Using a generator to charge 2 batteries

Summary
Hello,

I have a 24 volt motor connected to two 9V rechargeable batteries in series and a battery connected to a switch then back to the motor. I want to be able to charge the batteries directly from the motor when the switch is off & have the battery power the motor when the switch is on. Motor/generator. When the switch is on the motor runs, when the switch is off the rechargeable batteries don’t charge, any idea to get them to charge?
My project is to create a circuit for a stroller that can store the mechanical work into electrical while moving around and with the push of a switch, use the electrically stored energy to assist the movement of the stroller. I’ve tried capacitors but it ended up not being enough voltage and I didn’t want to deal with the danger, so I’ve been using two 9V/175mAh batteries, however when I set up the circuit and add a switch in the closed position, even though, the connection is closed the batteries can’t be charged while the switch is off and when the switch is on the batteries power the motor, any ideas to solve this issue? Thank you!
 

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berkeman

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Welcome to the PF. :smile:

The overall goal of the project seems good, but there are so many things wrong with your initial thinking, it's hard to sort them all out.

  • You want to drive a 24V motor with 9+9=18V worth of batteries?
  • What current does the motor draw when driven with its rated DC voltage?
  • You need some conversion circuit between the motor (when it's a generator) and the batteries to act as a charging circuit -- what is your experience designing battery chargers and DC-DC converters?
  • Standard 175mAhr 9V batteries are not a good match for driving an electric stroller (but I'm not confident in recommending an appropriately sized battery for this project, because so far you don't seem to have the background to work with that kind of energy safely)...
 
Welcome to the PF. :smile:

The overall goal of the project seems good, but there are so many things wrong with your initial thinking, it's hard to sort them all out.

  • You want to drive a 24V motor with 9+9=18V worth of batteries?
  • What current does the motor draw when driven with its rated DC voltage?
  • You need some conversion circuit between the motor (when it's a generator) and the batteries to act as a charging circuit -- what is your experience designing battery chargers and DC-DC converters?
  • Standard 175mAhr 9V batteries are not a good match for driving an electric stroller (but I'm not confident in recommending an appropriately sized battery for this project, because so far you don't seem to have the background to work with that kind of energy safely)...
1) yes

2) 0.5 Amps

3) Nothing, this is a senior project for my mechanical engineering field so as for building extensive circuits I’m basically a novice

4) And yes your not wrong, I cant use a 12 volt battery or anything like that anyway, since I have to worry about the weight of the system. I know the capacity is low but I really more worried about the configuration necessary to charge the batteries when the switch is off, I connected 1.2 batteries in series and spun the motor by hand to recharge the batteries directly so I know it’s capable but for that to be accomplished the batteries would have to have a connection to the power and the ground so batteries will be powering the motor as long as the batteries have a connection to ground. So you’ll definitely know more than me concerning all of this, any ideas come to mind?
 

berkeman

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My project is to create a circuit for a stroller that can store the mechanical work into electrical while moving around and with the push of a switch, use the electrically stored energy to assist the movement of the stroller.
You do understand that when the batteries are being recharged, the stroller will be difficult to push, right? Are you familiar with "regenerative braking" used on electric vehicles?
 

russ_watters

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Look into the specs for self propelled golf bag caddies.

Also, are you doing this project alone or with a team? For my senior design project we had multidisciplinary teams.
 
You do understand that when the batteries are being recharged, the stroller will be difficult to push, right? Are you familiar with "regenerative braking" used on electric vehicles?
No sorry I’m not familiar, beside the basic of it; anything I should see to help with this experiment?
 

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Look into the specs for self propelled golf bag caddies.

Also, are you doing this project alone or with a team? For my senior design project we had multidisciplinary teams.
Okay thanks I will, and I have a partner but he’s only good for splitting the price of part, so alone for this experiment
 
Look into the specs for self propelled golf bag caddies.

Also, are you doing this project alone or with a team? For my senior design project we had multidisciplinary teams.
And as for the self propelled golf bag caddie, does it recharge from mechanical input or just a basic one in general?
 

berkeman

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3) Nothing, this is a senior project for my mechanical engineering field so as for building extensive circuits I’m basically a novice
IMO, it will be very difficult for you to complete this prototype (or even a paper design) with the help of an EE student, or at least somebody who understands basic circuits fairly well. It is typical for such ME senior projects to involve electronics like this?
 
