# How to transfer charge from a higher voltage cap into a lower voltage cap

#### Mark Flint

Joined Jun 11, 2017
145
Noob question. In my application I have a cap that charges to 250v. I want to transfer this charge to a 22.5v supercap. What are my options to do this?

Joined Mar 10, 2018
4,057
A partial transfer of charge can occur by using a switch to parallel the caps.
Many considerations to tackle.

First was is the capacitance of both ? They will share charge proportional to their
capacitance.

Q1 = C1 x V
Q2 = C2 x V
or Q1/C1 = Q2/C2 for equal V.

If you want to truly transfer all charge from one to the other then a buck
converter might be an approach, basically use it to charge the super cap.
Some charge lost thru circuit losses.

Regards, Dana.

#### Mark Flint

Joined Jun 11, 2017
145
Thanks Dana,
The 250v cap is 660uF and the 22.5 cap is 1F.
I am aiming at 75-80% efficiency. I thought of using a buck converter circuit but don't have the skills to design such a thing.

#### Mark Flint

Joined Jun 11, 2017
145
Dana - thank you. Some nice pieces there.
I just did a test there by putting a boost converter that kicks in at 10v onto the 250v cap, and set the booster output to 22.5v for the 1F cap. It seems to nicely regulate the voltage into the 1F cap and does not charge it past that voltage. It pegs the 250v cap at 10v where the booster kicks in, and so the 250v cap does not charge higher than 10v while charging the 1F cap. Once the 1F cap reaches 22.5v the 250v cap starts to charge up again past the 10v because the boost converter has no potential difference on it's output I presume. This seems to be something I will e able to work with. I have not figured out the efficiency yet though,
Cheers, Mark.

#### -live wire-

Joined Dec 22, 2017
959
Do you know about proper safety precautions? Because a cap that size can easily deliver a fatal shock, even when disconnected. It is concerning when people do not seem too knowledgeable about this stuff but then are around such high voltages. While you seem more serious than others, you need to be aware of the dangers and have a thorough understanding before you do stuff around such dangerous voltages.

Joined Mar 10, 2018
4,057
As -live wire- prudently points out working with HV can be a heart stopper,
no pun intended.

Just google "hv safety practices", lots of resources worth saving a life, yours.

Regards, Dana.