Analog cell balancing circuit

Thread Starter

Circuitbeginner12V

Joined Apr 23, 2023
2
Hello,
I need help with redesigning the circuit given below for a lithium ion cell of maximum charging voltage 3.5V approx. It should be an active solution and should only use analog components. Functions such as automatic charging- discharging switches should be there along with charging optimisation.

Since only analog components are allowed use of Microprocessors and microcontrollers stands invalid. I looked up a couple of papers and use of op-amps seems necessary.

Attached image is my initial design and needs to be developed upon.
 

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crutschow

Joined Mar 14, 2008
34,682
Since only analog components are allowed use of Microprocessors and microcontrollers stands invalid. I looked up a couple of papers and use of op-amps seems necessary.

Attached image is my initial design and needs to be developed upon.
Sure does.

What's with all the LEDs?

We don't do homework here, only help with it, so start by looking up some charging circuits with op amps.
 

Irving

Joined Jan 30, 2016
3,980
Welcome to AAC.

I don't see any li-ion cells in your schematic, only capacitors and LEDs, which gives me no insight into your thinking. Maybe you should start by describing your understanding of the li-ion charging regime and the meaning of balancing?
 

Thread Starter

Circuitbeginner12V

Joined Apr 23, 2023
2
The capacitor will be used as batteries for charging and discharging
Led can be removed, that was just another part of what I was making

balancing transalates to BMS functioning
 

Irving

Joined Jan 30, 2016
3,980
The capacitor will be used as batteries for charging and discharging
Are you saying they are just a placeholder for the batteries or are being used instead of batteries (for real or just for simulation?). Batteries and capacitors behave very differently during both charge & discharge.
balancing transalates to BMS functioning
Balancing is a part of a BMS' function, true - but do you understand, and can explain, the purpose of balancing & how you plan to implement it?

As has been said, we can help, but won't do your work for you - you have to demonstrate your knowledge, or at least your attempts to learn...
 

Juhahoo

Joined Jun 3, 2019
302
Search Google for; "Fixed shunt resistor cell balancing circuit", this is as simple as it could get, with side effects of course..
 

Audioguru again

Joined Oct 21, 2019
6,758
An older Lithium-Ion battery cell is 4.2V when fully charged. It is less than half-charged at only 3.5V.

Nobody makes a "3.5V" LED, the part number for a blue or white LED has a range of forward voltages from 2.8V to 3.6V and you do not select the voltage when you buy one.
 

MrAl

Joined Jun 17, 2014
11,565
Hello,
I need help with redesigning the circuit given below for a lithium ion cell of maximum charging voltage 3.5V approx. It should be an active solution and should only use analog components. Functions such as automatic charging- discharging switches should be there along with charging optimisation.

Since only analog components are allowed use of Microprocessors and microcontrollers stands invalid. I looked up a couple of papers and use of op-amps seems necessary.

Attached image is my initial design and needs to be developed upon.
Hello there,

I can give you a hint, but because this is homework you must show your first attempt to solve this yourself.

The basic principle behind cell balancing when the cells are in series is usually just a shunt mechanism. That means that you have to design a circuit that shunts the charging current around the cell when the cell becomes fully charged so that cell no longer gets any further charging. The other cells can still charge until such time that their shunt mechanisms shunt their currents as well.

It is interesting that your circuit contains capacitors and two LEDs in parallel for each capacitor, and I'll tell you why.
First, capacitors are often used as a quick emulation for batteries to see what happens as they charge up.
Second, if the LEDs were high power LEDs and they had a conduction voltage of some exact value like 4.2 volts, then when they begin to light up the current would be shunted around the cell, thus limiting the charge current after that. There are no such type of LEDs though so this is just a very silly example, just to illustrate how this works.

For a better example of how it SHOULD work, say the cells are to be charged at 1 amp. If the top cell charges first, the current is shunted around the cell, and this means that only the other cells get 1 amp now, that one cell does not get any more current. The top cell is charged so doesnt need any more current, but the ones below that still need a charging current so they are the only ones to get that current now. As they too get to the full charge level, the current will be shunted around them also, and eventually they are all charged up.

Your job is to design such a shunt mechanism, and it has to be fairly accurate because the spec on an Li-ion cell has to be observed to a close tolerance (look that up).

See what you can come up with.
 
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