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I'm trying to make a simple passive battery balancing circuit using logic level n-mosfet driven by MCU. However, I've got some issue on how to drive the mosfets by the digital output of the mcu, and also having isolation between mcu ground and the source legs of the mosfet. Otherwise, in the case of the following circuit, the 2nd battery would be short circuited.

simple battery balancing circuit

Looking forward to hear your thoughts or maybe another alternative to drive the mosfet. I was thinking to use optocoupler, but then I need another isolated supply. Thanks

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    \$\begingroup\$ The way that schematic is drawn, the bottom of BAT1 is shorted to GND via a four-way junction at the source of Q1, bottom of R3 and top of R4. That doesn't seem right for a 2-cell series configuration. \$\endgroup\$ – Krunal Desai Mar 16 '16 at 7:37
  • \$\begingroup\$ well, that was the problem. I need the mcu's ground to drive the mosfet. That is why I am searching the solution to have kind of "floating" drive mosfet. \$\endgroup\$ – Jabar Mar 16 '16 at 7:50
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    \$\begingroup\$ Ah, right. Then I'd look at the answer below, that mentions an optocoupler as a potential solution. Barring that, you need to switch to PFET, or implement a charge-pump. On a similar design with a 2-cell series battery pack, I used optocouplers such that a MCU could trigger them both using NFETs. \$\endgroup\$ – Krunal Desai Mar 16 '16 at 8:03
  • \$\begingroup\$ thanks krunal for your response. Did you trigger your FETs using additional supply (different ground)? I was thinking to use optocouplers, but I dont want to add another source to drive the FETs. Would you share your idea on implementing that? thanks \$\endgroup\$ – Jabar Mar 16 '16 at 8:36
  • \$\begingroup\$ Nope -- for my balancing circuit, I did the following. Top optocoupler when active would tie VBATT+ to the gate of the transistor. This will be (for Li-Ion) 3-4.2V above the "center" voltage (the node between BAT1/BAT2). Likewise, the bottom opto would simply tie VBATT_CENTER to the gate of the bottom transistor when active, enhancing it to 3-4.2V above GND. It's late, but tomorrow morning I can post an Imgur link or schematic. \$\endgroup\$ – Krunal Desai Mar 16 '16 at 9:08
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A low-part-count solution would be using an optodriver such as the TLP190B, which allows you to drive a N-FET referenced to a different potential than your local ground without the need for an extra isolated supply. (The IC works by illuminating a small LED that points at a small photodiode, generating the current to enhance an N-FET's gate).

A possibly cheaper (but higher-part-count solution) would use N-FETs to pull down on the gate of P-FETs. Depending on your P-FETs and how many batteries are in your stack, you would need to be clever with the inclusion of zener diodes to stay within the V_gs,max of the part. The scheme I'm suggesting might look something like the schematic on this page.

Finally, there are a whole slew of battery management ICs that would work, depending on the constraints of your application. Searching for a "two cell battery management ic" would put you on the right track.

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  • \$\begingroup\$ thanks max for your quick and helpful answer. I would think about the optodriver. For the 2nd circuit, I dont think it is the best choice since I need to have supply higher than vgs. Which also means I can't use the voltage of single cell battery (3.7V). right? Yes, there are plenty of battery management IC that would extremely help me. But I was thinking to have individual cell balancing circuit, so that I can extend to whatever cells I want. \$\endgroup\$ – Jabar Mar 16 '16 at 8:28
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Try to use the Opto driver between MCU and MOSFET because MOSFET are voltage control device,to trigger the MOSFET must be required minimum Vge as per the mosfet specification.

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