Using a generator to charge 2 batteries

[Toeman123]

Now the diode and the resistor just do nothing because there is a cable.

Here is a simple circuit. The thin arrows indicate the direction of current flow in the two modes. You can replace the two switches by a single three-way switch if you always want to be in charging or discharging mode.

View attachment 242629

@mfb hello, just wondering, would there be an issue of the batteries powering the the motor immediately when the switch is closed on the both the charging and discharging sections?

Thank you!

 

[gneill]

My project is to create a circuit for a stroller

What happens to the polarity of the voltage provided by a motor acting as a generator if the stroller is pulled backwards?

Just a thought that occurred to me when I perused the thread.

 

[Toeman123]

What happens to the polarity of the voltage provided by a motor acting as a generator if the stroller is pulled backwards?

Just a thought that occurred to me when I perused the thread.

Well, all I know is that the voltage on the multimeter will read negative, so the voltage will display the same value but current doesn’t flow through, atleast from my perspective with this project, so I’m sure someone else can explain it better

 

[sophiecentaur]

I just didn’t find any 12 volt rechargeable batteries that was anything but a car battery.

When I look on Amazon and elsewhere, there are countless adverts like this one. 7Ah is just an example. I searched on "Small 12V batteries". Hardly an advanced search tactic.

the energy is 5,670 J

What would that energy achieve - e.g. how many metres would it lift a 40kg load (child plus stroller)? The actual numbers in an exercise like this are very very relevant. The other points in this thread are also important but the actual Energy storage is crucial.

No it truly just the senior project professor i have, I’ve heard he failed 70% of his students

This guy seems to have high expectation of you if you are not at a reasonable level of EE knowledge, which could be looked upon as unfair. But he can certainly criticize you if you don't consider how much Energy is needed and how much is stored.

 

[Toeman123]

When I look on Amazon and elsewhere, there are countless adverts on like this one. 7Ah is just an example. I searched on "Small 12V batteries". Hardly an advanced search tactic.What would that energy achieve - e.g. how many metres would it lift a 40kg load (child plus stroller)? The actual numbers in an exercise like this are very very relevant. The other points in this thread are also important but the actual Energy storage is crucial.This guy seems to have high expectation of you if you are not at a reasonable level of EE knowledge, which could be looked upon as unfair. But he can certainly criticize you if you don't consider how much Energy is needed and how much is stored.

Haha I can show you my search results, the only affordable batteries are the car batteries & most didn’t have prime so it would’ve been a waste of time, I don’t live in the uk but that would definitely be an improvement from 175 mAh

& I will be adding calculations to this, like power, how much energy to fully charge the batteries, the amount of output voltage from the motor, rate that it charges with specific rpm

& I see what you mean, but for the most part, I would need the actual system for find the energy needed/the energy stored, so that why I’m finalizing the circuit to get those values, unless you mean, I should be able to calculate these with the theory alone?

Thank you by the way!

 

[sophiecentaur]

unless you mean, I should be able to calculate these with the theory alone?

Theory has to be based on something. Some initial experiments would give you an idea of the actual energy needed to take a stroller out on the road and to make a trip to the shops, up (and down) a local hill with various road surfaces etc..That would involve a Force meter and a baby-sized load in a real stroller. [Edit: I mean without a motor or generator - just pulling and pushing it.]
You would need to find out how much energy is actually available from going down hill; it is not necessarily obvious that the extra (weight) load of the system will make the installation worth while. OR at least indicate the sort of conditions where it would be worth while; if the stroller were to be used on a very flat route, the inefficiency of the system would mean it would be better to do without it, for instance whereas, if there were a very steep hill, the help from a motor could be very useful.
Have you researched the existing systems that are used for electric bicycles and scooters, for instance?

 

[Toeman123]

Theory has to be based on something. Some initial experiments would give you an idea of the actual energy needed to take a stroller out on the road and to make a trip to the shops, up (and down) a local hill with various road surfaces etc..That would involve a Force meter and a baby-sized load in a real stroller. [Edit: I mean without a motor or generator - just pulling and pushing it.]
You would need to find out how much energy is actually available from going down hill; it is not necessarily obvious that the extra (weight) load of the system will make the installation worth while. OR at least indicate the sort of conditions where it would be worth while; if the stroller were to be used on a very flat route, the inefficiency of the system would mean it would be better to do without it, for instance whereas, if there were a very steep hill, the help from a motor could be very useful.
Have you researched the existing systems that are used for electric bicycles and scooters, for instance?

Yeah I see what you mean, I’ll find out if the school has a force meter, I doubt it but that would help; the system is only to assist, & not to completely drive it on it’s own, so as long as the motors shafts are rotating on the wheels, from the design of the circuit, the experiment should be fine, since if I did have a force-meter, is be able to calculate the force with the batteries and without, going uphill.

