More volts less amps ? Im slightly confused.

So 5T mxus at 120v with 45amps vs 5T mxus at 60v 90amps. I would think the motor would be much hotter on the 90amp setup right ? On the grin simulator they heat up at the same rate ? power in = heat , but i would have thought the amount of amps would effect this. surely 90amps is going to heat things up quicker.

In practice I ran the 120v configuration this morning, there was very little heat compared to the 90 amp run.

This would mean really we should look for highest volts and lowest amps we can get with a highest wind motor ? 120v is pretty lethal though, which is the only downside ?

80kmh top speed at 120v and very very quick acceleration with low heat is nice, but is it the best setup for the mxus. If i can keep heat down its nice, or im gunna die from shock ?

The one with lower amps will be accelerating slower, so it will be drawing near the max amps for a longer amount of time. the one with higher amps will be accelerating faster, and to a much slower top speed. So....it will be drawing max amps for a shorter amount of time.

Once you reach top speed, the amps will slide down to a "cruise" level. Even then the higher voltage system will have a higher top speed, so maintaining speed will draw more amps during cruise phase, compared to the lower top-speed system (wind resistance?). That is my guess...

spinningmagnets said:

The one with lower amps will be accelerating slower, so it will be drawing near the max amps for a longer amount of time. the one with higher amps will be accelerating faster, and to a much slower top speed. So....it will be drawing max amps for a shorter amount of time.

Once you reach top speed, the amps will slide down to a "cruise" level. Even then the higher voltage system will have a higher top speed, so maintaining speed will draw more amps during cruise phase, compared to the lower top-speed system (wind resistance?). That is my guess...

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Wrong....


If you start from a 5,000vdc battery drawing 1amp of battery current, or a 50v battery drawing 100amps of battery current, what the motor gets for phase amps is identical, and if the input power is the same and the copper fill is the same in the motor than the heat production is identical.

Phase Amps stay the same but volts are higher, thus higher watts and motor behaves like a 3t on lower voltage . Or I'm wrong ?

Though I should be able to push more watts in at a given phase amp due to voltage, meaning less heat? Motor side volts go up but phase stays the same yeh, so more watts in with less heat ?

Jestronix said:

Phase Amps stay the same but volts are higher, thus higher watts and motor behaves like a 3t on lower voltage . Or I'm wrong ?

Though I should be able to push more watts in at a given phase amp due to voltage, meaning less heat? Motor side volts go up but phase stays the same yeh, so more watts in with less heat ?

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Whatever the input voltage going into the controller, if the power in is the same, and the motor is the same, the phase amps and phase voltage is the same (it inherently has to be).

Remember, the motor controller and the motors windings together are a buck converter. Google what a buck converter is if you dont know.

Think in terms of power when feeding all electro-mechanical devices. Some inputs are just a little more controller friendly than other. The motor doesn't know or care either way.

Motors are current devices, amp-turns around the tooth is what makes them work, think of voltage has just the unfortunately required force to induce the current to flow and overcome BEMF etc.

For the motor (with adequate inductance for the controller pwm frequency), getting a 5% duty cycle of 1000v or getting a 50% duty cycle of 100v is both able to provide exactly the same amp-turns in the tooth, make exactly the same torque, exactly the same heating, exactly the same efficiency etc.

The difference is that the 1000v controller is going to more lossy and expensive and complex and the battery and management will be more expensive and heavy and have life safety hazards etc.

Thanks , this clears things up for me. I think .

So really high volts is nicer on batteries and wiring , motor end its phase amps and same heat, business as usual.

I find with 120v the motor gets just as hot, which makes sense. the powerband is much wider and more usable, And we'll the smile on today's ride was a big one, hitting 7kw or so at 120v vs 76v is totally different on the motor.

So a high wind motor and high volts really mix well. Easy on battery c rating, easy on controller, slightly more efficient due to less heat in batteries and controller. Only downside is electrocution.

So I have 10AWG wiring right up to my hub, but not in the hub, max kW I can safely use on a mxus v2 ? I'm guessing 12kw will be too much ?

Higher voltage doesn't give batteries an easier time - for the same capacity pack you simply end up rearranging the series/parallel connections for the same number of cells. In reality, voltage is worse for the battery, because the engineering required for a high-voltage pack is more onerous.

Really? So running 50amps @ 100v is worse than 100amps @ 50v. I'm not sure this is completely true? Each cell keeps its same voltage in series, they act as one to produce high voltage, cell volts stay the same. This means even in a 50v pack your saying the edge batteries are getting damaged due to higher volts ?

