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I'd like to add multiple Li-po batteries to my octocopter without worrying about different voltages across different batteries while also allowing regenerative braking.

I have seen designs where people use 'ideal diodes' to isolate batteries so they don't charge each other when there's a voltage difference. However, this also means that ESC can't push energy back to batteries when decelerating via regenerative braking.

I think this problem can be solved with a skimped BMS design.

Specifically:

  1. Drone ESCs' power are controlled by high power MOSFETs (Q1,Q2)

  2. Q1, Q2 are controlled by an MCU that's powered by 'LOW POWER+'.

  3. LOW POWER+ is always available.

  4. Diodes (D1, D2) are added to the LOW POWER route to prevent the batteries from charging each other. Power loss over diodes should be minimal since current in the LOW POWER section of the circuit is expected to be small.

  5. Q1 and Q2 are only simultaneously closed when BT1 and BT2's voltages are close to each other (+- 0.05V for example).

  6. The battery with the highest voltage are prioritized to be connected to the circuit.

  7. All batteries are expected to have the same chemistry and amperage ratings and etc. This ensures the current draw from the batteries are shared equally when multiple batteries are powering the ESCs.

schema

I need some second opinion on this. I'm sure similar designs have been done elsewhere. DJI's matrice 200 drone supports multiple battery but I don't know how they implemented.

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  • \$\begingroup\$ "Drone ESCs' power are controlled by high power MOSFETs (Q1,Q2)" - well, not really, those body diodes are always going to be conducting. And to turn on the MOSFETs you would need a gate voltage higher than the battery voltage or a purpose-built high side driver. \$\endgroup\$
    – Finbarr
    Sep 12 at 11:36
  • \$\begingroup\$ Thank you for pointing that out. I think you are referring to MOSFET's zero gate voltage drain current? Those are typically in uA range so I think it's generally safe to assume the MOSFET is an open circuit when Vgs < Vgs_threshold? Could you explain why the gate voltage needs to be higher than battery voltage? The condition for 'turning on' the MOSFET is Vgs > Vgs_threshold right? \$\endgroup\$
    – KWang
    Sep 12 at 19:27
  • \$\begingroup\$ No, I'm talking about the body diodes between S and D that you can clearly see on your diagram, they will be forward biased all the time. Turning the MOSFET on will allow current to flow in the other direction, i.e. from DRONE ESC+ into your batteries. \$\endgroup\$
    – Finbarr
    Sep 13 at 9:36
  • \$\begingroup\$ Vgs(th) is the point at which a MOSFET just STARTS to turn on and is normally specified for a drain current of 250uA. I suspect your drone needs a lot more. And since S is at battery voltage, the gate voltage needs to be at least Vgs(th) HIGHER than battery voltage for it to turn on. \$\endgroup\$
    – Finbarr
    Sep 13 at 9:38

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Personally, I would make this simple by making sure all batteries are at a very close voltage level before I connect them in parallel.
Once they are connected in parallel, you don't have to worry about voltage difference between them and they naturally share their currents in both directions.

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  • \$\begingroup\$ This is the right answer. They will behave as one big battery. \$\endgroup\$
    – user253751
    Sep 12 at 9:02
  • \$\begingroup\$ Making sure the batteries have the same voltage is a viable option and also eliminates the need of diodes (D1, D2) in my schematics. But I simply find it a hassle to do. This is especially true if I have say 4 batter or even more on the craft at once. Besides, I think this design allows hot-swapping batteries with different voltages without worrying about voltage differences. Naturally the circuit will eventually bring all the batteries to a similar voltage level and use the all at once \$\endgroup\$
    – KWang
    Sep 12 at 11:18
  • \$\begingroup\$ @KWang Perhaps a simple modification to the circuit is to make it work like a balancer which upon equalizing the voltages keeps MOSFETs shorted (like closed switches) so that they work in both directions. That way you don't have to worry about manual balancing hassle. \$\endgroup\$
    – Edin Fifić
    Sep 12 at 12:11
  • \$\begingroup\$ @Edin Fifić Can you tell me more about this balancer? The balancer I know (like the ones on BMS) only works with cells in series. \$\endgroup\$
    – KWang
    Sep 12 at 19:50

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