# How to limit voltage charge of supercapacitor

For a DIY project, I plan to charge an 100F 2.7V Supercapacitor using a 3.7V Lithium-ion 18650 battery. However, I need it to charge only until 2.25V.

Is there a simple circuit that I can build to limit the voltage\charge?

I basically need the stored energy to be ~250J.

• "Are there any adjustable boards that I can buy" makes it a shopping question - not allowed Mar 1, 2017 at 16:45
• Buck converter + series resistor should do it. Mar 1, 2017 at 16:46
• How much current you can afford to be taken from your 3.7V battery?
– user136077
Mar 1, 2017 at 17:08
• @user287001 I'd only need to charge and discharge about 10-15 times per session, so I should have enough. Mar 1, 2017 at 17:23
• The max allowed current is needed for "how long the charging will take" If you can afford say 100 mA, then the full charging takes 38 minutes. 1A => 3 min 48 s, etc...
– user136077
Mar 1, 2017 at 17:39

If you have a power PNP Darlington or can make one with a power transistor and driver then get 2 Ultrabright RED LEDs and any old NPN signal transistors. These make good low voltage zener references.

Current sensing is on 330m should be 2 to 3 W. The Upper LED turns on DIM when PNP Darlington is ON and then when output LED turns on DIM it switches off the series regulator and with 2A it takes <120 s.

Java Sim design

Here is a very basic circuit that would limit the cap voltage to 2.7V.
The zener diode is used so that the 2.7V reference is fairly constant regardless of the battery voltage.

simulate this circuit – Schematic created using CircuitLab

If you use an op-amp as shown, the charging rate will decrease as the cap approaches the reference voltage. If you were to replace the op-amp with a comparator, this shouldn't be an issue. To use a comparator, I believe a pullup resistor would need to be connected from the comparator output to the positive end of the battery.

Here is a simulation that uses an op-amp.

• Just use a Z-Diode and a resistor as voltage reference. With this modification the charging voltage is stable and does not depend on the input voltage. And add a series resistor after Q1 to limit charging current.
– auoa
Mar 1, 2017 at 18:28
• @auoa You beat me be 2 seconds!
– Bort
Mar 1, 2017 at 18:29
• haha, now you've to add the resistor, too :D
– auoa
Mar 1, 2017 at 18:30
• Are you sure that's how this circuit works-- To me, it looks like the cap will see voltages all the way up to a $V_{CE}$ drop of the battery when the comparator is off. Mar 1, 2017 at 18:31
• Have you actually simulated it? I think you want an op amp in negative feedback, the 2.7 volts at the positive input and not a comparator. That would cap the voltage of the cap at 2.7V. Note also that 2.25V was specced in the question. Mar 1, 2017 at 18:34

The best would be a synchronous buck switching constant current source, with a voltage-limited output. If you don't find a pre-made one, you can use a switching LED driver and a comparator to stop it when target voltage is reached.

• this would be most efficient sol'n Mar 1, 2017 at 20:19