And not much (for electric bikes/scooters) since the charging portion is always separate & I need to make a circuit that can charge and then with a flip of a switch use the stored power, I’ve been focused on the circuit since I’m not sure the exact configuration to accomplish all he requires.

 

[sophiecentaur]

I’ll find out if the school has a force meter,

By "Force Meter" I mean spring balance (old term for it). They are used throughout a school Science course and have ranges from 10N, 50N and higher. They are cheap as chips, too

if I did have a force-meter, is be able to calculate the force with the batteries and without, going uphill.

You could measure the forces, rather than calculate it. You really have to base this on actual requirement and not something snatched out of the air.

Something that I have not found in this thread and that is the need for some sort of speed control. That could be lossy unless you use a switch mode circuit which doesn't waste power in a dropper resistor.
Edit
BTW, for this thing to be useful and not to give the passenger a hard time, there needs to be a smooth transition and proportional control between charging and motor assist.

 

[Toeman123]

Oh a spring balance, what would be the appropriate way to measure the Nm on a spring balance?

And if the purpose is to be smooth, wouldn’t having two switches accomplish that? So when you turn off the top switch, the charging operation is fully complete, and nothing is driving the stroller except you own energy; until you turn on the bottom switch, when the batteries power the motor?

Thank you!

 

[sophiecentaur]

Force times distance is work.

 

[sophiecentaur]

. . . . or, also very relevant, Force times speed is Power. With speed control, the user could choose to go up a hill slowly or fast. Ask a user of a stroller for what would be required. Observe / measure user habits for an idea about typical speeds. (Watch people in the high street - not in a creepy manner ). All this prelim work has just as important a place as the circuit details and it should actually drive the choice of motor power, battery capacity etc. It is the sort of thing that a high scoring student study would require.

So when you turn off the top switch, the charging operation is fully complete,

The switch would need three positions at least. If the user is getting fatigued and the battery has run down, it is necessary to turn the whole system off. A clutch to disconnect the drive mechanically would avoid wasted energy pushing against wheels with the accompanying whine of the free running mechanism (always a drain of power, however efficient, because of the necessary low gear) Alternatively, a bicycle freewheel system could take care of that problem.

 

[Toeman123]

Force times distance is work.

Haha yes yes, I guess when I heard, the description of the measuring device, I isolated it to one type, I can just get the weight of the stroller for one calculation and add the weight of the electronic components for the other calculation, to determine how much energy would be required to go up a hill, just a note, would it be wise to give an angle in the calculations when describing the up hill motion, for example; “going up a 30 degree hill shows that I would 1000J for every meter” & then show for maybe 40, 50, & 60 degrees so that a trend can be created to show the benefit of the system

 

[mfb]

Make sure you consider realistic values, no one goes up a 30 degree slope with a stroller outside of bad textbooks.

Rolling resistance is typically not negligible, but hard to predict - an experiment would help.

 

[Toeman123]

. . . . or, also very relevant, Force times speed is Power. With speed control, the user could choose to go up a hill slowly or fast. Ask a user of a stroller for what would be required. Observe / measure user habits for an idea about typical speeds. (Watch people in the high street - not in a creepy manner ). All this prelim work has just as important a place as the circuit details and it should actually drive the choice of motor power, battery capacity etc. It is the sort of thing that a high scoring student study would require.

The switch would need three positions at least. If the user is getting fatigued and the battery has run down, it is necessary to turn the whole system off. A clutch to disconnect the drive mechanically would avoid wasted energy pushing against wheels with the accompanying whine of the free running mechanism (always a drain of power, however efficient, because of the necessary low gear) Alternatively, a bicycle freewheel system could take care of that problem.

And honestly thank you, your absolutely right, I went straight to the circuit, without a formal idea of the parts I would actually require, not wise; when measuring the speed I how many rpm’s I have in a minute with a stroller going up a hill to get the speed or should it be more focused if there’s a way to convert calorie lost to speed because I’m sure they have an average for that, which can give me an idea

And yes someone else said the same about the three way switch, he just commented actually haha, so basically a safety switch will always be need, understood

 

[Toeman123]

Make sure you consider realistic values, no one goes up a 30 degree slope with a stroller outside of bad textbooks.

Rolling resistance is typically not negligible, but hard to predict - an experiment would help.

