Battery question : determining the sweet ratio V/A for range

[Vanarian]

A new thread again, but a relevant one!

Do you guys know how to determine the sweet spot between V and A for a fixed power limit, to obtain the best range possible?

My common sense makes me think that for the same amount of cells with a fixed C discharge rating, you should obtain the same final Watt-hours capacity right? But as I've seen, common sense can make me misunderstand electrics sometime.

I want to get the maximum out of 60 Samsung 25R cells. Each cell is 3.7V 2.5Ah (max charge rathing is 4.2). Here are all the setups I can make with these :

10s6p = 37V 15Ah

12s5p = 44.4V 12.5Ah

15s4p = 55.5V 10Ah

20s3p = 74V 7.5Ah

If my calculations are correct, for each config I obtain 555Watt-hours for 60 cells.

What is the best combo between all these? Please correct me if I'm wrong!

Thanks

 

[torqueboards]

I think if they are all 555 watt hours. The highest voltage would be the most efficient since you would use less amps and deliver more power sooner.

However, I don't think you would want to use 15S or 20S on an electric skateboard. If you do that's beast lol. I guess you could and throttle it down also.

 

[onloop]

It doesnt matter how you configure it.

WAtt hours are the same. Same same.

With higher voltage you have less amps. Less heat losses.


I would go 10s or 12S setup.


This battery is going to be huge For an eboard though. It will also weight like 2.5kg... maybe too big?

How will you mount it onto the deck? It will have to be two layers of cells stacked on top of each other.

Also think about if you will ever want to use hub motors.... anything over 10s might be difficult to wind a useable KV. Unless you want to race cars.

 

[Vanarian]

Thank you guys for the lights

You're right 2.5kg is a bit on the heavy side but if the power and range compensate it I can accept the added load ; it is still nowhere near the weight of a 6pack water bottle. Should be carriable without much sweat.

It's good to know that Watt hours remain the same, this makes me want to go either 12s or 15s. I plan to couple it to 63mm 130 or 170Kv motors from APS, is the KV low enough?

I'm not exactly putting the batteries on a board deck, I'm building inline skates so I'll stack the batteries around the boot and inside the deck of the skate. And you have a point, I will probably be forced to put them in at least two layers so I hope it won't be too bulky and ugly. Should receive a decent 3d printer by september (finally! ) so I can try custom Nylon enclosures to protect them. Don't know if it will be able to handle carbon like materials, I'll figure it when it comes home.

Well I won't be racing cars (unless I learn the bad habit), rather small 2-wheels vehicles of the 50cc likes ? The reaction of a car driver would be priceless if he was left behind but I fear drivers in my country are not educated enough about ecologic commuting vehicles such as longboard, inlines, to behave without putting me at danger. They only know bikes. You have probably already met the type of guy who thinks that even if there will be a redlight in 100 meters, even if you're actually going fast, he HAS to pass you absolutely because he drives a car and you don't. There are plenty where I live unfortunately.

You're also right about hub motors, actually I might make some slimmed 80mm from the leftover units a guy here is selling (still waiting for him to contact me and give me his prices), depending on what these can take I might try to wind them as low as possible? For hub motors the larger is better, but as I can't put large sizes for inlines, instead I should go bigger diameter?

 

[torqueboards]

Doubt you would need that much power. 50mm motors would be sufficient. 63mm would be bigger if you don't have the space you wouldn't need it. 80mm motors are overkill as some give out 4-8kw. Unless you have batteries to deliver 4-8kw it would be a waste. 15S * 100amps = 55.5v * 100 amps = 5550 watts. That's pretty difficult to do.

 

[arkmundi]

An interesting question I'm sure can adequately be buried in the physics, but mostly irrelevant. Its the resistive force of the wiring that will determine how efficient a setup is, hence less loss of those watts. So its not just battery, but motor, controller, connections, etc. that all matter. Putting too many volts (battery) behind a motor/controller ill equipped to handle the additional amps will lead to power loss through heat dissipation. Hence, configure your battery to your motor/controller, using their specifications.

