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I have two 3.7v cells in series to make a 7.4v battery. I want to make my own active balancer because I don't have access to buying any cell balancers.

I'm using a transformer with 2n turns on the primary side connected to the 7.4v part, and I have 2 different secondary coils with n windings on each, which are connected to diodes which are connected to the individual cells.

From my understanding, if one cell is less than the other, the total battery voltage divided by two should charge the cells at different rates since one cell will pull more current from the transformer because of its lower voltage drop.

I'm not that good at electronics as I'm limited, but I'd like some input on this.

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    \$\begingroup\$ If I understand you right you are going to charge what appear to be lithium cells with half or full-wave rectified DC with no voltage monitoring and no current control. Can you think of any problems with this design? (Hints: smoke first, then fire and maybe explosion.) \$\endgroup\$
    – Transistor
    Feb 12, 2021 at 15:11
  • \$\begingroup\$ Say I throw in zener diodes across each cell, so the voltage never passes the 4.2v full charge of each cell \$\endgroup\$ Feb 12, 2021 at 15:14
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    \$\begingroup\$ Say you read up on battery management and try best practice rather and safety first? \$\endgroup\$
    – Transistor
    Feb 12, 2021 at 15:18
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    \$\begingroup\$ Bongani, lithium cells can catch fire or explode if charged improperly. They are very delicate and fragile - and cannot be recharged from a transformer. \$\endgroup\$
    – rdtsc
    Feb 12, 2021 at 15:55
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    \$\begingroup\$ My suggestion is to place it in a fire pit outside, turn your charger on and take pictures it will be better than fireworks. I highly recommend either putting the project aside or be sure your insurance is up to date. You can add us as beneficiaries. Try this link: batteryuniversity.com/learn after reading what they have to say and follow there links you will probably understand what we are trying to say. \$\endgroup\$
    – Gil
    Feb 12, 2021 at 18:38

2 Answers 2

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Your proposed balancer is similar to the one you posted in your own answer.

Your understanding of its function seems to be correct - the charge current is distributed to the cells depending on the discharge state of each cell. A cell with lower voltage will get more current than a cell with higher voltage.

In that sense it will work.

The folks who wrote the article Overview of cell balancing methods for Li‐ion battery technology seem to think so as well.

This is figure 9 from that article:

enter image description here

It shows pretty much the same concept as what you have been discussing.

The problem is that it is a concept. It is not a complete solution.

It is missing all the stuff it takes to safely charge a lithium ion battery.

In comparison, here's a full up system that will safely charge your cells and balance them:

enter image description here

It includes the needed protection for each cell: charge control, discharge control, balancing, etc.

That diagram also leaves out a lot of details. The details are in the datasheets of the named ICs.

What I'm trying to say is that a concept is a long way from a usable circuit and that lithium cells are not friendly subjects for learning through experiments.

By all means, continue with your concept. You will learn a lot.

Keep the cells in a fireproof container, and be prepared to spend more in replacing damaged equipment than you save in building your own equipment.


Here's a paper in which a group at a university designed and built a balancer based on an IC from Texas Instruments.

Take note of the equipment they used in the development:

enter image description here

You are starting a difficult project. It will teach you a lot.

I wish you luck.

Stay safe.

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https://www.mdpi.com/2076-3417/8/8/1278/htm

Image source: Figure 8 from Bui, Thuc M.; Kim, Chang-Hwan; Kim, Kyu-Ho; Rhee, Sang B. 2018. "A Modular Cell Balancer Based on Multi-Winding Transformer and Switched-Capacitor Circuits for a Series-Connected Battery String in Electric Vehicles" Appl. Sci. 8, no. 8: 1278. https://doi.org/10.3390/app8081278

So what the commenters are saying is that they have no knowledge of cell balancing, only what happens to batteries if they are not charged well!

Anyway I researched and stumbled on this...

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  • \$\begingroup\$ There seems to be a lot missing from that diagram. Transformers don’t work well with DC, so there has to be some magic that we’re not seeing. \$\endgroup\$
    – Kartman
    Feb 13, 2021 at 7:28
  • \$\begingroup\$ Obviously the DC is pulsed! Why does everyone here like to point things out as if other people don't think about them, \$\endgroup\$ Feb 13, 2021 at 8:41
  • \$\begingroup\$ There’s a lot of information missing from that schematic, I don’t think it’s wrong to point that out. \$\endgroup\$
    – Frog
    Feb 13, 2021 at 9:16

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