IMO, it will be very difficult for you to complete this prototype (or even a paper design) with the help of an EE student, or at least somebody who understands basic circuits fairly well. It is typical for such ME senior projects to involve electronics like this?
No it truly just the senior project professor i have, I’ve heard he failed 70% of his students (from him haha) I think I’m going to use a second motor to power the batteries exclusively, I just need to make sure to protect the batteries from overcharge, which is another project by itself so I’m just gonna set up the experiment and give reason why it won’t work with & add calculations involving energy, power, ill ask if that’s enough.
 
Thank you both for all your help though, I think no matter what I’ll be working on this project more so I’ll definitely be checking out alternative methods of storing the power safely
 

anorlunda

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Design of battery chargers can be very complicated. There are chargers for rechargable batteries that are meant for mobile use and can run on 12V. That may be your best option., push the stroller to generate 12V ##\pm## 4V and use that to power the mobile charger. Hopefully, the charger can work with a range of input voltages.

You also need switches to completely disconnect the batteries from their motor driving function and connect them to the charger when charging.

Some motors can be used as generators, but there are many types. What type of motor do you have?

@berkeman gave you a great clue when he mentioned regenerative braking. What you propose is very similar to that. If you don't have the electronics background, it could take many months or even years to complete the project. Do you have that much time?
 
Design of battery chargers can be very complicated. There are chargers for rechargable batteries that are meant for mobile use and can run on 12V. That may be your best option., push the stroller to generate 12V ##\pm## 4V and use that to power the mobile charger. Hopefully, the charger can work with a range of input voltages.

You also need switches to completely disconnect the batteries from their motor driving function and connect them to the charger when charging.

Some motors can be used as generators, but there are many types. What type of motor do you have?

@berkeman gave you a great clue when he mentioned regenerative braking. What you propose is very similar to that. If you don't have the electronics background, it could take many months or even years to complete the project. Do you have that much time?
Thank you! At this point I’m not going to extend too much beyond what I have but for the future, ill definitely check it out; yeah I was thinking the same with the charging. sequences; i think for my case I’m going to use a second motor with the sole desire to charge the batteries with a resistance to hinder the batteries from starting the second motor, & a switch on both on both the motor path to signify what will be performing next; it’ll be hard to push but at this point, it is what is for the ground floor scenario.

I’m using a brushed motor.

No only the span of a semester, so from February to now haha, I used up a lot of time earlier trying to use capacitors, however without a step up converter, it wouldn’t produce anything reasonable for the project and a bunch of other issues since the increase wasn’t very linear due to the discharging & also the danger; then I moved to rechargeable batteries, but in the end I’ll produce a prototype on a wheelchair to test it but, I’ll try to see if this can be kept more theoretical since it’s pricy
 

Tom.G

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with a resistance to hinder the batteries from starting the second motor
I recommend you use a Diode instead of a resistor, or at least in addition to a resistor. If the diode is wired in the correct direction, it will block the battery voltage from the motor but allow the generated voltage to reach the battery.

A resistor, if used, will decrease the current flow from the generator (second motor) to the battery, making the whole thing easier to push.

Cheers,
Tom
 

russ_watters

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And as for the self propelled golf bag caddie, does it recharge from mechanical input or just a basic one in general?
Just basic, but it will be a good gut check on battery and motor sizing.
 
Oh right, because V=IR is so simple yet I never use it when I actually need it haha, thank you for that diode tip @Tom.G it really helped a lot. Thank you as well @russ_watters thats very helpful for the next stage of the project. Thank you ! I ended up getting it to work, I’ll end up using a motor on both the wheels, so one motor will be used to charge the batteries when the bottom switch is off and the top switch is on & then the system will be powered by the batteries when the bottom switch is on and the top switch is off; circuit down below. I’m waiting for my partner to 3D print some pieces so that we can have it fit on the wheel comfortably; so I’ll announce if it succeeds but I tested the “charging only” part of the prototype circuit I have with two D rechargeable batteries and it performed perfectly so I’m hopeful
567F3371-9286-4B58-803D-4146F75D1D96.jpeg
 
I drew the one of the diodes the wrong way on the wrong way but you get what I mean haha
 

Tom.G

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I drew the one of the diodes the wrong way on the wrong way but you get what I mean haha
A few questions.
  • Could you mark the battery polarity? That will help us sort out what the design assumptions are.
  • The "Inverter" is shown in series with the drive motor. That is not likely to work well. Did you want it across the batteries?
  • Which Diode has the wrong polarity? Actually, I think there is an extra one!