 

[Vanarian]

Doubt you would need that much power. 50mm motors would be sufficient. 63mm would be bigger if you don't have the space you wouldn't need it. 80mm motors are overkill as some give out 4-8kw. Unless you have batteries to deliver 4-8kw it would be a waste. 15S * 100amps = 55.5v * 100 amps = 5550 watts. That's pretty difficult to do.

I probably oversized the battery but summer is coming, I live on the coast, and the roads are made so you can litteraly visit 4 different cities while staying on the sea border, just by riding into bike lanes! There are also many hills to climb too, I saw too big but I'd like to test the total power consumption in these various conditions to know what exact ranges I can achieve. Plus this many cells allow some effortless bursts of powers, if I datalog the peak powers too I can finally downsize the battery to gain over weight.

For the hubs I plan to obtain them by cutting 2 Turnigy C80-10800 to a very slim size, these things are as large as 100mm and I'll strip them from 70mm at least. I should end up with 80x40 hubs instead of 80x100, if I can slim them more I'll try that too.

It is true that these motors are rated up to 7Kw but that's only with standard specs... I think that I won't even get up to 5Kw with this much magnetic surface stripped from them, the hubs will be burned out from insulation way before they take this much power. My only expectations are to at least success in reaching something between 2Kw and 3Kw ? Maybe I'm wrong but that's what I meant by "since I can't go larger, I try to compensate with bigger diameter"
If I solve this successfully I will be next to Eto with his razerblades hub motors!

As for the raw power of 63mm from APS, you're right I don't need this, but I'm confident I can find the right space at the heart of the frame to put them under my feet, and to ride these will be madness! And if 63mm find enough space, then I'll be 100% sure to find a way to plug 50mm motors under some Doop skates :wink: Maybe even 42mm with gear reduction can work to get something usable in city with lightweight.

An interesting question I'm sure can adequately be buried in the physics, but mostly irrelevant. Its the resistive force of the wiring that will determine how efficient a setup is, hence less loss of those watts. So its not just battery, but motor, controller, connections, etc. that all matter. Putting too many volts (battery) behind a motor/controller ill equipped to handle the additional amps will lead to power loss through heat dissipation. Hence, configure your battery to your motor/controller, using their specifications.

Thank you for the resistive force info ! Actually I want to stay at the same power level, just raise total voltage and proportionnaly decrease max amperage. Though I was wondering, how can I measure the voltage spikes that inductive load might throw through the system?

For example if I were to use 12S (44.4V), how much spikes would I reach at 3200W? Is also the resistance of wirings the dertermining factor?

As far as voltage go my controller is rated at 60V max. Amperage is rated around 80A continuous with way more for bursts of seconds

 

[torqueboards]

Cool going to be beast. LOL Remember to post about it

 

[arkmundi]

I'm sure there are some academic types interested in those relations. I'm more on the practical side of things.

Think of R as a constant, though wire gauge and inexpedience of connections are design choices. Also, you're wanting to keep P constant. Ohm's law states the necessary relation that V & I are directly proportional to power. But its not the battery that decides, but the motor & controller. Well, the C-rate of the battery does factor into max-amps.

You've got a controller capable of delivering 80 amps. What is the max-watts of your motor? Define the voltage of your battery to deliver those watts to the motor. Period. Range is then a consequence of the applied amps, via throttle/controller. And the efficiency of your setup, which may be most apropos.

 

[agraham]

The reason to power a motor at higher voltage is to drive amps, including max-amps. Lower voltage gets you lower amps:

You've got it backwards. Lower voltage doesn't get you lower amps it gets you lower watts. At lower voltage it's easier to hit the amp ceiling. At a higher voltage you can achieve the same power with less amps.

Also, you're wanting to keep P constant. Ohm's law states the necessary relation that V & I are directly proportional.

Backwards. V and I are multiplied together to make P thus they have an inverse relationship. More V means less I for the same P. More I means less V for the same P.

More volts means less amps to achieve the same output wattage. You want to run the highest voltage your electronics allow in order to get the amps as low as possible for the least losses and then pick your motor kv and transmission to match.