You’re not wrong haha, so even for the slope, there should more research on what type of slope we’re dealing with, in this case would a singular slope be more beneficial; the slope the experiment would be taken on, instead of the multiple scenario slopes

And yes friction is usually on the sidelines for for all textbooks, I’ll make sure to create a free body diagram for this section as well, to see how much energy the rotation of the motors will provide & to see how much energy I would need to provide to the stroller while it’s in freewheel compared to when the motor is off

 

[sophiecentaur]

get the weight of the stroller for one calculation and add the weight of the electronic components for the other calculation

Not so much the weight but the actual force needed to pull it (Force meter will work for any force!). Put some potatoes or a paint can in the seat to represent y our electrics. Rolling friction will be a significant factor as will the friction is the gearbox etc. I repeat - it is essential to be able to disconnect the mechanics from the 'pusher'. The sad fact is that enough effort to produce effective charging over a significant time will make the pusher very annoyed - particularly if they are near home and can charge it as soon as they get in the door.

to see how much energy the rotation of the motors will provide & to see how much energy I would need to provide to the stroller while it’s in freewheel compared to when the motor is off

You should take some time to measure absolutely everything you can think of associated with this project. At some stage you will find that things are not going to plan an that information will help you to think your way out. All the retired engineers on this forum will be nodding their heads at this point! (And even the young ones)

the parts I would actually require, not wise; when measuring the speed I how many rpm’s I have in a minute with a stroller going up a hill to get the speed

Yep. Your experimental pushes will tell you the power and the torque needed (Force times wheel radius). Then you will need to decide on a motor AND the appropriate gearing / transmission. It will have to be something that is off the shelf. Perhaps a toothed belt from motor to the axle with two appropriate pulleys. You can try various combinations of pulleys easier than using an actual gearbox (despite it probably being more efficient)
Which brings up the question of whether you can realistically use the same (brush) motor as a generator. An alternator is much more efficient - particularly at low at low turning speeds. Look at what's used in small wind generators - always alternators. So you would also need to couple to the generator. Sharing the same belt may not give the optimum charging as driving.

PS This may not be what you want to hear but electric wheel chair users get on fine with a beefy battery pack and charge it at home. I have a friend who has a range of four or five miles from one charge and chair plus user are much heavier than a stroller plus kid. In the conclusion to your final write up, that option could be mentioned if you want Engineering Credibility.

You could also Google Electric Powered Stroller. Loads of ideas there.

 

[Toeman123]

Make sure you consider realistic values, no one goes up a 30 degree slope with a stroller outside of bad textbooks.

Rolling resistance is typically not negligible, but hard to predict - an experiment would help.

You’re not wrong haha, so even for the slope, there should more research on what type of slope we’re dealing with, in this case would a singular slope be more beneficial; the slope the experiment would be taken on, instead of the multiple scenario slopes

And yes friction is usually on the sidelines for for all textbooks

Not so much the weight but the actual force needed to pull it (Force meter will work for any force!). Put some potatoes or a paint can in the seat to represent y our electrics. Rolling friction will be a significant factor as will the friction is the gearbox etc. I repeat - it is essential to be able to disconnect the mechanics from the 'pusher'. The sad fact is that enough effort to produce effective charging over a significant time will make the pusher very annoyed - particularly if they are near home and can charge it as soon as they get in the door.

You should take some time to measure absolutely everything you can think of associated with this project. At some stage you will find that things are not going to plan an that information will help you to think your way out. All the retired engineers on this forum will be nodding their heads at this point! (And even the young ones)

Yep. Your experimental pushes will tell you the power and the torque needed (Force times wheel radius). Then you will need to decide on a motor AND the appropriate gearing / transmission. It will have to be something that is off the shelf. Perhaps a toothed belt from motor to the axle with two appropriate pulleys. You can try various combinations of pulleys easier than using an actual gearbox (despite it probably being more efficient)
Which brings up the question of whether you can realistically use the same (brush) motor as a generator. An alternator is much more efficient - particularly at low at low turning speeds. Look at what's used in small wind generators - always alternators. So you would also need to couple to the generator. Sharing the same belt may not give the optimum charging as driving.

PS This may not be what you want to hear but electric wheel chair users get on fine with a beefy battery pack and charge it at home. I have a friend who has a range of four or five miles from one charge and chair plus user are much heavier than a stroller plus kid. In the conclusion to your final write up, that option could be mentioned if you want Engineering Credibility.

You could also Google Electric Powered Stroller. Loads of ideas there.

Okay I see, so for example I’d have a sack with some weight to represent the baby, and I’d have a spring balance, but I’m picturing that for a spring balance, that the sack needs to hang off spring balance and be hanging off the stroller to simulate the force, can the force still be calculated when the sack is on the seat? Just set up wise?

