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Previously I made same scheme (attached) but with two relays and it worked pretty well. Now I want to rebuild it with transistors (MOSFETs).

The main purpose as I mentioned in the subject is to switch battery connection type, from series, which I need to power my scheme, to parallel, for charging with USB. (Arduino will switch MOSFETs states.)

But I have some questions:

  1. Is it okay to use M1 or should I use NPN BJT to drive M2?
  2. How to pull up M2 gate?
  3. Will M3 be closed by default or I should use scheme like on the second attachment. (M1, M4: N-channel MOSFET; M2, M3: P-channel MOSFET)

schematic 1

schematic 2

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    \$\begingroup\$ Consider using Q instead of M for your transistors (as the lower schematic does). Q is standard practice for transistors (of any type), where M is not. \$\endgroup\$ – JYelton Oct 18 '19 at 17:30
  • \$\begingroup\$ Consider using a charger that does two cells in series, or keep the cells in parallel and use a boost converter on the output. Either will be simpler (and probably cheaper and more reliable) than what you are trying to achieve. \$\endgroup\$ – Bruce Abbott Oct 18 '19 at 19:16
  • \$\begingroup\$ @BruceAbbott I tried using boost converter on the output, but i worked pretty bad with radio module. When current consumption increase in a short time(motor startup) i observe problems with radio transmission. \$\endgroup\$ – Serhii Yurchenko Oct 18 '19 at 23:41
  • \$\begingroup\$ Then use a 2 cell charger (or a better boost converter, or isolate the radio supply from the motor supply). Don't try to work around your problem with a dodgy solution that could be more trouble than it's worth. \$\endgroup\$ – Bruce Abbott Oct 19 '19 at 0:53
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MOSFETs only block current in one direction. That's why the symbol contains that parasitic body diode. You have a short which may be unfixable without significantly more circuitry to support back-to-back NMOSFETs sharing a source connection so current in both directions is blocked. Just using a step up converter and series cells or step down converter and series cells is simpler since a floating gate drive supply is required for each back-to-back NMOSFET pair.

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I've actual made a schematic of this for a project I'm working on for the same purpose. You can take a look at it if you want to use it as a reference. enter image description here Q1 & Q3 break the series connection of the batteries. When the Gate of Q2 is LOW, Q1 & Q3 conduct. When the Gate of Q2 is HIGH, Q1 & Q3 stop conducting. The gate of Q2 is controlled by the arduino.

Q8 & Q9 work just like Q1 & Q3. Their purpose is to break the connection from the main circuit. The only difference is that the gate of Q12 is controlled by the input voltage of the USB connection.

Q4 is controlled by the arduino and it connects the negative terminal of B2 to GND.

Finially, Q11 & Q5 (also Q13 & Q6 since they're the same) are switches that lets current flow from the output of the charging IC. In this schematic, each battery has it's own charging IC, so keep that in mind. These FETs are controlled by the gate of their respective N-FET. When the Gate of Q10 is LOW, Q11 & Q5 are not conducting. And when the Gate of Q10 is HIGH, Q11 & Q5 will conduct. The Gate of Q10 is also controlled by the arduino.

In the program, I have it to where I set specific gates high in a specific sequence to prevent any short circuits: digitalWrite(A0, HIGH);

delayMicroseconds(50);

digitalWrite(A1, HIGH);

delayMicroseconds(50);

digitalWrite(A2, HIGH);

And when the USB Power supply is removed, the way I return from parallel to series is as follows: digitalWrite(A2, LOW);

delayMicroseconds(50);

digitalWrite(A1, LOW);

delayMicroseconds(50);

digitalWrite(A0, LOW);

I hope this helps you with your project.

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