# How much current can this circuit output?

#### Ian0

Joined Aug 7, 2020
8,947
That circuit does have a bit of a problem - at the moment of power up, the capacitor is immediately charged to the battery voltage at as much current as the wiring can sustain, which may well be rather too much for that weedy little BA159 1Amp diode.
maximum current =1 Amp - it's limited by the diode
ISL78268 - No - it's a buck regulator so it steps the voltage down not up.

I also wonder why one would want to charge the super capacitor to its absolute maximum voltage, and not simply let it charge to the battery voltage.

#### Papabravo

Joined Feb 24, 2006
20,615
That circuit does have a bit of a problem - at the moment of power up, the capacitor is immediately charged to the battery voltage at as much current as the wiring can sustain, which may well be rather too much for that weedy little BA159 1Amp diode.
maximum current =1 Amp - it's limited by the diode
ISL78268 - No - it's a buck regulator so it steps the voltage down not up.

I also wonder why one would want to charge the super capacitor to its absolute maximum voltage, and not simply let it charge to the battery voltage.
I think you know the answer to that question.

#### Papabravo

Joined Feb 24, 2006
20,615
You would of course need to do this on a printed circuit board. Doing this on a breadboard of any description might not work out so well.

Joined Nov 21, 2018
857
That circuit does have a bit of a problem - at the moment of power up, the capacitor is immediately charged to the battery voltage at as much current as the wiring can sustain, which may well be rather too much for that weedy little BA159 1Amp diode.
maximum current =1 Amp - it's limited by the diode
ISL78268 - No - it's a buck regulator so it steps the voltage down not up.

I also wonder why one would want to charge the super capacitor to its absolute maximum voltage, and not simply let it charge to the battery voltage.
Thanks.
1. If changed BA159 as a large diode say SB550, can it increase the current to 5A?
2. when mentioned the ISL78268, I can use a step down circuit， can I use its large current?
3. How do you think to charge the supercapacitor [SC] to certain Voltage, can the SC keep that voltage stable?
In fact, the base capacitor allowable voltage is only 2.7V.

Joined Nov 21, 2018
857
You would of course need to do this on a printed circuit board. Doing this on a breadboard of any description might not work out so well.
Thanks.
I'll do PCB after tested.

#### DickCappels

Joined Aug 21, 2008
10,104
I think what Papabravo was trying to communicate is now that you are up into multiple amps a plastic plug-in breadboard might not handle the current without an unacceptable voltage drop or damage to the breadboard itself.

Joined Nov 21, 2018
857
I think what Papabravo was trying to communicate is now that you are up into multiple amps a plastic plug-in breadboard might not handle the current without an unacceptable voltage drop or damage to the breadboard itself.
Thanks.
I will test the circuit in a small current first?

#### Papabravo

Joined Feb 24, 2006
20,615
Thanks.
I will test the circuit in a small current first?
You don't understand what I am trying to say. Testing at small currents may lead you to draw unwarranted conclusions, especially when it comes to running at higher currents and higher frequencies. Unfortunately you CANNOT slow walk this process. You have to cut printed circuit boards and learn by doing. There are special things you do in PCB layout and fabrication for SMPS's with high current and high frequency. You cannot learn what they are or appreciate them by experimenting with lower currents and frequencies. Doing it the right way might take 3-4 board turns. Doing it the wrong way might take you 10.

Joined Nov 21, 2018
857
You don't understand what I am trying to say. Testing at small currents may lead you to draw unwarranted conclusions, especially when it comes to running at higher currents and higher frequencies. Unfortunately you CANNOT slow walk this process. You have to cut printed circuit boards and learn by doing. There are special things you do in PCB layout and fabrication for SMPS's with high current and high frequency. You cannot learn what they are or appreciate them by experimenting with lower currents and frequencies. Doing it the right way might take 3-4 board turns. Doing it the wrong way might take you 10.
Thanks.
OK, I'll do on pcb, which one better left or right?

#### Papabravo

Joined Feb 24, 2006
20,615
Thanks.
OK, I'll do on pcb, which one better left or right?

View attachment 241206
Neither one has a layout on it. When I say a Printed Circuit Board. I mean one with a circuit etched on it. I can see you're bound and determined to do it your way, so do that and best wishes.

