Charging efficiently a bigish capacitor from a battery

Thread Starter

kpram

Joined Aug 28, 2023
2
Hello,

The above subject is highly discussed before but unfortunatelly i cannot find a suitable solution after 2-days web-searching :-(

I want to charge a rather big capacitor (i.e. 6800uF) at 12V from a Li-SOCL2 battery (3.6V, <100mA). But I want to do it efficienlty, meaning rather fast (< 1s) and not wasting too much energy (i.e. on current limitting resistor) and finally at a rather low cost (i.e. <2-3$).

The perfect idea would be a rather cost-efficiect boost converter with input currect limitting below 100mA. Unfortunatelly i haven't managed to find one. Alternatively i was thinking to use a soft-start boost converter and then a very small current limiting resistor (i.e. 10ohms) and a mosfet that i could control using the on-board uC to pulse charge the capacitor. Or a bigger current limitting resistor (i.e. 400ohms) for the initial charging and the a mosfet in parallel that would do the rest of the charging.

I guess there are simpler solutions than the above but i am not any good at power electronics. However i am looking for a solution as reliable as possible and preferably with a low component count.

Any help/idea would be appreciated... regards,

Kostas
 

LowQCab

Joined Nov 6, 2012
4,075
Why do You want to charge the Capacitor ?

Why do You think the Current needs to be limited ?

Is this a "one-time", "start-up", "Inrush-Current-Limiting" scheme ?,
or, will this charging event be repetitive ?, and if so, at what frequency ?

Please provide a Schematic-Diagram and Specifications of all Components.
.
.
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Ramussons

Joined May 3, 2013
1,409
Hello,

The above subject is highly discussed before but unfortunatelly i cannot find a suitable solution after 2-days web-searching :-(

I want to charge a rather big capacitor (i.e. 6800uF) at 12V from a Li-SOCL2 battery (3.6V, <100mA). But I want to do it efficienlty, meaning rather fast (< 1s) and not wasting too much energy (i.e. on current limitting resistor) and finally at a rather low cost (i.e. <2-3$).

The perfect idea would be a rather cost-efficiect boost converter with input currect limitting below 100mA. Unfortunatelly i haven't managed to find one. Alternatively i was thinking to use a soft-start boost converter and then a very small current limiting resistor (i.e. 10ohms) and a mosfet that i could control using the on-board uC to pulse charge the capacitor. Or a bigger current limitting resistor (i.e. 400ohms) for the initial charging and the a mosfet in parallel that would do the rest of the charging.

I guess there are simpler solutions than the above but i am not any good at power electronics. However i am looking for a solution as reliable as possible and preferably with a low component count.

Any help/idea would be appreciated... regards,

Kostas
Assuming the intended boost convertor is 100% efficient, 100 mA @ 3.6 V => 30 mA @ 12 V.

So use a CC boost convertor limited to 30 mA.

If the convertor is 80% efficient, limit the output current to 30 * .8 = 24 mA.
 

BobTPH

Joined Jun 5, 2013
8,989
But I want to do it efficienlty, meaning rather fast (< 1s) and not wasting too much energy (i.e. on current limiting resistor)
In a boost (or buck) converter, the current is limited by an inductor and the on time. Unlike a resistor. An inductor dies this while dissipating no energy. Si resistor always loses energy as heat.

You cannot charge a capacitor efficiently without using an inductor.
 

Alec_t

Joined Sep 17, 2013
14,321
To charge 68000uF to 12V in 1 second requires an average current of ~ 0.8A. So a 3.6V battery powering a theoretically 100% efficient boost converter would have to provide a current of 0.8A x 12/3.6 = 2.7A. If your battery can only source 100mA it's not enough.
 

WBahn

Joined Mar 31, 2012
30,065
Hello,

The above subject is highly discussed before but unfortunatelly i cannot find a suitable solution after 2-days web-searching :-(

I want to charge a rather big capacitor (i.e. 6800uF) at 12V from a Li-SOCL2 battery (3.6V, <100mA). But I want to do it efficienlty, meaning rather fast (< 1s) and not wasting too much energy (i.e. on current limitting resistor) and finally at a rather low cost (i.e. <2-3$).

The perfect idea would be a rather cost-efficiect boost converter with input currect limitting below 100mA. Unfortunatelly i haven't managed to find one. Alternatively i was thinking to use a soft-start boost converter and then a very small current limiting resistor (i.e. 10ohms) and a mosfet that i could control using the on-board uC to pulse charge the capacitor. Or a bigger current limitting resistor (i.e. 400ohms) for the initial charging and the a mosfet in parallel that would do the rest of the charging.

I guess there are simpler solutions than the above but i am not any good at power electronics. However i am looking for a solution as reliable as possible and preferably with a low component count.

Any help/idea would be appreciated... regards,

Kostas
First, let's consider some basic bounds.

From an energy standpoint, to charge a 6800 µF capacitor to 12 V, you need to transfer CV²/2 energy to it, so that's just under 500 mJ. To do that in 1 s, requires an average power of 500 mW. A 3.6 V battery that is limited to no more than 100 mA can provide 360 mW of power. That's with 100% efficiency.

So while you are not too far out in left field, you are still chasing something that is unicorn-adjacent. If you allow at least 2 s for the charge time, you might be in the ballpark.
 

WBahn

Joined Mar 31, 2012
30,065
To charge 68000uF to 12V in 1 second requires an average current of ~ 0.8A. So a 3.6V battery powering a theoretically 100% efficient boost converter would have to provide a current of 0.8A x 12/3.6 = 2.7A. If your battery can only source 100mA it's not enough.
The cap is 6,800 µF, not 68,000 µF. Easy to miss.

Aside from that, the situation isn't quite this bad, because the boost converter only has to output 12 V at the very end. Initially, it is outputting nearly 0 V, so it can deliver a lot of current for that 100 mA of battery current. If it is outputting 12 V all the time, then half of the energy will have to be given up as heat during the charging process.
 

Thread Starter

kpram

Joined Aug 28, 2023
2
Thank you all for the replies..

You are right that it is not possible to charge such a capacitor at 12V in 1 second with 25mA.

Meanwhile i came across a constant current limiting circuit using an LM317 or 1117 regulator that seems quite simple and reliable.

I did a spice simulation with it and shows a nice behaviour and that it would take around 2.5 seconds to charge the capacitor to more than 11 volts which is OK for my case. BTW, the idea is to drive a latching solenoid with it.

Thank you again for your time.. regards,

Kostas
 
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