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# Circuit for electric vehicle? Regenerative braking / free-wheeling? Answered

I'm working on a circuit for an electric trike I plan to build for the local EV challenge competition. I plan to incorporate regenerative "free-wheeling" to recharge the batteries on the down hill sections.
S1 closed and S2 open to accelerate the vehicle.
S1 open and S2 closed to recharge the batteries while freewheeling down hill.

Batteries: sealed lead acid batteries 12V or 24V (depending on motor)
Motor: brushed DC motor

Would this kind of circuit work? Is it possible to recharge SLA batteries in such a way?
Any help and/or suggestions is appreciated.

PS. I know it would also be possible to implement regen braking using a full H-bridge, but i think we would be doing more freewheeling than braking on the race track.

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A motor connected directly to a battery will pull current whilst driving, and automatically charge the battery while "undriving" or going down hill. that presumes you don't have a switch disconnecting the battery from the motor.
You can only get current to flow through a motor when a load is presented to the motor shaft. If you "drive" the motor shaft mechanically harder than it wants to go, you reverse the current flow, and the battery starts to charge.
Remember that putting 100watts into a battery only gets about 80 watts stored, and demanding 80 watts from a battery requires around 100 watts.
Charge and discharge always has a cost of about 20%. You only get a portion out from what you put in - The loss is heat on the battery.

I really knows about how does the battery electric vehicles works? please send me full information with schematic diagram and what is the purpose of regenerative braking?

what is the application of the power electronics in battery electric vehicles?

Regenerative braking is specifically the PWM process of pulsing the motor on/off in reverse, the motor is turned on for a pulse - which builds up a magnetic field in the windings, and then put in reverse just long enough for the back-emf to reverse the polarity and stall the motor. The left over energy is then fed back into the battery. Regenerative braking requires a H-bridge circuit - which is clearly not what i have in my diagram.

What I want to know is whether the electricity generated by spinning the motor while freewheeling will be fed back into the battery (S1 open/S2 closed).

Wildly wrong. You need to do some serious research.

I've had some screaming arguments on this site with people who think there is something magical and free about the energy you get from runnning a spinning motor as a generator. There is NOT. You have a HALF bridge circuit. Despite the popular usage, engineers do not recognise "H" bridge. ALL full bridge circuits are configured so they look like an H

aren't those H bridges are to make motor spin in reverse direction? I think op tried to make some electricity using motor as dynamo but needs to make more volts than battery already has (that is why there is only one diode) or use boost converter...

you want to use it like a dynamo, right?

s1 open s2 closed
looks like motor would short, even if not, then diode wont allow current to pass in reverse. either way no current will pass on battery so it wont charge.
will this work?

No, regenerative braking is the process of using the back EMF of the motor to provide braking action - In very simple terms - The back EMF is created because the current produced by the battery when driven as a generator is in opposition to the current required to drive the motor as a motor - This opposition gives the braking effect.

If you take a DC motor and short the brushes to each other it will be almost impossible to turn, and as you try harder the opposition force will increase - try is you only need a small toy motor although the effect is bigger at slow speeds with a bigger motor.

As for your diagram, to recharge the battery you will need to attach the positive output of the "generator" to the positive of the battery - Not what you diagram currently shows.

Thanks for the continued help rickharris

What I really want to know is weather the circuit will work to recharge the battery while freewheeling (I've scrapped the regenerative braking idea).

I had originally planned to use PWM with higher duty cycles as the vehicle reached top speed so that in essence the motor will be powered for a smaller percentage of time than if it were powered continually. But after reading you reply I'm wondering if it wouldn't be better to boost and coast after reaching top speed, any suggestions?

There's no difference between "Freewheeling" and regenerative braking !!! Where does the energy come from to charge the battery ????/

I say again, WEIGHT is your biggest enemy - every gramme you can save will be an advantage to you. After weight drag becomes important if you get over 20 MPH. For every MPH you increase speed the drag will be cubed. Below about20 MPH this isn't as much of a problem that's why you don't see standard bicycles with fairing but you do see recumbent bikes and velomobiles fully faired and doing up to 70MPH down hill.

