Supercapacitor paralleled with battery..how to pre charge

[Fuss]

How do you pre-charge your Supercapacitor before paralleling it with your battery? Or.., do you just use a 1, 2, both switch and have a huge number of amps flowing?

[GordMay]

I’m not personally familiar with the use of parallel supercapacitors, so not certain how helpful the following will be.
[I look forward to learning more, here]

Supercapacitors are transported fully discharged. At initial installation or commissioning, they will be at 0 V.
Therefore, a precharge circuit may be needed to reduce inrush currents, as supercapacitors can appear to be a short circuit, due to their inherently low ESR.
Also, the rectifier, PCS or power electronics may accommodate firmware changes, to slow ramp up voltage during initial charging, to help reduce the inrush currents.

How to Quickly and Safely Charge Supercapacitors
➥ https://www.ti.com/lit/an/sluaao7/sl...oogle.com%252F

Supercapacitor Pre-Charge/Discharge DIY Circuit
➥ https://diysolarforum.com/threads/su...circuit.69275/

[Fuss]

Thanks Gord for this info.
I've been a little concerned to just connect it to the 500a 1,2,both switch.
What I did so far is connect a 1ohm resistor from the battery positive to the supercap positive pole and also a 21w 12v small stop lamp in parallel to the resistor to the same poles.
This is similar to the diysolar solution ablovee

Once the lamp goes dim then i know I can use the 1,2, both switch to parallel the battery and the supercap.


Maybe there are some better solutions....

[CaptainRivet]

Originally Posted by Fuss

Thanks Gord for this info.
I've been a little concerned to just connect it to the 500a 1,2,both switch.
What I did so far is connect a 1ohm resistor from the battery positive to the supercap positive pole and also a 21w 12v small stop lamp in parallel to the resistor to the same poles.
This is similar to the diysolar solution ablovee

Once the lamp goes dim then i know I can use the 1,2, both switch to parallel the battery and the supercap.


Maybe there are some better solutions....

you use a 1 and 2 switch, on 1 you connect the precharge curcuit and on 2 the supercapacitor+battery. so you switch for to 1 wait 10sec or till your light dims then 2 to combine both. you can also use on/off switch and a timer relay to automate that.


Thats also how i have my inverters connected with a 1 and 2 switch with just 2 10w 6ohm resistors i had lying around connected to contact 1 and glued into case of switch with hot glue.

[Fuss]

Thanks,
I don’t understand how this would work with the 1 2 both switch.
I’ll try to write it as simply as I can without leaving out any details.
There is the precharge circuit connected to pole1 (Connecting battery + to capacitor + through a resistor)

There is a battery connected to pole 2
There is a common that connects pole 1 and pole 2
When the switch selects pole 1 then the precharge circuit is connected to the common and the capacitor is connected to the battery and therefore is precharged (over thin precharge wires).
When the switch selects pole 2 then the battery is connected to the common
When the switch selects Both then the battery is connected to the common and also the precharge circuit is connected to the common so the circuit is not capable of a large amperage as the the connection still just the thin precharge wiring not thick battery cabling.


So unless I am misunderstanding something, this will not work.


Can you go into more detail?


Ok, Now the on/off switch with the timer relay to automate the process.
How would this work, I would imagine that a timer relay capable of handling 600+ amps would be very expensive.
Can you give some examples of the parts required and the circuit design?

[Fuss]

Has anyone else on the forum done this?. Surely I can’t be the first. I just want to know your experience as the supercap is the ideal starter battery backup if the dangers/concerns surrounding the precharge can be solved.

[Jammer]

Originally Posted by Fuss

How do you pre-charge your Supercapacitor before paralleling it with your battery? Or.., do you just use a 1, 2, both switch and have a huge number of amps flowing?

Through a higher wattage power resistor or an incandescent lamp. You can use a diode in series if you do not want the supercap to discharge back through the charging source.

[wholybee]

Originally Posted by Fuss

Thanks,

When the switch selects Both then the battery is connected to the common and also the precharge circuit is connected to the common so the circuit is not capable of a large amperage as the the connection still just the thin precharge wiring not thick battery cabling.


So unless I am misunderstanding something, this will not work.

What problem are you trying to solve with a supercapacitor? It seems to be a solution looking for a problem.

To answer your question, why are you using a 1/2/both switch? I would use 2 separate switches, but that is just me. But even with a 1/2/Both switch, it will not be a problem.

