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Hybrid 2 stroke RC car lithium battery/power management system design questions

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jpoopdog

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Hi,

Im going to build a rather large RC car using a specific 2 stroke engine i have, the purpose of the project is to make a vehicle using this engine but also implement various features you might see in actual automobiles, its unavoidable in case anyone wanted to say i should just do 100% electric.

Anyway, when i say hybrid, i mean that at times it may get electronic assistance, or, just operate entirely with the electric motor. an example might be, and bearing in mind certain aspects of the design are still rather fluid and open to change, the ability to reverse may be entirely electric, depending on wether or not i can implement more than 2 gears (reverse would be prioritized after having a high/low).


While having two lithium batteries in the car, one for controlling the on board electronics (1S battery on boost converter) and the other just a big bank for motors is doable, id rather everything just operate with one.

What i want is for the battery to be charged ( can manage to make a circuit that will manage safe charging), and balanced which should be doable with one of these https://www.aliexpress.com/item/Aih...ery-Protection-Circuit-Board/32831693152.html , but i dont think its deisgned to be charged and discharged at the same time (im not fussed about balancing, there will be plenty of time for it to balance.

the problem i have, which a second battery is for in the first place, is i have no idea if i can make a power supply that runs off an alternator, that is responsive and reliable enough to always maintain power like how a real car can.


Id like some suggestions (besides implementing a lead acid battery instead of the li-ion 4s pack i plan to use) on how to manage the power. I dont need to fast charge but the battery will be installed permanently, so everything needs to be internal and only charged by the car.
Ideally id like to make a lithium equivalent to a lead acid battery, in that it simply has a postive and negative line and can charge and discharge though it, like a lead acid would. so, it has a charge manager, balancer, and a constant power switcher which will allow instantaneous changing from charging to discharging, for use such as when the charging source is not especially stable.

Most likely ill be constructing my own alternator so i have no idea how well i can manage the power. given the microocntrollers used, a cut in power could make the car partially unresponsive for 10-20 seconds.


Also, i have other reasons to learn how to make a power supply like this, offering constant power supply where the charging current may be suddenly turned off without warning, like in a laptop.
 

dknguyen

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Can you use A123 or LiFe instead of lipos? Safer failure modes.

Simultaneous charge and discharge of the battery will only appear as one or the other as the difference between the two (aka charger could be supplying the direct power to the loads bypassing the battery), whichever is greatest at the time. I wouldnt worry about that too much tbh.
 
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dknguyen

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the problem i have, which a second battery is for in the first place, is i have no idea if i can make a power supply that runs off an alternator, that is responsive and reliable enough to always maintain power like how a real car can.
You can. Dead easy. Hook up an outrunner as a generator to a three phase diode rectifier and smoothing cap. Add a smps regulator. Done. Its used all the time on model gasoline helicopters to power the onboard radio.

Add a small nimh buffer if you're paranoid but my understanding is that the rc helis omit the battery...and they need the power to stay on a helluva lot more than your car does.

If you size your outunner to always overvoltage for your idle rpm, you can step down using a hystertic regulator which is simple, inherently stable, and fast. Otherwise you will need a SEPIC so you can step up or down as required.
 
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jpoopdog

Member
if i cant charge and discharge at the same time thats fine, i just need to to switch between charging and discharging instantaneously as needed, and if possible, utilize the alternator as much as possible for anything it can handle. The charging circuit is actually relatively low current so, having it always on and just disconnecting the battery when its not needed wont work, ill want to be able to actively and quickly switch between the alternator (via smps regulator as you said) and the battery.

Also no, i have no intention of changing batteries. I dont have LiPo's though, they are a 34C 4S li-ion pack. Im only using them because i have them, otherwise id try to make do with a 6-10AH SLA since this whole process would be much more simple that way.
 
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ClydeCrashKop

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Typically in a car, you may have the headlights and heater using battery voltage. The alternator is keeping the battery charged up.

If you consider that charging and discharging at the same time, then, yes you can.
 

camerart

Active Member
Hi,
I was thinking about something like this, and designed a generator, just for constant battery charging, so no flywheel or output shaft.

I thought it would be running and charging the battery all of the time that the electric car is out, so it's size would be designed around an average distance needed.

Here is an animation to illustrate the idea:

I hope it's of interest.

Camerart.
 

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large_ghostman

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The outrunner is a good idea, Pommie did some calcs for a drone using a 4stroke gas engine to power the electric motors, its something I am also looking into. They seem the best small option and power to weight solution.

@
dknguyen
Thanks thats useful info for the drone project, we will be pulling alot more power than the helicopters need. But knowing its common for something else gives me more avenues to explore.
 

dknguyen

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Most Helpful Member
The outrunner is a good idea, Pommie did some calcs for a drone using a 4stroke gas engine to power the electric motors, its something I am also looking into. They seem the best small option and power to weight solution.

@
dknguyen
Thanks thats useful info for the drone project, we will be pulling alot more power than the helicopters need. But knowing its common for something else gives me more avenues to explore.
Those gasoline helis fly for so long that the battery for the electronics runs out before the fuel does and those guys don't want to have to land to replace something so frivolous.
 

large_ghostman

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Most Helpful Member
Those gasoline helis fly for so long that the battery for the electronics runs out before the fuel does and those guys don't want to have to land to replace something so frivolous.
The downside for me is the 30-50% nitro in the fuel, we cant get it (actually i can but i am rare), its real expensive however. I wasnt aware the serious heli guys used what is basically a alternator, for my needs it has to pump out alot of juice but as pommie was saying you get a problem with phase timing for what we want. I am pretty sure the heli guys will have come up against this and might have some good ideas to look at.

Drones and planes I fly or rather flew for a while, all hand built before they were off the shelf, but helicopters i just never fancied so didnt know much about them. I am pretty sure they run flat out and use high Nitro, apart from that i know zero on them. Anyway thanks again thats a useful avenue to explore.
 
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