Look up velomobiles on Google to get some design ideas. Many of these will easily outperform your electric car!! and a human can only provide about 270 - 300 watts

You can't recharge the battery without producing electrical resistance/drag/braking from the motor - work requires energy. In this case your getting the energy from your cars kinetic energy. If it is light as it must be that figure isn't so big and that's why there isn't enough energy to significantly charge a SLA.

In full sized electric cars they have a lot of weight and therfore kenitic energy, they also drive on roads where they need to stop and start a lot so regenerative braking is a little hlepful. Most also use LIPO batteries which will charge faster than SLA.

Looking at the GreenPower website there are some very impressive builds, but i dont think my team is going to make any thing on that kind of level. We are trying to make something for under R5000 (+-350 Euros).

But back to the question: S1 and S2 will be ralays or mosfets controlled by an arduino, will the circuit work? Will it recharge while freewheeling?(S1 open, S2 closed). and what sort of precautions should i take not to fry anything?

Any help's appreciated - I'm not very good on the electrical stuff, more of a software guy.

Your motor alone could cost that. On average the greenpower cars cost around £700 to £1500 to build - The battery set is usually used for 2 years then replaced.

Those cars are designed and built in schools. 11 to 16 years old. Most of the drivers are 11 or 12.

Back to what Steve says - If you drop the motor across the battery directly you will pretty much stall the motor locking the wheels. In addition you will get minimal recharging for the amount of braking your going to use.

You will need to build a control circuit to vary the amount of regenerative current that the battery takes - and be able to adjust it according to needs. Potentially there could be a lot of current flowing. It really isn't worth it. IF you can build a very light car, use a very light driver all you need is a 70 amp circuit breaker - essential, a solenoid switch and a push switch to activate it to be able to pulse the motor on and off as required.

To get the max from the battery you need to do what the Shell marathon gas powered cars do a boost and coast- Power up to a pre set speed then let the car coast down to another preset limit before boosting again. This way you best manage the battery usage -

Trust me, I managed the team through dozens of "races", some of which we won. There is always some team that go to using gears, fancy electronic speed control, fancy data links to check battery condition, radio links to driver etc, BUT the basic simple and LIGHT cars are never far behind them and often in front.

Thanks for the feedback.

So what you are saying is that it would be pointless to implement this circuit, because i wont be getting much gain from it? I know the team that won last year had a regen braking system. Just out of interest would this circuit work?(I'm not sure if i have the diode in the right way), also overheating might be an issue.

Just to clarify the competition rules state that the battery must be SLA, maximum 32V, and have maximum rating of 432Wh. And the winner is the vehicle that does the most laps in 1 hour.

The race our car was built for was over 4 hours with a change of battery during the race. 2 x 70Ah SLA and 500 watt motor - the top cars were doing 120 t 170 miles in the 2 hours.

Light weight, Aerodynamics are pretty key. Our races were on full race tracks - mostly pretty flat but some cars were capable of 40 MPH. No regen braking allowed in fact brakes were rarley used air drag is significant for most cars at 30 MPH.

www.greenpower.co.uk is the URL for the competition - lots of pictures there.

Generally you need electronics to "do" regenerative braking, because there is a lot more to it than slamming the motor against the battery. Do it wrong and the battery can explode, whilst you find your self subjected to very severe braking as your wheels lock up.

1. . With an SLA battery your charging in the down hill will make little difference at all - you need Lipos oe LTFE to gather any serious charge quickly

2. Depending on how fast you think you need to go you will need something like a 500 watt battery to get to 25 - 30 MPH on the flat - This will mean that a 70Ah battery will last roughly 2 hours.

3. trying to recharge the battery will supply much more braking effect than you want in a race. At these moderate speeds you wlll hardly touch the brakes.

4. when disconnected from the battery the motor will free wheel pretty well.

5. 500 watts at 24 volts is approx 20 amps at 12 volts it will be 40 amps - You need to lower amperage so at least 24 volts.

6. the battery and you should be by far the heaviset component in the car.

https://www.instructables.com/id/Building-an-electr...