# best way to fast charge large capacitor

#### Mullins

Joined Dec 31, 2021
179
hi, i have a couple of large electrolytic capacitor. 6 volt 630 F.
actually i use them (in series) to start heavy diesel engine and they work pretty good.
Can you tell me which is the best way for fast charging of this kind of capacitor i read somewhere that the capacitor must be charged whit 1/50 of the inrush current. Since I dont know this value I use a 12v 55W lamp in series to charge them but I'm looking for a faster solutions.
I dont know the ESR of the capacitor.

#### Ian0

Joined Aug 7, 2020
10,025
Do you know the maximum permitted current?
If it will start a diesel engine (say 500A) then you can probably charge it will a similarly large current, but check the datasheet first.
How fast would you like to charge it?
I would suggest a buck regulator.

#### Mullins

Joined Dec 31, 2021
179
I can immagine that to start this engine the battery must have 1000A inrush current.
Ineed to recharge il as fast as possible but I don't want to damage the capacitor. Please tell me more about this buck regulator and how I can use it?

#### Ian0

Joined Aug 7, 2020
10,025
What are you charging it from?
Is there a maximum current specified on the datasheet?

#### Mullins

Joined Dec 31, 2021
179
I'm using car battery with a 55W light in series with the capacitor. I dont have the datasheet but I saw better and find ESR 1.02mOhm 630F

#### Alec_t

Joined Sep 17, 2013
14,374
Will you be charging the two caps individually, or in series ? Either way could cause problems unless you take precautions to keep both cap voltages below their maximum rating (6V), bearing in mind that a fully charged car battery voltage is greater than 12V by some way.

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#### Mullins

Joined Dec 31, 2021
179
Will you be chargung the two caps individually, or in series ? Either way could cause problems unless you take precautions to prevent both cap voltages below their maximum rating (6V), bearing in mind that a fully charged car battery voltage is greater than 12V by some way.
I charge them in series. I use them in series.

#### Mullins

Joined Dec 31, 2021
179
I tryed this but the charging time appear too much long

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#### Danko

Joined Nov 22, 2017
1,842
Can you tell me which is the best way for fast charging of this kind of capacitor i read somewhere that the capacitor must be charged whit 1/50 of the inrush current. Since I dont know this value I use a 12v 55W lamp in series to charge them but I'm looking for a faster solutions.
Use device like this for charging by constant current: https://www.ykpowersupply.com/product_show.asp?id=4614

#### Ian0

Joined Aug 7, 2020
10,025
A lamp is actually quite a good solution, as its resistance will reduce as the capacitor voltage gets close to the battery voltage, shortening the charge time. For a simple, robust solution just use more lamps.
One thing it doesn't do is prevent the capacitor voltage exceeding 12V.

I can't help but wonder why you don't just start the diesel engine with the battery?

#### WBahn

Joined Mar 31, 2012
30,242
hi, i have a couple of large electrolytic capacitor. 6 volt 630 F.
actually i use them (in series) to start heavy diesel engine and they work pretty good.
Can you tell me which is the best way for fast charging of this kind of capacitor i read somewhere that the capacitor must be charged whit 1/50 of the inrush current. Since I dont know this value I use a 12v 55W lamp in series to charge them but I'm looking for a faster solutions.
I dont know the ESR of the capacitor.
What kind of capacitors are these? Do you have a make and model number?

What is the voltage across the two cells before you start the engine?

What is it after you start the engine?

That will tell you how much charge needs to be replaced.

How fast do you need to charge them? What is the longest time that is "fast enough"?

To just throw some numbers out, let's say that you charge them to 12 V (not recommended, but let's go with it) and they discharge to 8 V when starting. That means that about 1200 C of charge was used. If you charged them as just 10 A, that would be replaced in just two minutes. Are you likely to need them to restart the engine in that amount of time? If not, then is there a reason that two minutes isn't an acceptably fast recharge time?

What voltage do you charge them to? I would recommend NOT charging them each to 6 V, if that's what they are rated for. You will get substantially longer lifetimes if you stay well away from this.

You should also charge balance them in some way, either actively or passively. If you charge them in series, one will take more of the voltage than the other because of differences in their parasitic characteristics. This could lead to one of them seeing so much of an overvoltage that it destroys the capacitor.

#### DickCappels

Joined Aug 21, 2008
10,227
It is somewhat rude to connect an unltracap across and power supply. My solution is to use a switch mode current source, in particular this one which charges the capacitor at a limited current and stops at 2.5 volts. A look at the MC34063 datasheet will show how the circuit values were obtained.

This thing charges the capacitor right up, with no sparks and no worries.

#### MisterBill2

Joined Jan 23, 2018
18,970
I see that the TS is unwilling to state even a guess at the current charging time.

#### Mullins

Joined Dec 31, 2021
179
@Danko
Thank you but I'm looking for a 12v solution. I need to be able to charge it from a 12v battery.
@Ian0
I don't literally run the same engine over and over again. I use it to ignite diesel or petrol engines. Capacitors don't always discharge completely. I try to use this method as little as possible because I know I'm not treating capacitors right. I would like to implement their use with a Charging circuit and a diode to prevent them from being charged by the engine that has just started.

@WBahn
I can wait even 2 minutes for charging.

@DickCappels
I think I Need more Amps... Or do you think I can use multiple circuits in parallel?

Sorry for replying late but I don't have much free time. Thanks everyone for your interest.

#### DickCappels

Joined Aug 21, 2008
10,227
Use a higher current switching regulator. How many amps do you need, or how many volts per second do you want to change across what capacitanc ?

#### MisterBill2

Joined Jan 23, 2018
18,970
Once again, the TS has not told us how long it takes to charge the capacitors using that 55 watt lamp as a current limiter.
It is clear to me that this application is using the charged capacitor to do a "Jump Start" for systems that have a weak battery that can not deliver enough cranking power. For a functional engine the actual cranking time is only 2 to 3 seconds, and so the capacitors will be able to handle that inrush that lasts less than a second.
The problem with fast charging is heating as the current flows through whatever resistance is in the circuit.. So the challenge will be providing a resistance that will limit the current to 50 amps as the capacitor charges and the voltage forcing the current decreases. One very "Low Tech" scheme would be connecting directly to a 12 volt battery using high-temperature-insulated wires, about ten foot leads of #14 wire might be adequate. An actual current regulator circuit will be a much better choice, and more efficient, but also will be a lot more complex. So the selection of the scheme will be a trade-off no matter what choice is made. (CAUTION: the wire size and length, and resulting resistance, are estimates, not calculated values)