Again I'm slightly confused :?

Punx0r said:

Higher voltage doesn't give batteries an easier time - for the same capacity pack you simply end up rearranging the series/parallel connections for the same number of cells. In reality, voltage is worse for the battery, because the engineering required for a high-voltage pack is more onerous.

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This is correct.

I think it might be more realistic to compare 72V/20A to 36V/40A. There are a lot of caveats to the equation. Are both motors the same Kv? Do the chosen cells easily provide the specified amps, or are they straining?

A 6T at 72V should have roughly the same RPMs as a 12T at 36V? If both used the same diameter tire on flat land, and they both used a high-current cell that can easily provide 40A?

In the first post, both of the motors were 5T...(I'm willing to learn)

liveforphysics said:

Punx0r said:

Higher voltage doesn't give batteries an easier time - for the same capacity pack you simply end up rearranging the series/parallel connections for the same number of cells. In reality, voltage is worse for the battery, because the engineering required for a high-voltage pack is more onerous.

Click to expand...

This is correct.

Click to expand...

MMm any more on this? Take some 1860 cells, designed to run at what ? 18v for laptop batteries. The chemistries are designed to be ok with 80v or so in a stealth ? We are just expanding cell counts but not the voltage of the cells, but the pass through is more the issue i take it. but pass through voltage stays the same right ? the cells never see this higher voltage, lots of little voltages making one big voltage shouldn't see any difference at the cell level. Engineering of the pack, you mean the interconnections and fusing needed ?

Lots guys running over 100v, seeing batteries dying early death ? vs high amp users.

Let a caveman jump in Higher voltage allows a battery cell ( one ) to stay at a higher voltage because of less amp draw ?
Cave invented battery Not Egyptians.
This is False ?

whatthefuuuu? I heard wat you were saying tron, curious as well.

Think again in terms of power (just like with a motor), and you find it all balances.


Say you have 16 cells, each 10Ah.

If you group them 4p4s, you have a ~14.8v 40Ah pack. If you want 1kW of power from this, it's ~67.5A from the pack, or ~16.9A/cell.

If you group them 2p8s, you have a ~29.6v 20Ah pack. If you want 1kW of power from this, it's ~33.8A from the pack, or ~16.9A/cell.

If you group them 1p16s, you have a ~59.2v 10Ah pack. If you want 1kW of power from this, it's ~16.9A from the pack or ~16.9A/cell.

Simply divide the number of kW of power demand by the number of kWh of pack capacity and you find the C-rate is identical no matter how you configure he cells.


Motor turns also all perfectly balances, it happens to have a squared function with the relationship between phase current and winding resistance, where the square is balanced by resistance increasing at the square of the number of turns on the motor, because with every turn around the tooth the wire gets longer (giving one X) and thinner (giving the second X). This is why motors with identical copper fill (meaning the percentage of slot that is packed with copper) all make exactly the same amount of heating per unit of torque, same efficiency per amount of torque or power, capable of same performance etc.

There are no free lunches in motors or batteries. The best you can do is avoid needless complexity and expense and safety in BMS and pack weatherproofing and corrosion protection needs by using the lowest pack voltage you can find a motor and controller to suit for meeting your performance needs.

Personally, for all systems below say 5kW, I would run <30vdc, ideally around 6S due to the wide range of dirt cheap and easy to implement 1 chip BMS solutions (designed for laptops). A couple feet of a little thicker wire is so much cheaper and more reliable and more compact and easier to work with than adding a big rats nest of BMS wiring and trying to vibration protect it all and environmentally protect it all etc.

Free lunches, is that a widely known thing? I learned about 'tanstaafl' in economics, but assumed it wasn't 'main' curriculum (raised by wolves) -crap it's on google, I really was raised by wolves :lol:

Anyway, a standing question indicated by punxor? Do series'd batts start to have wear related to voltages higher than originally intended? An interesting question I thought, how could this just be associated with the wiring, but at what degree would it be evident and to what detriment?

nutspecial said:

Free lunches, is that a widely known thing? I learned about 'tanstaafl' in economics, but assumed it wasn't 'main' curriculum (raised by wolves) -crap it's on google, I really was raised by wolves :lol:

Anyway, a standing question indicated by punxor? Do series'd batts start to have wear related to voltages higher than originally intended? An interesting question I thought, how could this just be associated with the wiring, but at what degree would it be evident and to what detriment?

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Punxor said the pack engineering is more difficult as the cell string count grows, not that the cells care or know what voltage string they happen to be in.

If you have X amount of pack design/construction talent/resources available, the less cells in series the better shot that pack has at being done right and hence lasting.