And yeah I’m thinking that the motors won’t have enough power to make a significant dent because when I look up the watts to move the tires of a wheelchair it tells me 100W & 20-50V & as for the gearbox/pulley scenario, my partner will be 3D printing out an attachment so the wheels and the motor move together

So from the looks of it, the motor and the battery may need to be larger to produce the amount of torque & your absolutely right about the beefy motors haha, it reality this seems like it would be given to people who willing want to be active but I’m definitely going to look at the motors/batteries they use for strollers
Thank you again!

 

[sophiecentaur]

The potatoes sit in the stroller (in place of the kid). You pull the stroller forward with the meter and that tells you how hard the user has to push and the force needed from the wheels.
A steady 100W is probably more than needed for this task. 100W corresponds to working quite hard. An adult in a wheelchair is a massive load compared with a kid in a stroller.
This is why I suggested doing actual measurements.
Using 3D printing is a good idea if the material is strong enough. Will you use gears or a belt?

 

[Toeman123]

Oh really helpful thank you! I’ll ask the electric and physics department tomorrow since the mechanical department didn’t have one.

And yeah since then I’ve been looking up motors used for electric bikes and the motors that were used only had 12 volts/19 watts so I’m sure after I calculate the forces, it will be more in that range of necessity

In this case gear, just as long as positioning the motor with the flywheel checks out without too many hiccups & that’s true, hopefully it will be but I’ll just add fill if it’s too much of an issue

 

[sophiecentaur]

You may find that positioning the motor and shaft is hard to achieve with gears (spacing and orientation) A belt (or chain) drive would allow for a big range of pulleys and an idler pulley would mean the same belt could be used. Google Images of Belt Drive Transmission for a range of ideas and layouts. Printed pulleys could be stronger than gears, too.

 

[Toeman123]

You may find that positioning the motor and shaft is hard to achieve with gears (spacing and orientation) A belt (or chain) drive would allow for a big range of pulleys and an idler pulley would mean the same belt could be used. Google Images of Belt Drive Transmission for a range of ideas and layouts. Printed pulleys could be stronger than gears, too.

Haha yeah I’ve found out first hand the difficulties, definitely going to use a belt to drive it thank you! if I was to fit a belt on the shaft of the stroller wheel (and if it rotates, I need to check) I think I could make a simple belt with items in the lab; if not I’ll ask about a 3D printed belt, to see how well the 3D printer will manage

 

[sophiecentaur]

if I was to fit a belt on the shaft of the stroller wheel (and if it rotates, I need to check)

OMG yet another problem. All the strollers I've come across (and other such trolleys) have a solid axle and the wheels have individual bearings. This is actually essential to allow the stroller to follow a curved path without the wheels scuffing. Powering the wheels is actually non-trivial and would require extensive re-engineering with a differential drive or perhaps two separate motors. Just powering one wheel would be problematic because it would make the stroller go in circles.
This is beginning to give me sleepless nights.

 

[Toeman123]

OMG yet another problem. All the strollers I've come across (and other such trolleys) have a solid axle and the wheels have individual bearings. This is actually essential to allow the stroller to follow a curved path without the wheels scuffing. Powering the wheels is actually non-trivial and would require extensive re-engineering with a differential drive or perhaps two separate motors. Just powering one wheel would be problematic because it would make the stroller go in circles.
This is beginning to give me sleepless nights.

LOL i know how you feel , you’ve helped me plenty to be honest; thank you for everything! I won’t be offended if you sleep for days to avoid this haha. It took a while but the 3D printed flywheels on the motor shaft fit/move perfectly with the wheels (& yes you were completely right about the shaft), so now all I have to do is build the circuit tomorrow and start measuring the voltage increase of the batteries at an average rpm & it’s seeming like no one carries a spring balance, so I may just end up buying it if the physics department doesn’t have a spare; and I’m most likely going to test the system with the two 9 volt batteries, even if it doesn’t have enough power so I can test the circuit, but after the force necessary to push the wheels is calculated, I’ll buy a new motor

 

[sophiecentaur]

& yes you were completely right about the shaft

So the shaft goes from side to side and rotates? Thats a help. Strollers are made light weight, consistent with strength. The shaft will not be 'fat' and may flex when driven by a belt. It could be a good idea to arrange that the drive is connected on one side, near the shaft bearing to reduce the flexing or you could put a long sleeve around the shaft to relieve the stress from the pulley. Easy to arrange if you are prepared in advance.
Spring balances are available for 'non-Science' use. You can get them in fishing tackle shops and in many stores that sell travel goods (for checking your luggage weight allowance). You are not after good accuracy - just ease of use.

I may be over-engineering this in my mind and your design will be more of a proof of concept model, rather than aimed at production. That makes life somewhat easier as long as you don't miss out the really important bits.