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I am trying to design a circuit for a device that will only run for around 20s at a time. For that 20s, however, I need to power two brushed motors (with connected propellers) that together pull around 5a @ 9v under load.
For my application I need a near constant static thrust for those 20s. I think the easiest way to do this would be to hold the voltage across the motors constant. I am looking to minimize weight, however, and need a power source that is rechargeable, and only has a little bit more than the energy required for this application.

What circuit/power source should I choose?

One option might be to use a voltage regulator to keep a constant voltage for those 20s and choose the smallest possible battery that would not drop below the regulator's minimum input voltage for the duration of the 20s (with some headroom).

To eliminate the voltage regulator is there any power source that is both rechargeable and can output near constant voltage for the majority of its life cycles? I'm guessing no.

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  • \$\begingroup\$ You need constant current and not constant voltage if you want constant force/torque/thrust. eg ..if you power a DC motor by 12 V, then holding the spindle will increase the current up to a maximum of the stall current. \$\endgroup\$ – Jack Creasey Jan 21 '18 at 19:39
  • \$\begingroup\$ But if the load isn't changing wouldn't a constant voltage across the poles of the motors yield a constant current? What else besides changing voltage or a changing load (as you said, resisting the motor by holding the spindle for example) could change the current through the motor? \$\endgroup\$ – Murey Tasroc Jan 21 '18 at 19:47
  • \$\begingroup\$ How is the constant thrust measured? and how many reps. before the battery is dead?. You probably need 3 LiPos. \$\endgroup\$ – Tony Stewart EE75 Jan 21 '18 at 20:09
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    \$\begingroup\$ You'd be better controlling the PWM input to a motor controller (ESC) and not worrying about variations in battery voltage. \$\endgroup\$ – user_1818839 Jan 21 '18 at 20:49
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    \$\begingroup\$ You probably want a closed loop control that seeks a target RPM. If you use a brushless motor and modify brushless ESC firmware, you can probably do this without even needing to add any sensors, since a brushless motor control knows the speed it is electronically commutating at. \$\endgroup\$ – Chris Stratton Jan 21 '18 at 21:04
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The most simple and least expensive solution to a "power source that is both rechargeable and can output near constant voltage for the majority of its life cycles" must be a battery and a voltage regulator.

A constant voltage applied to the motor terminals may be a reasonable solution to providing constant static thrust from a propellor. I assume that "static thrust" is thrust that does not move the platform upon which the propellor is mounted. You would need to consider changes in the medium that the propellor acts on. Does its temperature change? Does it flow in a closed path? You also need to consider how constant is "constant."

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  • \$\begingroup\$ The air temperature does not change. By static thrust I do mean thrust when the motor is held in place relative to the atmosphere (no wind and the motor is not moving). I am saying as constant as is practical (so rpm drops as little as possible during these 20s). \$\endgroup\$ – Murey Tasroc Jan 21 '18 at 21:42
  • \$\begingroup\$ The battery and regulator solution should provide thrust as constant as a desk fan plugged into the wall or a battery alone for the first 20-second intervals, perhaps more constant. If that is what you need, use a regulator. \$\endgroup\$ – Charles Cowie Jan 21 '18 at 21:55
  • \$\begingroup\$ A regulator able to handle several amps is non-trivial. Available, yes, but not what an inexepericenced reader will likely end up with if they go shopping for "a voltage regulator". \$\endgroup\$ – Chris Stratton Jan 21 '18 at 22:31
  • \$\begingroup\$ Most things sold as "voltage regulators" will not work for this purpose. Absent specifics in this proposal, it is highly misleading and only likely to cause the asker frustration. \$\endgroup\$ – Chris Stratton Jan 22 '18 at 0:03
  • \$\begingroup\$ It looks as if you missed the "and only has a little bit more than the energy required for this application." part. But that's OK since he's not going to get it. \$\endgroup\$ – WhatRoughBeast Jan 22 '18 at 1:45
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  1. You will need independent constant current sources for each motor. The power source should be 12 VDC. Google "constant current supplies" and you will find many off the shelf, ready to use.

  2. You need a 12 volt source because by definition a constant current source puts out whatever voltage is needed to maintain a certain current setting. You still have adjustable speed control but now it is done by adjusting the current to the motors.

  3. Due to resistance and internal voltage drop to be sure your motors get 9 volts (if they need it) the voltage source should be around 12 volts or so.

  4. You can use most any 12 volt battery, from a marine deep-cycle battery (very heavy if 100 A/H type) to 3 Li-Po batteries in series.

  5. For a given type of battery be sure to have the correct charger. Li-Po batteries must be charged individually as they need a charger specific to that battery voltage (3.8 volt, 4.2 volt, etc). The 12 volt marine battery will need a more conventional 12 volt charger, but may not charge as fast as the Li-Po batteries using a fast charge mode.

  6. At 5 amps of current for both motors (I assume that will be a normal setting you would want) the marine battery will last for days, But you may have weight and space issues with such a heavy battery. The Li-Po batteries will weigh much less but last only as long as their amp-hour rating divided by the expected run time before you must charge them.

  7. The safety issue is not so much the marine battery but the Li-Po batteries. They cannot be charged in series as one cell will overcharge while the others are under charged. A over charged Li-Po battery can get hot enough to catch on fire, hence the reason for more elaborate charging procedures.

EDIT: Others have the opinion that for best performance you should get motors designed for a servo-loop where current and voltage are based only on the motor RPMs you want or need. Another suggestion is a PWM constant wattage supply, which may or may not be cheaper. I wrote my answer assuming you wanted to keep the motors you had and control motor speed in a safe way.

5 amps at 9 volts is only 45 watts. With some conversion losses a 60 watt power source would be enough, and that is not going to be that expensive. No more than a 100 A/H marine battery plus its charger.

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  • \$\begingroup\$ You don't want a current source for this. Instead, you want a PWM motor driver (which is pretty common once you start talking propellors) and a closed loop control system that seeks something like a fixed RPM. \$\endgroup\$ – Chris Stratton Jan 21 '18 at 21:03
  • \$\begingroup\$ @ChrisStratton. I agree that is the more ideal solution, but the OP seems concerned with cost issues. A CC supply will at least protect the motors. Water would have a constant load factor based on propeller RPMs. I think the OP wants to stay with the motors they have. \$\endgroup\$ – user105652 Jan 21 '18 at 21:10
  • \$\begingroup\$ A PWM RC hobby ESC is likely the cheapest suitable driver that can be purchased and is probably what their motors and props are meant to be used with - high power constant current sources aren't nearly as common. \$\endgroup\$ – Chris Stratton Jan 21 '18 at 21:12
  • \$\begingroup\$ @ChrisStratton. Agreed, but the OP claims that both motors together only pull 5 amps of current at the speed setting she uses. That is a mere 60 watts. Still think an adjustable CC supply is best, unless the OP is willing to buy motors designed for a servo-loop. \$\endgroup\$ – user105652 Jan 21 '18 at 21:18
  • \$\begingroup\$ But they don't need to buy new motors. It's overwhelmingly likely that motors someone sources to drive propellers are from the RC hobby market, and are normally driven with PWM RC hobby ESCs. All that it takes to give those closed loop control is a little software commanding the ESC. \$\endgroup\$ – Chris Stratton Jan 21 '18 at 22:29

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