The voltage limits on motors for the most part I think don’t matter they are after all just coils of wire inside a metal case with magnets in there. So if you know the resistance in Ohms of the wire and you know the voltage you can calculate the current, the BLDC ESCs are opening/closing the gates pretty much constantly though so the current is only happening when gates are open (also percent of gate open time essentially creates an average voltage lower than the battery incoming voltage).
As current flows heat builds up if no air is flowing to take away the heat it just collects until enamel coating on wires or other things start to melt and then you have shorts and real issues with the current spiking and more things melting at that point. Voltage is the ‘pressure’ and will at high enough voltage jump across small air gaps (but talking thousands of volts for a cm air gap) but through a component it could be far less to make the jump/short out.
I’m actually looking at a new battery solution for my next board after my trusty old DIY Board finally went up in flames towards the end of charging the other day.
It was a 4 wheel drive board with 2 electrically independent home made, 12S3P battery packs with no BMS. One 12S3P for front wheels and one for rear wheels.)
For this next board, I think I’m going to go with pre-fabricated Packs with BMS.
But I am trying to figure out if I can keep the board a little lighter than it was before with 2 Packs of 12S3P, or keep the power similar from a lower cost vendor on Alibaba or something like that. The best they seem to have are 30Amp nominal with 60Amp surge. But I was using Sony VTC5A before and I think they had quite a bit more amperage than that, especially with no BMS…
heh okay gotcha just good to have some context sometime people just want to maximize numbers but don’t know what the big numbers mean good to hear you have some experience even if not 100% positive
I went with Lipo setup on my first deck (to keep it cheap) and that one is still rocking and pretty happy with it, took me like 2 sets of lipos to find ones that are a good compromise on size and max discharge but anyway that one is good for budget friendliness (I take apart two 5S batteries that are put in series to make it 10S and charge them separately or in parallel with a hobby balance charger)
More recently grabbed one of the batteries from metro-boards, I haven’t exactly stress tested them but will go take a quick ride here and post some datas for Science!
^^ maybe worth a look/comparison too, it has some sort of balance circuitry built in from what they tell me but couldn’t get details on the BMS and they sell a power supply for charging them (about $50-60 for that but you need it as far as I can tell)
Nice, sounds familiar. I saw some good looking LiPo Batteries and was thinking of trying those. They were specified as delivering great Maximum discharge Amperage, but had low Milliamp Hour ratings. Was thinking this would be OK since my rides are typically brief. I’m thinking i’m going to try Alibaba for the first time, will post some info if anything notable occurs…
Will check back in days and weeks ahead and hope to see some info about your new Metro Boards Battery performance!
Thanks will definitely get out and try my hexacopter to test out the GPS I’m sure that will be memorable
Would focus in on the dark blue line that is voltage there and the yellowish line that is esc temp. I was just going a few blocks here 1/4 mile basically with couple intersections in between. Can see how much the voltage is dipping as I ask for more current, can also use the arrow on the graph to turn on ppm/pwm display to show the control input and corresponding amperage over time.
Something that comes up repeatedly that I don’t really get is how motor amps can be greater than battery amps. Surely if I limit my battery to 60A, the motor cannot draw more than 60A no matter what. I’m not sure how this is implemented in the VESC but I would imagine it would be like a regular constant-current supply that just uses PWM to create an average current flow.
The only thing I can think is that maybe motor amps is a measure of the instantaneous current flow during a period of “on” time? But that would make no sense since a) we only care about average current flow through the motors, not current flow during an “on” period, and b) the current flow during an “on” period would be a natural result of voltage and motor resistance; the duty cycle is the control.
In my mind the only use for a battery amps limit is if you’re driving two motors on a single-MCU dual-output VESC. Otherwise battery amps and motor amps have identical function and only the lower of the two would be taken into account.
Essentially the motors are only drawing that current for half the time, so effectively the battery only sends half of that current to the VESC and the VESC sends 2x that current half the time to the motors.
Honestly that graph did nothing but increase my confusion lol. My problem is that it shouldn’t be possible for motor amps to be higher than battery amps (where is the extra current coming from??)… but then I saw this explanation by @MysticalDork:
In this way, the ESC’s switching, combined with the inductance of the motor winding, acts as a buck converter to drop the battery voltage down, while at the same time increasing the current to the motor.
This is how you can have 100A flowing through the motor, but only be pulling 12.5A (P=I*V, 100A * 5v = 500W, and 500W / 40v = 12.5A) from the battery.
So the motor becomes a buck converter. That explains everything. Not really sure why that’s not the first thing people say when they explain this; would make it a lot easier to understand.
Yeah but on its own that doesn’t explain it. For instance, if I have an LED running at 50% duty cycle, it will be dimmer than if it were running at 100% duty cycle. The LED will never have more current flowing through it than the battery can supply.
Intuitively, if I run the motor at 100% duty cycle, it will have no more than the maximum battery current flowing through it (because the VESC would actually reduce the duty cycle to maintain the battery current). Then the faster the motor spins, the less current would flow because the less the effective phase voltage becomes due to back-EMF. This is what I was confused about.
That seems like an awkward way to think about it. The battery only supplies variable amps because the ESC varies the duty cycle to the load (the motor). The ESC can’t control the amps coming out of the battery except by changing the load duty cycle.
Not by itself though, surely? It would rely on the motor’s inductance right? I’m far from an expert on switched-mode power supplies so maybe I don’t know what I’m talking about.
Current flows through the battery only half the time, while current flows through the LED 100% of the time regardless of duty cycle, so the average current through the battery is lower.
At this point people should just give up trying to understand how it works and just remember these two concepts
Motor Current = Maximum torque - how fast you’ll accelerate Battery Current = Maximum power - maximum speed/climbing rate
The average current through the battery and through the LED would be the same, and the current would only flow through the LED 100% of the time if you have some sort of filter circuit on the switching. Am I not right? This is assuming a simple DC circuit with a single switching transistor.
This is not how inverters work. The load is inductive and there is either a catch diode or synchronous complimentary transistor. This is also not how 99% of LED drive circuits work either. They use a buck converter to deliver a continuous current.
In the case of something resistive, like an LED, during the on time the LED sees rated current, the value of this current is the analog of “motor current” in this example. During the off time the led sees no current. The battery sees half the rated current. Inductive loads do not behave this way. Interestingly enough most LED circuits use a buck converter to deliver a constant current to the LED because the forward voltage never matches the supply voltage. Current through an LED determines brightness. Current through a motor winding determines torque. LED forward voltage determines power output. Motor BEMF voltage determines power output. LED forward voltage is always lower than supply voltage. Motor BEMF voltage is always lower than supply voltage. Supply voltage divided by power output is supply current. LEDs are driven by a buck converter. Motors are driven by a buck converter.
Current cannot change by large values during a switching cycle due to the inductance.
Draw the current paths during different parts of the switching cycle for an inductive load.
There’s got to be a good water analogy illustration or video about how voltage/current/duty cycle in magic boxes works. I think I get it, but it still feels like a mind bender to try to visualize the relationships between the battery side and motor side. I do also want to understand it better though.
really eloquently explained.