On "both" there will be 2 paths, one through the precharge circuit, and one direct. The precharge path will have a resistance added, and the direct will be near 0 resistance. This forms a current divider where practically all the current goes through the direct path, and a minuscule amount goes through the precharge path. So the small wires are not a factor there at all.

https://en.wikipedia.org/wiki/Current_divider

[wholybee]

FWIW, instead of a 1/2/Both switch, I would do it like this, with a normal high current on/off switch, and a small push button. Push the button and hold for a few seconds, then turn the switch on.

[Jammer]

Originally Posted by Fuss

Has anyone else on the forum done this?. Surely I can’t be the first. I just want to know your experience as the supercap is the ideal starter battery backup if the dangers/concerns surrounding the precharge can be solved.

Leave out the "both."


If you want to use an A-B switch that's OK, find a non-shorting one, A is battery, B is the supercap. Or just wire the starter to the supercap without an A/B switch.


For charging the supercap, there are various more complex ways of doing it that are faster and more efficient, but the easy way is that you build a circuit with a fuse (or breaker), a switch, a resistor (or incandescent lamp), and a diode all in series. The battery connects to one side and the supercap connects to the other.


You size the components for the corner case of the supercap being completely discharged which is when the most current will flow. With a resistor, the power dissipated in the resistor will be 12^2/R, that is 144 divided by the resistance in ohms.


Here's one example that can be ordered in quantity 1 and fits the situation:


https://www.digikey.com/en/products/...P2R2JE/3314069


It's a 2.2 ohm, 350 watt resistor. Big but it will work and you can buy one for $45. The smaller ones tend to be minimum quantity of 10. Anyway, that will cap the current at roughly 6 amps and the power at 75 watts. It will charge your supercap in less than 10 minutes which is good enough because you can leave you supercap charged all the time so it's there when you need it.


Diodes, lots of choices, inexpensive, the industry has gone to surface mount even for higher currents but these can be adapted with careful soldering. See for example:


https://www.digikey.com/en/products/...060DJF/5965310

[Jammer]

It is helpful to put a charge-off-discharge switch in the charge line, wired so you can ground out the supercap through its resistor for maintenance. Any three-position toggle switch that can handle 5A or so will work.

[Jammer]

Originally Posted by wholybee

FWIW, instead of a 1/2/Both switch, I would do it like this, with a normal high current on/off switch, and a small push button. Push the button and hold for a few seconds, then turn the switch on.

With a newer battery, you'll melt the switch, because even after a few seconds, you'll get over 100 amps.

[wholybee]

Originally Posted by Jammer

With a newer battery, you'll melt the switch, because even after a few seconds, you'll get over 100 amps.

Yeah, I was considering it similar to a precharge for a large inverter, but the capacitor is much larger. So instead of a pushbutton, you need a switch and more time.

I am still wondering what problem a supercapacitor solves.

[Fuss]

Originally Posted by wholybee

Yeah, I was considering it similar to a precharge for a large inverter, but the capacitor is much larger. So instead of a pushbutton, you need a switch and more time.

I am still wondering what problem a supercapacitor solves.

Thanks for the replies...I'll study these before giving feedback.


But to answer the question about what problem this solves...


In my lead days my starter only started the engine. Now my Lifepo4 starts my engine and participates as part of the house bank. This is made possible by the 280ah capacity and not needing to be fully charged each cycle.
With lead this would run the risk that the engine was not able to be started as a partially charged lead battery loses lots of CCA.
However, I have no backup now without disassembling part of the house bank, not too difficult but time consuming.
(I had no real backup in the lead days as my motor needed 2 x 12v to start)

So now I want a backup and I do not want lead. So a supercap will provide this. It sits where the 2nd lead used to sit and will use the 1 2 both switch to connect once its precharged.
The supercap is small, fully charges from a discharged lifepo4 and many truckers use them.
It makes sense. AGM puts me back into the world where things need to be fully charged, I want to get away from this.

all my batteries are the same cell type LF280K, there are 24 of them which provided huge 24v redundancy and the only exception will be the supercap which provided immediate, emergency backup and can also participate in starting the engine.
I hope that makes it clear what I am trying to do.

[Fuss]

Thanks again for the replies.
Ok, so what I will do is connect a 1ohm resistor from the battery positive to the supercap positive pole and also a 21w 12v small stop lamp in parallel to the resistor to the same poles.
(in testing, this then speeds up the precharge process to around 5 mins before the lamp begins to dim)
The capacitor is Maxwell 500f 16v.
For this precharge circuit I will use a separate switch small bep on/off.
Once the lamp goes dim then i know I can use the 1,2, both switch to parallel the battery and the supercap.
I'm not totally happy with this concept but this is where I will start.