the engine had started, but sadly my tablet had lost communication with the devices again.

According to Harolds calculations, if I assume current requirement of .5A, and reduce time to 4 secs, should this not work?, or are there other variables a noob like me is not aware of.
ATM, I do not have any diodes or resistors in circuit.

I am not an electrical engineer, so am only capable of very basic circuit construction.

My alternative to all this is to use a timed relay to switch to a permanent 12V source when trigger voltage drops during cranking.

Thanks

pootler

 

[Frankchie]

Pootler,
Yes, at .5amps the super caps should work well. At 1 amp draw the caps should keep the voltage above 4 volts for maybe 7 seconds. Further, you can put your caps on the output of the convertor and only 1 resistor and one diode is needed. I recommend that you use a Schottky diode for their low voltage drop, maybe a 1N5820 that's capable of 3 amps. A 10 ohm 3 watt resistor should limit the charging current to reasonable 1/2 amp. See attached dwg.

I assume the hub is a USB hub that supplies power to the other devices. If it was an Ethernet hub I would think recovery would be automatic.

P.S. It's probably not required but another Schottky diode on the output of the convertor might be needed. Maybe others here can comment.

 

[Bluejets]

My alternative to all this is to use a timed relay to switch to a permanent 12V source when trigger voltage drops during cranking.

Your best alternative is, as I already stated, to get a battery that doesn't drop the voltage as much during cranking.
The way this is achieved is a CCA batery to suit your vehicle and then some.
Might be a good idea to get the starter and it's main cabling checked while you are at it.
Starters do deteriorate over time.

 

[ratstar]

Pootler,
Yes, at .5amps the super caps should work well. At 1 amp draw the caps should keep the voltage above 4 volts for maybe 7 seconds. Further, you can put your caps on the output of the convertor and only 1 resistor and one diode is needed. I recommend that you use a Schottky diode for their low voltage drop, maybe a 1N5820 that's capable of 3 amps. A 10 ohm 3 watt resistor should limit the charging current to reasonable 1/2 amp. See attached dwg.

I assume the hub is a USB hub that supplies power to the other devices. If it was an Ethernet hub I would think recovery would be automatic.

P.S. It's probably not required but another Schottky diode on the output of the convertor might be needed. Maybe others here can comment.
View attachment 51426

Thanks for that, nice circuit I can see whats going on now! I dont know why you would need a super capacitor, an electrolytic can fill a time gap up like that as well.

 

[Harald Kapp]

I dont know why you would need a super capacitor, an electrolytic can fill a time gap

Check size vs. capacity and compare the two of them.


@pootler : Your circuit is a nice variant with less components than mine. Whether it works under the given circumstances depends on the behavior of the adapter, however: If the output voltage of the adapter drops and the output resistance of the adapter is very low (as it should be), the capacitor will force current into the output of the adapter (no blocking diode as in my circuit).
In real life this can be verified with just one simple test by measuring the direction of current with the capacitor being fully charged, then dropping the output of the adapter.

 

[ratstar]

Check size vs. capacity and compare the two of them.


@pootler : Your circuit is a nice variant with less components than mine. Whether it works under the given circumstances depends on the behavior of the adapter, however: If the output voltage of the adapter drops and the output resistance of the adapter is very low (as it should be), the capacitor will force current into the output of the adapter (no blocking diode as in my circuit).
In real life this can be verified with just one simple test by measuring the direction of current with the capacitor being fully charged, then dropping the output of the adapter.

I see now, sorry for not recognizing your circuit before, it looks like half of a charge pump, nift-o!

 

[pootler]

Your best alternative is, as I already stated, to get a battery that doesn't drop the voltage as much during cranking.
The way this is achieved is a CCA batery to suit your vehicle and then some.
Might be a good idea to get the starter and it's main cabling checked while you are at it.
Starters do deteriorate over time.

Thanks Bluejets

Not sure- The battery was changed about 14 months ago .
I actually did choose one which was much heavier duty, and had even more CCA than the OEM.
Car starts first time every time, and only turns over for about two seconds or less before it starts - regardless of weather.
I am in the UK, so don't really get any extremes
It is a diesel Honda ( 2.2 cdti )

Thanks

pootler

 

[Martaine2005]

I’d keep it simple and be inclined to run directly from the battery and an inline fuse. Plus switch if needed.
You can buy aux fuse boxes for this this very scenario.

Martin

 

[Frankchie]

Actually, ratstar may have a point (gulp).

That is, now that pootler's measurements show that the convertor is not shut off during cranking, maybe the problem is short dropouts and a small electrolytic capacitor might bridge the gaps. I would try bridging the 5v output with say a 1000uf cap and see if that does the trick.

Frank

 

[flippineck]

Been working on a mini digger recently, electrics have been a nightmare, all became clear eventually - several leads despite being visually perfect & checking out at below 1 ohm on the multimeter, were going up to high resistance but only when larger currents were applied. Worth checking for heat damage to connections, corrosion, water ingress etc on the vehicle. Also I've noticed car batteries are easy to kill rapidly under the right circumstances when you almost wouldn't expect

 

[Bluejets]

were going up to high resistance but only when larger currents were applied

Used to be common on starter lead at the start solenoid lug, mainly from heat.
The starters in cars these days tend to be rather small motors with a large gearing so start current not so high.
Although Op does not say WHAT CCA ( secret I assume), it may be sufficient to start and run the vehicle but not protect against brown outs on sensitive electronic equipment plugged in as an auxiliary.
Also I did not see where the OP has stated just where the "tap" was done and how, including any earth return.
Possible any "twist and tape" or similar would give the brown-outs described.

 

[pootler]

Used to be common on starter lead at the start solenoid lug, mainly from heat.
The starters in cars these days tend to be rather small motors with a large gearing so start current not so high.
Although Op does not say WHAT CCA ( secret I assume), it may be sufficient to start and run the vehicle but not protect against brown outs on sensitive electronic equipment plugged in as an auxiliary.
Also I did not see where the OP has stated just where the "tap" was done and how, including any earth return.
Possible any "twist and tape" or similar would give the brown-outs described.

Specification-78ah 680 C.C.A TYPE EXTRA HEAVY DUTY

Power obtained from add a circuit fuse adapter - separate 5A fuse.
Earthed using dedicated earthing point.

Cheers

Pootler