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I am willing to use my 7.4 volts 1000mAh lithium polymer battery to power a motor that works with 3.7 volts, so I will input the 7.4 volt to a high efficiency buck converter to get an output of 3.7 volt, which is connected to the motor, Assuming that the motor draws 1 Amp, my question is will I will still get 1 Amp for only 1 hour from the output of the converter or I will get 1 Amp for two hours ??

The total watt hour for my battery is as I know 7.4 watt hours (V*Ah) but my motor draws 1 Amp at 3.7v so watt hours of 3.7 watt, so If I divided the watt hour of my battery by the watts my motors draw I get 2 hours, Is these calculations correct or there is another rules or factors that affects it ?

I am a school student by the way so, please if you answer me, answer me in full details as if I don't know anything.

Thanks in advance.

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Assuming that the motor draws 1 Amp, my question is will I will still get 1 Amp for only 1 hour from the output of the converter or I will get 1 Amp for two hours ??

You'll get 1A for 2 hours (approximately).

The total watt hour for my battery is as I know 7.4 watt hours (V*Ah) but my motor draws 1 Amp at 3.7v so watt hours of 3.7 watt, so If I divided the watt hour of my battery by the watts my motors draw I get 2 hours

Correct, but you have to substract losses in the DC-DC converter. If your converter is 90% efficient and uses a 7.4Wh battery as power source, 7.4*90% will be available on the output, or 6.66 Wh. Basically, conservation of energy (input energy = output energy + losses).

Note that you don't need a DC-DC converter. You can use a MOSFET instead:

enter image description here

A permanent DC motor does not require accurate voltage regulation, and it is quite inductive so PWM works very well. If the question was about using a 110V rated motor on 230V we should definitely check if the wire insulation would withstand the extra voltage, but for 7.4V I don't think this would be a problem.

This simple schematic uses PWM to apply 7.4V to your 3.6V motor with 50% duty cycle, so the average voltage on the motor is 50% of supply voltage, or 3.6V. This saves a buck converter.

Efficiency calculation: suppose the PWM is fast enough so we can neglect the current variations in the motor, it will be an average of 1A. Since DC motors have quite large inductance it will smooth the current.

Duty cycle 50%, so D=0.5

When mosfet conducts: it has resistance RdsON, and dissipates \$ R_{dsON} I^2 \$

When diode conducts, it has forward voltage Vf, and dissipates \$ V_f I \$

Note that mosfet conducts with duty cycle D and diode conducts with duty (1-D), so:

Total losses: \$ D R_{dsON} I^2 + (1-D) V_f I \$

You can add switching losses which are estimated by (Switching time) * Frequency * I*V but at motor PWM frequency they will be pretty small.

With realistic values RdsON = 0.02 ohm, Vf=0.5V using a schottky, D=0.5, Losses=0.26W, Motor power=VI=3.6W, efficiency=93%.

Another option is to wire the battery in accordance with the motor voltage. Since your battery is made of two 3.6V elements in series, if you wire them in parallel instead, you got 3.6V 2000mAh instead of 7.4V 1000 mAh (the energy capacity in Wh stays constant of course). But you may need a new charger, protection pcb, etc, so that may not be an option.

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  • \$\begingroup\$ Thanks so much for your answer, but I have a small question, will the efficiency of the mosfet be better than the efficiency of the buck converter ? \$\endgroup\$ – Shams El-Deen Jan 7 at 13:04
  • \$\begingroup\$ @ShamsEl-Deen This is basically a buck converter already, but upside-down, and using the motor itself as the coil. A neat trick! \$\endgroup\$ – user253751 Jan 7 at 14:01
  • \$\begingroup\$ Efficiency depends on MOSFET and diode losses, I've added a short explanation. And yes it is just like a buck converter using the motor inductance. \$\endgroup\$ – peufeu Jan 7 at 15:46

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