Cheers,
Tom
 
A few questions.
  • Could you mark the battery polarity? That will help us sort out what the design assumptions are.
  • The "Inverter" is shown in series with the drive motor. That is not likely to work well. Did you want it across the batteries?
  • Which Diode has the wrong polarity? Actually, I think there is an extra one!
Cheers,
Tom
Hello, @Tom.G
I added some labels
B2FAC530-CE89-4299-B88B-D13EAB58ACF3.jpeg

And the inverter won’t work there? So should the inverter be in parallel with the batteries, like this?
CF7ECEB7-EF38-42EB-A9E9-9E4D765C28F1.jpeg

if that’s the case, I would probably need to add another switch at the inverter so it doesn’t take away from the battery charging process

And yeah the diode labeled flip can actually be taken out entirely, since it won’t change anything
 

Tom.G

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And the inverter won’t work there? So should the inverter be in parallel with the batteries, like this?
Yes.
if that’s the case, I would probably need to add another switch at the inverter so it doesn’t take away from the battery charging process
Maybe. From a total energy usage standpoint during charging it is better to use the energy directly rather than storing it in a battery. This is because there are losses in both charging and discharging the batteries. So putting the energy in the batteries just to use it later for something (charging a phone) that can be done at any time wastes a little bit of energy.

However a switch for the inverter is a good idea because it won't be needed all the time... and you don't want to drain the batteries if no one is charging their phone!
And yeah the diode labeled flip can actually be taken out entirely, since it won’t change anything.
It is actually the other diode I'm thinking is not needed. The diode labelled "flip" may or may not need to be flipped! That depends on whether both motors are mounted on the the same side of the vehicle or opposite sides. Can you see why flipping depends on where the motors are mounted? Or maybe I'm missing something. Can you explain the need for that lower diode?

from my earlier post:
If the diode is wired in the correct direction, it will block the battery voltage from the motor but allow the generated voltage to reach the battery.
Cheers,
Tom
 
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@Toeman123: I don't think an inverter does what you expect it to do.

In your circuit nothing can charge the batteries.
 
The motors will be on opposite sides of one another on the stroller

And relooking at the circuit, to be honest the diodes won’t be necessary, since the switch’s cut off power already, making the issue of the batteries powering the motors involuntarily while charging irrelevant; I believe I only added them when I was trying to determine a circuit with only one motor, that could perform the charging system and also be powered by the batteries

And hmm in this case wouldn’t the first motor (next to the flywheel) not add to anything since the switch will be off when the charging operation is happening, but the diode on the charging section, if flipped can only allow the motor to provide electrical energy to the battery and not vice versa so I understand what you mean there, however I am confused on the side that the motors are placed on can change what the diodes position is, my assumption is that when the motor is charging the batteries, the motor next to the fly wheel is not providing any assistance since the second switch is off, so I would think the side wouldn’t matter since they are performing separate operations right?
 
@Toeman123: I don't think an inverter does what you expect it to do.

In your circuit nothing can charge the batteries.
What do you mean? The batteries charge when the top switch is closed/ bottom switch is open and then the battery powers the system when the switch on the bottom is closed and the top switch is open.

And I only need the inverter to show my professor that the energy stored can be used to power a phone charger and something around that
 
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The diodes prevent any charging current. In both parts you can only discharge the batteries and drive the motors.

Edit: And it looks like the motors would prefer the opposite polarity.
 

Tom.G

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The motors will be on opposite sides of one another on the stroller
however I am confused on the side that the motors are placed on can change what the diodes position is,
Starting from behind the stroller, move to the right side of it. If the stroller were to move forward the wheels would turn clockwise.

Now move to the left side of the stroller. Which direction do the wheels turn if the stroller moves forward?

The rotation direction a DC motor depends on the polarity of the applied voltage... and the polarity of a DC generator changes depending on the direction of rotation.

As for the upper diode being useful, yes it will work without it. However the usability of the stroller will be enhanced with it. I highly recommend that you have a diode on hand to try in the circuit to see any effects for yourself.
 

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