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Since I currently use a 24V battery system for my house using 16 12V car batteries (2 parallel banks of 8 in series with each other) and use the lower 12V side for additional devices, (as well as due to the difference in age between batteries and occasional removal of one) there is an imbalance when charging and discharging the batteries.

schematic

simulate this circuit – Schematic created using CircuitLab

My requirements are to have a simple circuit made with easily available materials offering up to 1A balancing current and drawing as little as possible of the quiescent current. After a lot of drawing and brainstorming to come up with the simplest solution that works, trying to use as few transistors as possible (at the expense of having a slower and less efficient circuit) I have eventually realized it can't get simpler due to the scale of voltages involved and balances and protections needed.
So, I have resorted to the classic complementary pair MOSFET driver solution controlled by a TL494 (or KA7500) IC and came up with the schematic below:

schematic

simulate this circuit

All that I need is for the capacitor to be switched about 1000 times per second between upper and lower parallel bank, which would automatically, without any measurements and logic, keep them at equal voltages.
This is accomplished by switching M1 and M3, and then M2 and M4 MOSFETS at the same time.
A simpler circuit could be made but it would be less efficient:

schematic

simulate this circuit

This circuit requires fewer transistors but it has somewhat poorer performance. I haven't done detailed calculations, just rough estimates which should be enough for my needs here.
Both of these circuits took a few days and many "rewirings" or rearrangements as some details are not apparent until a first schematic is drafted and looked over carefully a few times, finding an issue until all potential problems are addressed.

If anyone has better, simpler, more efficient idea or sees a problem with my circuits, feel free to suggest improvements.
I don't know if there is a more efficient IC (significantly lower quiescent current than the TL494), so those suggestions are welcome as well.
In fact, any suggestion, even including a complete IC solution, is welcome, though discrete solutions are preferred.
A simple discrete flip-flop (like an astable multivibrator typically made with 2 transistors) which includes a pause (dead-time) between opposing pulses would be great too.

EDIT: The way my circuit works is by merely switching M1 and M3 on at the same time to connect the capacitor across the lower 12V bank. Then they are turned off and M2 and M4 are turned on at the same time to connect the capacitor to the upper 12V bank. Then the cycle repeats 1000 times per second, transferring energy from the bank with higher voltage to the bank with lower voltage automatically. Basically, it's a switched capacitor design. There is a "dead time" between the ON times for the 2 pairs of MOSFETs, and they are all OFF when there are no output pulses from the TL494 IC.

EDIT 2: The batteries are charged by solar panels. The intent of this circuit is to experiment with simple battery/cell balancing circuits and principles in order to later upscale it to more batteries or cells. Also, to share a simple, workable schematic with the rest of the world.

HINT: If you want to suggest a modification to my schematics, the easiest way to do it is by opening my schematic, selecting all of its elements (Ctrl+A), copying them (Ctrl+C) and pasting them (Ctl+V) into your schematic editor in your answer. That way you don't have to redraw anything from the scratch.

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    \$\begingroup\$ I haven't figured out how your circuit is supposed to work but if I had the problem I would consider a different system: 1. Use a buck converter for 24 V to 12 V for the 12 V loads. This solves the unequal current draw. 2. Decide on the maximum float voltage allowed per bank and add a shunt regulator on each bank to prevent the voltage rising above the safe level on each bank. \$\endgroup\$
    – Transistor
    Commented May 28, 2023 at 14:47
  • \$\begingroup\$ @Transistor A high efficiency buck converter WOULD help with the unequal load, though it would need a feedback so that it keeps both banks equal, thus naturally balancing them. \$\endgroup\$ Commented May 28, 2023 at 15:03
  • \$\begingroup\$ The way my circuit works is by merely switching M1 and M3 on at the same time to connect the capacitor across the lower 12V bank. Then they are turned off and M2 and M4 are turned on at the same time to connect the capacitor to the upper 12V bank. Then the cycle repeats 1000 times per second, transferring energy from the bank with higher voltage to the bank with lower voltage automatically. Basically, it's a switched capacitor design. There is a "dead time" between the ON times for the 2 pairs of MOSFETs, and they are all OFF when there are no output pulses from the TL494 IC. \$\endgroup\$ Commented May 28, 2023 at 15:07

3 Answers 3

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Here are a few, simpler and much better solutions:

  1. Use two 12 V chargers, one for each battery, instead of a single 24 V charger
  2. Buy an off-the-shelf 24 V / 12 V battery equalizer / balancer
  3. Remove the tap between the two batteries; power the 12 V load with an off-the-shelf 24 V to 12 V DC-DC converter / buck regulator
  4. Connect the two 12 V batteries in parallel instead; get a 12 V charger; power the 12 V loads directly; power the 24 V loads with an off-the-shelf 12 V to 24 V DC-DC converter / boost regulator
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  • \$\begingroup\$ I forgot to mention these batteries are charged with solar panels. I need 24V to reduce the loss in my home wiring (mostly light loads). I could buy a done solution, but I want to make my own solution, both for the learning experience, as well as for a more complete control over all details. I also want to make charging easier and possibly more efficient by using a single 24V charger. Using solar energy makes every watt important, so lower losses/fewer conversions ar preferred. \$\endgroup\$ Commented May 28, 2023 at 15:16
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How about a half-bridge balancer, as commonly used in li-ion battery packs? In the simplest implementation, just a fixed duty cycle and the battery with the highest voltage will "passively" transfer energy to the other battery. With a regulation loop, you can choose to switch with just one to speed up the process or if you go all out, active rectification by using both and a control scheme to maintain a midpoint.

schematic

simulate this circuit – Schematic created using CircuitLab

M3 is a PMOS for simple gate drive here and the schematic is very simplified.

Here is a better but more complicated approach:

schematic

simulate this circuit

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  • \$\begingroup\$ That's a very good suggestion! Simple and effective. As I understand, the first example makes the upper MOSFET work in a buck converter configuration, while the lower MOSFET works as a boost converter. But it seems the voltages would not be exactly equal due to the body diode drops unless active rectification is used. Can you explain a little bit how they work, especially the lower one? I assume the op-amp changes the width of the upper or lower MOSFET duty cycle based on which battery has lower voltage. \$\endgroup\$ Commented May 29, 2023 at 19:35
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Battery balancing is a little tricky. I would get a DC to DC buck power supply. (24V to 12V) Set the output to 12V or is it 14V? You know that during charging the 12V might get to 14V. But with a buck power supply you can set the output to what you want and it will stay there. Alie This will only output 4A but is a good example. enter image description here Here is a 30A version from the same place. enter image description here

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  • \$\begingroup\$ Thank you for the suggestion. I know that balancing is tricky, and I am aware of such potential solutions. My intent here is not merely to keep my system in balance, but to make a circuit that will work in other similar setups as well, scaling it up to 3 or 4 batteries in series, and possibly making a similar balancer with many more lithium cells in series. So, this both for my own problem and for other future solutions. This is basically like a humble beginning. I am also giving these schematics out to those who need to solve a similar problem. \$\endgroup\$ Commented May 28, 2023 at 21:14

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