#### Alec_t

Joined Sep 17, 2013
14,009
Even 5A would be pushing it if you intend using perf-board. You can connect strip-board tracks in parallel to handle multi-amp currents, but there are limits.
Track inductance and inter-track capacitance also become significant at high switching frequencies.

#### Ian0

Joined Aug 7, 2020
8,947
Taking this capacitor as an example
https://uk.rs-online.com/web/p/electric-double-layer-capacitors/1845030/
it has a DCR of 30mΩ.
Connect six in series to a 12V battery (I'm assuming a SLI battery with the capacity to deliver >1000A), through a diode and the current will be 67A, and a 70F capacitor will take some considerable time to charge - over a second at that current.
You're going to need rather a lot of strips of perfboard in parallel to deal with that.
Don't forget that the circuit has no way of limiting the current until the capacitor voltage exceeds the battery voltage.

#### LowQCab

Joined Nov 6, 2012
3,594
The whole premise of this project is not very practical.
If You charge the Capacitor(s) to ~16V, you then have to add an
extremely high-Current Voltage-Regulator to match the Voltage of the Battery.
You then have Charging-Circuit losses, plus, Regulating-losses,
adding up to a lot of wasted Power.

Batteries and Capacitors operate on completely different principles.
The Voltage will be more consistent, for a longer period of time, with 2-Batteries.

A second Battery, would not only be much cheaper,
but would also provide more than double your current run-time, without all the complexity.
( the run-time will more than double because the discharge-rate on the Batteries will be cut in half,
and therefore, there will be fewer internal losses in each Battery )

Your Boost-Converter will need a Inductor with at least ~14-Gauge-Wire, (12ga. would be better),
have you done the calculations for building a 25-Amp Choke ?
It's not as easy as it looks.
.
.
.

Joined Nov 21, 2018
857
Neither one has a layout on it. When I say a Printed Circuit Board. I mean one with a circuit etched on it. I can see you're bound and determined to do it your way, so do that and best wishes.
Thanks for the experienced advice.

Joined Nov 21, 2018
857
Even 5A would be pushing it if you intend using perf-board. You can connect strip-board tracks in parallel to handle multi-amp currents, but there are limits.
Track inductance and inter-track capacitance also become significant at high switching frequencies.
Thanks.
'strip-board track' yes, it is what I was planing to use.

Joined Nov 21, 2018
857
Taking this capacitor as an example
https://uk.rs-online.com/web/p/electric-double-layer-capacitors/1845030/
it has a DCR of 30mΩ.
Connect six in series to a 12V battery (I'm assuming a SLI battery with the capacity to deliver >1000A), through a diode and the current will be 67A, and a 70F capacitor will take some considerable time to charge - over a second at that current.
You're going to need rather a lot of strips of perfboard in parallel to deal with that.
Don't forget that the circuit has no way of limiting the current until the capacitor voltage exceeds the battery voltage.
Thanks.
It's good idea to connect them in series and charging in a higher voltage, even actually used in parallel. About the current control before capacitor voltage exceeds the battery voltage, I have had a question asked here:
I'll try the circuit suggested?

Joined Nov 21, 2018
857
The whole premise of this project is not very practical.
If You charge the Capacitor(s) to ~16V, you then have to add an
extremely high-Current Voltage-Regulator to match the Voltage of the Battery.
You then have Charging-Circuit losses, plus, Regulating-losses,
adding up to a lot of wasted Power.

Batteries and Capacitors operate on completely different principles.
The Voltage will be more consistent, for a longer period of time, with 2-Batteries.

A second Battery, would not only be much cheaper,
but would also provide more than double your current run-time, without all the complexity.
( the run-time will more than double because the discharge-rate on the Batteries will be cut in half,
and therefore, there will be fewer internal losses in each Battery )

Your Boost-Converter will need a Inductor with at least ~14-Gauge-Wire, (12ga. would be better),
have you done the calculations for building a 25-Amp Choke ?
It's not as easy as it looks.
.
.
.
Thanks.
I'll use two-battery unit.
I don't have a Inductor yet, where can buy it?