Selecting a battery pack for a self driving car - Page 1
Forum section: electronics / beginners

author: Nikola Raic
author_rank: Newbie
timestamp: April 04, 2024, 11:27:05 am
content: Hi,I have a 3 kW 3 phase BLDC (24 - 36 V) motor around which I plan to create a self driving car that I got from AliExpress. Besides the motor the battery needs to power raspberry pi 5 and a small steering motor.I'm trying to find a battery that will give me enough power to comfortably power all components and leave some room for additional stuff down the line.I was looking at 36 V LiPo batteries, Im a bit concerned about motors wire thickness (16 awg) so I want to have as low as possible current going to it.Main question is the 20 A continuous current discharge rating of the battery pack, will that be enough to have some headroom for design changes if needed?How much power should i expect the motor to draw during use?Edit: Its a rc car format, expected to weight at approximately 20 kg, and to have it run on paved flat ground.

author: Ice-Tea
author_rank: Super Contributor
timestamp: April 04, 2024, 11:49:21 am
content: Quote from: Nikola Raic on April 04, 2024, 11:27:05 amQuote

author: Zero999
author_rank: Super Contributor
timestamp: April 04, 2024, 11:52:52 am
content: P = V*IP = 3000WV = 36VI = P/V = 3000/36 = 831/3A

author: Nikola Raic
author_rank: Newbie
timestamp: April 04, 2024, 12:04:02 pm
content: What can I expect the power draw to be in normal use. I dont plan on running the motor at 100% all the time, if ever.

author: Zero999
author_rank: Super Contributor
timestamp: April 04, 2024, 12:52:39 pm
content: Quote from: Nikola Raic on April 04, 2024, 12:04:02 pmHow are we supposed to even guess, when you haven't provided enough information?

author: Nikola Raic
author_rank: Newbie
timestamp: April 04, 2024, 01:14:24 pm
content: What information is needed to make an estimate of the power draw of the motor in the system?I'm still in requirement gathering stage of the project so I can't measure it.

author: BrokenYugo
author_rank: Super Contributor
timestamp: April 04, 2024, 01:24:50 pm
content: "Car" usually implies a 4 wheel vehicle with an enclosed cabin seating 1 or more adults and use in traffic on paved roads, I would think even a little ultralight single seat car in city traffic will be running 100% a lot of the time with a 3kW motor.Smaller gauge wires are often good enough for shorter distances, actual ampacity of wire is often higher than rules of thumb or electrical codes written around insulation rated for 50C indicate. To calculate the actual current a wire can handle one has to take into the account the insulation temperature rating, ambient temp, wire length, free air vs confined (how well the wire can dissipate heat), and probably a couple other variables I'm forgetting.

author: CaptDon
author_rank: Super Contributor
timestamp: April 04, 2024, 01:34:12 pm
content: I suppose it would be a fractional dirivative of the full load current? What about the load current at break-away (starting to move)? Would it be 3 to 5 times the full load current? A 3KW motor is what....5 horsepower maybe? On a self driving car? Must be a real small car, I think the King Midget had a 10Hp gas engine. I can't think of any production 'cars' with less than perhaps 18Hp and that includes all of the craptastic cars we had on Iceland!! I am thinking of Trabants, LADAs, and 2CVs. You have nice ambitions, that's great, but appear to have little understanding of electricity / electronics. That could be your biggest problem.

author: Nikola Raic
author_rank: Newbie
timestamp: April 04, 2024, 01:38:17 pm
content: Sorry it was a translation error. Its size of bigger rc car, but with no rc. I guesstimate it'll weigh at around 20 kg.Plan is to have it run on mostly paved flat ground.Apologies for confusion

author: CaptDon
author_rank: Super Contributor
timestamp: April 04, 2024, 01:41:52 pm
content: A 24 to 36 volt motor wound with #16 AWG wire giving 3KW of OUTPUT power??? Did they rate that as BHP Brake Horse Power or was that rating similar to the 9900MAH 18650 cells? In which case it would be Bullshit Horse Power. Maybe 3KW in 2KW out at the shaft + or - the smoke output from the overheated windings?

author: tszaboo
author_rank: Super Contributor
timestamp: April 04, 2024, 01:43:47 pm
content: Quote from: Zero999 on April 04, 2024, 11:52:52 amOn 16 AWG wires going to the motor, it will be about a minute before the isolation turns into smoke.It's probably a e-bike motor. They are undersized for cooling, since you can get them up to speed in seconds, and from humans you cannot really expect more than 3-400W so even 1KW feels like a lot.Quote from: Nikola Raic on April 04, 2024, 01:38:17 pmYeah, since would will be using some sort of ESC see what sort of settings you can set up on it is software. You can limit the input or output current or power in the ESC, and size the battery pack based on that. I doubt you will need 3KW, probably 2-300W will provide plenty of acceleration and speed.As I said, cyclist will use 2-300W peak power, to drive a 80KG person, and go 30-40Km/h easily.

author: Zero999
author_rank: Super Contributor
timestamp: April 04, 2024, 01:45:01 pm
content: Quote from: Nikola Raic on April 04, 2024, 01:38:17 pmIt depends on the aerodynamics and the required acceleration and top speed.

author: CaptDon
author_rank: Super Contributor
timestamp: April 04, 2024, 01:47:05 pm
content: O.K., a medium scale R.C. car, that makes more sense. Probably only need about 1 shaft horsepower even at fast speeds which would require the electrical energy input of around 1.4 to 1.5 KW. Do the ohms law power calculation. You know the estimated power required and the available voltage, do the math.

author: CaptDon
author_rank: Super Contributor
timestamp: April 04, 2024, 01:53:25 pm
content: You may want your control circuits power supply to be isolated through a diode from the traction battery and have a capacitor energy smoothing filter on your controls power supply. The noise on the traction battery could be huge and the voltage swings on the battery could also be large. Will your BLDC control package have regenerative braking? How would you stop the car from drifting if you want to stop on a hill?