Battery amps vs motor amps vs voltage

How-Tos & Esk8 mechanics Esk8 Mechanics
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Moar S count Moar top speed AND Moar torque!

So. I would hear this and be one of the people who thought this is BS if you just add more voltage you only get more rpm and you can trade that rpm by gearing down and getting more torque. so

more S count is more speed OR more torque.

Only I think i realized I’ve been wrong on this all this time. because torque is actually from motor amps and not battery amps and with a higher voltage more motor amps can be delivered per rpm… so you really do get

more speed AND more amps

here’s my take on the traditional max amps graph. but factoring in rpm at 2 different battery voltages.

Blue is 10s. Red is 12s.
Solid is motor amps.
Dotted is battery amps.

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Does that seem right? [ EDIT it does not. see below ]

Graphed on desmos you can play with the numbers there.

Thanks

@MysticalDork I finally get the curve now. it’s a 1/x style. :smiley:

thanks @JoeyZ5 for that initial graph. :slight_smile:

thanks @novamuto for the convo that led me to think about this.

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I’m ready for @MysticalDork to science us :eyes::penguin:

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There’s some information missing IMO:

Battery watts. Basically, what’s the product of your battery voltage multiplied by your maximum battery current.

This is needed, because some people will compare a 10s4p to a 12s4p (20% more cells, = 20% more power available) and see an overall increase in performance, whereas if someone takes apart a 10s6p and rearranges it into a 12s5p (same number of cells), the overall performance will not change as much, because the total wattage is the same.

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hah. I think i just came to the same conclusion … that I’m wrong about being wrong.

in the graph I did above. it assumed 12s vs 10s. with the same battery amps (35) setting. which would imply the same P groups … which means the 12s pack has more batteries and more watts available.

but… if I do the 10s6p vs 12s5p. same cells. then battery amps on the 12s has to be 5/6th of the 10s

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and then magically… the new green line (12s at 29 battery amps) lands on top of the blue 10s line. and there’s only more top speed.

so… I’m wrong about being wrong and was right originally. :smiley:

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Has been some discussion on this topic here

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You thought you were wrong, but it turns out you were mistaken. :stuck_out_tongue:

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I also had a discussion about this topic today. My argument was to Always use the maximum voltage that your ESC can support safely because higher voltage in the battery results in less current through your battery cables and BMS. -> Less power lost to heat.
If a 9s 4p would be enough for your desired speed and range. I would suggest a 12s3p and a software speed limit instead. Same amount of cells but lower current battery side.
Am i correct in my assumption that higher voltage would be equal or better in this scenario?

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I think you would need to gear down to same desired top speed and not software speed limit. otherwise you throw away torque the 4p could provide that the 3p does not.

otherwise. I agree.

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my understanding is that even if you don’t change the gearing (or cant change it) there still should not be a difference because torque is only limited by motor amps and battery power.
Motor amps is limited by your controller so it stays the same and
Battery power also stays the same because you have the same amount of cells.
Assuming you pull 20A from one cell:
12s3p: 60A x 44,4V = 2664W
9s4p: 80A x 33,3V = 2664W
so with identical max motor amps and identical battery powere were is the difference ?

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9s4p does not provide more torque than 12s3p

The ESC basically acts a buck converter

@Blise518B is correct

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I guess that’s actually what my green line above is saying.

I’m having trouble wrapping my head around that. that the same number of cells in different configuration can give more top speed but the same torque at any RPM.

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Now the question that’s left is:
Does the higher voltage difference of the 12s decrease the efficiency compared to the 9s or does the lower battery current make the 12s more efficient?

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Power loss is I SQUARED R

Resistance increases linearly (more cells in series), current decreases linearly (less current for same amount of power)

So overall the result is HIGHER efficiency with more cells in series.

That’s literally the whole point of using an ESC, it allows you to move ANYWHERE along the motor’s performance curves.

Always use the highest voltage your electronics safely allow.

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Thanks, I learned more than I thought i’d learn from today’s exercise.

updated desmos graph to allow comparing two battery currents along with two voltages.

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I may have written my question a little bit strange …
you said

buck converters decrease in efficiency if the voltage difference gets higher. In case that’s also true for our ESC a higher voltage battery without ever using it (speed limit never reaches full duty cycle) should also lower efficiency again. I don’t know if the efficiency gets worse with a bigger voltage difference so that’s what the question was meant to ask.

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Yes you are correct, the switching losses are higher, but at these power levels and configurations, it’s not that much of a difference.

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I haven’t been able to stop thinking about this. pretty excited to understand it better.

I did another version of the power curve comparator on desmos. parameterized it in terms of NsMp cell counts. and added a “gearing” component.

In the default comparison, 10s6p (blue) and 12s5p (red) the power curves are the same. Red has more top speed but the torque/amps continues to drop as the rpm/volts (dutycycle * volts) goes down. keeping max power the same. (!!!)

bingo light going offf for me. :slight_smile:

That power curve staying the same is why:

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Now,

if I were to gear it down to the same top speed. I do gain torque. but only where motor amps previously limited blue. and only where it stays under the max power curve. ( red is now virtual amps/torque relative to blue. but really it’s gearing )

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so higher S count’s extra speed can be traded for more torque but only where previously limited by motor amp max. it can’t exceed max power. This is the tradeoff I was thinking of. but now I can also see the max power curve and how it limits the effect of that trade off.

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But more series and less parallel would be more battery sags on startups and then quicker decrease in capacity/voltage/longevity, no ?

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Good point!
10S6P at 90A and 12S5P at 75A (in above example) result in the same 15A/cell current though.

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But the graphics above represent a “perfect” discharge scenario,
When you start powering an electric motor, the sag is 5 or ten time (?) The nominal amperage of the motor for a fraction of second.
The biggest the sag, the harder the strain on the cells, which impact the discharge speed in a non proportional way.
If you’re motor’s sag is of 200A, on a 10s6p pack it’s 33A by cell so a sag of 18A which is probably ok for sush a short time,
If you have a 12s5p, it’s 40A so 25A sag which is a lot less ok.
Or am i mistaken ? (It’s been a long long time I-ve seen this at school, i’m probably missing something)

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