With todays cells without paper math . My cells can only deliver so many amps per cost or danger.
What does this mean it not the voltage but the amps that will kill you.
Anyway my battery is 36v 40ah or 72v 20ah so what do I want at what price ? A 36v 80 amp controller how much ? Cost ?

So 24v 100ah battery verus 72v 25ah ?
And a 120amp 24v controller ?
Price as can't pick diamonds of out my
Money limits battery quality price ect thing.

Edit : No lipo ...
Luke your battery source in mine are different leagues.
Plus no franken bike. I live in a city shot first.

The C rating stays the same if you parallel your packs, this makes sense, but if im only buying enough cells for my pack serial only, im going easier on those cells.

100 amp from 18s vs 50 amps from 36s. then each cell in the 36s pack is under less stress yeh. But keeping all things equal running a parallel pack comes with its own complexity too.

pushing 12kw out of my 120v setup is much easier than getting hold of a 200a 60v setup, also what is the weight of a 120v 100amp vs a 60v 200amp setup, space and weight is premium on a bike.

Also dead cell in a parallel pack vs dead cell in a serial pack ? Im thinking in a parallel pack we would see an in rush on that bad cell ? serial we should just see a voltage drop and a sleeping cell ? Trouble is i run 30s 2p

Jestronix said:

The C rating stays the same if you parallel your packs, this makes sense, but if im only buying enough cells for my pack serial only, im going easier on those cells.

100 amp from 18s vs 50 amps from 36s. then each cell in the 36s pack is under less stress yeh. But keeping all things equal running a parallel pack comes with its own complexity too.

pushing 12kw out of my 120v setup is much easier than getting hold of a 200a 60v setup, also what is the weight of a 120v 100amp vs a 60v 200amp setup, space and weight is premium on a bike.

Also dead cell in a parallel pack vs dead cell in a serial pack ? Im thinking in a parallel pack we would see an in rush on that bad cell ? serial we should just see a voltage drop and a sleeping cell ? Trouble is i run 30s 2p

Click to expand...

This has already been discussed to death in at least a hundred threads... For maximum performance it comes down to the silicon availability in the controller. At this moment, 100v rated MOSFETs are the highest power density switching devices, followed by 75v MOSFETs, and then 60v MOSFETs are about tied with the power density of 150v MOSFETs.

Using 100V mosfets in the controller enables something like 40-60% higher power density, or 40-60% less heating and size of controller needed VS running 150v MOSFETs.

Your 120V setup is a needlessly large controller wasting a needlessly larger amount of heat than a controller using 100V parts in it over 150V parts in it.

Say you want 10kW of power. Using 20S (and hence 100V MOSFETs) will get you there with something like 2/3'rds of the parts and expense and losses etc as comparied to using 150V MOSFET based controller to make the same 10kW.


Regarding the battery, yes if you have bigger battery (more cells) it obviously reduces the per-cell loading, but this is unrelated to choosing a logical pack voltage. With respect to parallel cells in a pack and cell failures, I've seen an awful lot of packs that died from BMS related issues far more than defective cells.

ATB,
-Luke

eTrike said:

It is worth noting that 50V 100A will produce a lot more heat on the rest of the system (wires, etc.) than 100V 50A. Perhaps this is part of the initial confusion.

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Actually, it just needs 2x the copper cross section for the 1-2ft between the battery to controller. Then it's exactly identical heating.

The mass and cost of the wire is drastically reduced cost and hassle and engineering than having 2x the cell string to manage.

eTrike said:

Thanks for that, I should have added "all else being equal". I sometimes wonder if the internet has affected my real life communication to the point that I feel the need to explain every nuance so that someone grasps the larger view or hopefully preventing them from raising a cursory point based on nuance. I often find myself losing the original point in the explanatory cloud of information/explanation. Now what were we talking about? :lol:

ATB!

Click to expand...

No problem my friend.

Glad we've all moved to the same page.

36v 5kw controller , that's a lot of Amps and parallel packs. The top speed is sure to knock my socks off! For a daily commuter this would be fine at lower amps.

But I hear u on the safer lower voltages and simplicity of a low volt systems.


Thanks guys this has all made me slightly less confused 8)

Assuming the same amp rated controller;
More volts will give you better acceleration and a faster top speed.
More amps will give you better acceleration but the same top speed. Although a faster speed uphill.
You can also push more watts through the same wiring at higher voltages without overheating the wiring.

Sorry to poach your thread, but for you guys with more experience, what is a reasonable length of time a MAC can sustain 2k watts?