Let me start: I have no clue of electric engineering. However, I built an automated window blind controlled by my home automation.

It consists of a stepper motor, motor controller board, an ESP8266 and an Ikea battery pack (7.2V) which they use for their window blinds. So far so good, I can control those by software and by a switch. Every now and then I have to remove the battery and recharge it.

Sometimes these blinds move quiet slow and I lower them by hand. The motor shield protects the ESP circuit against the current produced by the motor. However, I wonder if it would be possible to adapt the circuit to charge the batteries instead.

As I said, I have no clue of electric engineering. Is this even possible? In my eyes this would be awesome to not "waste" the energy produced by manually lowering the window blinds. (A dream would be to balance the energy needed to raise the blind with the energy produced by lowering the blind by hand, but that's just a dream)

BTW: found this blog here Understanding regeneration

Hope I find a creative, helping spirit here :)

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    \$\begingroup\$ Back EMF can't be used for charging. You may have mixed it with the concept of regenerative breaking mechanism. \$\endgroup\$ – Sadat Rafi Jun 19 at 6:53
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    \$\begingroup\$ @SadatRafi BEMF can be used for charging. Regenerative braking utilises the BEMF generated by an aiding load \$\endgroup\$ – JonRB Jun 19 at 8:17
  • \$\begingroup\$ @JonRB So, would be possible to develop such a circuit which could simply be added to an existing charging circuit? Like some Arduino boards or ESP32 boards have? Or an additional one? \$\endgroup\$ – Wolfram Jun 19 at 8:36
  • \$\begingroup\$ @SadatRafi Isn't it the same thing/principal? In my car isn't it the same motor used to charge the battery when breaking? \$\endgroup\$ – Wolfram Jun 19 at 8:38
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    \$\begingroup\$ Regeneration requires boosting the voltage generated by the motor to a higher level so that it can charge the battery. The amount of energy produced by moving the covers by hand is just too small to make any significant difference in the charge of the battery, so building a circuit to do that would be a waste of time. \$\endgroup\$ – Dave Tweed Jun 19 at 12:05

Theoretically that's impossible. When a coil rotates in a magnetic field a potential difference is generated across it. It may be a DC generator or a motor, that doesn't matter.

Consider a DC generator. If you rotate the shaft, a potential difference will be generated across it. As soon as you connect a load across it, current will start to flow through it. This current will oppose the reason of it's creation. i.e. it will try to slow down your shaft. Then you have to apply more mechanical power to the shaft to maintain your desired frequency.

In case of motor, you are supplying electrical energy and the shaft rotates utilizing it. A soon as the motor starts to rotate, the generator effect stars to appear. Because a coil is moving in a magnetic field. A voltage is gradually developed across the motor which is known as the back EMF. It opposes the applied EMF.

Let's assume that you have connected something across the generator created inside the motor(i.e. back EMF). As you are drawing current from the generator it's shaft will be slow. That's actually the shaft of motor which is getting slow. So if you want your motor to run at the previous speed , then you have to increase your input power. Otherwise the motor will run slower.

So,Connecting a battery across motor won't utilize the back EMF. It will increase the line current.

You have given a link of regenerative braking. It's not simply like a battery connected across a motor. It uses the generator effect to slow down the wheel as soon a the motor is turned off.

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  • \$\begingroup\$ Ok. If I understand you right: BEMF != regenerative braking. Is that correct? Is it than theoretically possible to build such a circuit using regenerative braking (so drawing the window cover by hand)? \$\endgroup\$ – Wolfram Jun 19 at 15:32
  • \$\begingroup\$ You have nothing to do with back EMF or regenerative braking. What you have to is to provide much power to the motor. Use a 12 volt 5 amp DC power supply / 12 volt LiPO. And you should include motor model and specification in the question. And how you are driving your transistor is also a very important factor. You should also provide the proper circuit diagram. \$\endgroup\$ – Jack Danniels Jun 19 at 19:57

What you are asking about is termed "regenerative energy recovery" or "regeneration" for short.
Back EMF uses the same basic principle of energy generation but in a different context.

It would be theoretically possible to recover some of the input energy when the blind descends, but in practice you would find that the energy available was such a small proportion of the input energy as to make the effort not worthwhile.

Available regenerative energy = input energy - ascending energy - descending energy used
A blind lifting mechanism will almost always be of low electrical to mechanical conversion efficiency. Almost all the input energy is expended in friction in the gearing and cordage losses. Actual energy input to the blind proper
= mass x lift height x g.
For a say 1 kg blind lifting its centre of gravity 1 metre this is roughly
1 x 1 x 9.8 ~= 10 Joules.
A say 7.2V, 1000 mAh battery contains about 25,000 Joules.
[Joules = Ah x Vmean x 3600 s/h]
If it took equal energy up and down and efficiency was say 50% that should suffice for
25000/10 x 50% = 1250 cycles
or about 3 years use.
In practice you will get very much less than that.

In descent the blind mass provides less load on the motor so you are already getting some energy recovery in the form of energy not used.

You can get some idea of how efficient the current system is
by measuring motor energy to lift
= V_motor x I_motor x seconds = Joules used.
Compare this with Blind_mass x _height_cofg_lifted x 9.8 Joules
Divide blind energy gain by energy use to give efficiency.
Be suitably amazed at how low it is :-).

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