 

[made_in_the_alps_legacy]

[post deleted - no one cares -]

 

[arkmundi]

P = V * I. Watts required. More Watts, raise Voltage of battery, or raise amperage of controller, and most often both, in proportion. Running an 80 amp controller for a 3000 watt motor? Better be running that with at least a 72 volt battery, if not 96V. Its what I meant by directly proportional. Yes, V = P/I, and I = P/V. But don't get confused by the math. The higher S configuration in the original question is what I'd recommend. Just try running a 3000 watt motor on 12 volts at 80 amps and see if you don't smoke that battery in short order. Because current is also running through the battery. Still don't know the load for the application.

 

[vedder]

More volts means less amps to achieve the same output wattage. You want to run the highest voltage your electronics allow in order to get the amps as low as possible for the least losses and then pick your motor kv and transmission to match.

Exactly. A very good sentence that summarizes the answer to the question in this thread

 

[riba2233]

P = V * I. Watts required. More Watts, raise Voltage of battery, or raise amperage of controller, and most often both, in proportion. Running an 80 amp controller for a 3000 watt motor? Better be running that with at least a 72 volt battery, if not 96V. Its what I meant by directly proportional. Yes, V = P/I, and I = P/V. But don't get confused by the math. The higher S configuration in the original question is what I'd recommend. Just try running a 3000 watt motor on 12 volts at 80 amps and see if you don't smoke that battery in short order. Because current is also running through the battery. Still don't know the load for the application.

I don't agree, on lower voltage you will have battery proportionally with more Ah, and proportionally more current capability, so there is no difference, you can same have power at 36 or 72 V (or whatever), but you will have to have thicker cables on lower voltage. But also a cheaper BMS.

 

[made_in_the_alps_legacy]

It doesnt matter how you configure it.
WAtt hours are the same. Same same.
.

Exactly. A very good sentence that summarizes the answer to the question in this thread

 

[arkmundi]

I don't agree, on lower voltage you will have battery proportionally with more Ah, and proportionally more current capability, so there is no difference, you can same have power at 36 or 72 V (or whatever), but you will have to have thicker cables on lower voltage. But also a cheaper BMS.

Yes, I agree completely with that understanding. My own limited experience in making my setup work, including making my own batteries is that batteries are designed for a purpose. Increasing amp-hours, a capacity measure, most often also carries with it concomitantly, increasing the amps that battery is capable off. In that respect the C-rate of the cells and number in parallel. Yes, for efficiency, you want the lowest possible amps for the watts delivered, but there are practical limits. Lets take an extreme example - would you design a battery for the theoretical 3000 watt motor at 3000 volts and 1 amp? Unlikely, but that's what screwy math might suggest. Efficiency, wanting more range, a good thing, will have a great deal more with how you apply the throttle and how much pedalling you do. Again, its hard to make a specific recommendation without knowing the specifics - what exactly is the motor in question, its nominal wattage?

Practically speaking, there aren't that many choices. For my motor, a MAC 8T, run it a 36V or 48V, 30 amp controller or 40 amp? At 48V and 40amps, getting 2000 watts for a motor nominally rated for up to 1500 watts. How many MAC owners are running at 96V and 20amps? How many at 24V and 80amps? The watt-hours are the same. I would not recommend those setups, though they would probably work.

 

[riba2233]

Well for the specific motor you should follow manufacturers recommendation. That is the reason why there are many different winding options for mac motors, to suit the wider voltage range (36 to 72 v if I got got right). All have the same efficiency and power. If they made mac that would work on 3000 V and 500 mA (500T?) it would also have the same power and efficiency, but you would have problem finding other components. That's why we mostly use 36 to 72 v, but between those numbers, it doesn't relly matter much what voltage do you choose, you can get same power and efficiency. Higher power levels are kind of exception because controllers are made to work on 72 v or more.

 

[Vanarian]

Thank you for all the replies!

Well I initially sized my number of cells to be able to feed a 3200W 130/170Kv motor from APS in the standard specs (10S 80A peak power), equals 37/42V and 15Ah, with a C discharge rate of 3.4 C max to obtain the maximum power. As the Samsung cells are rated for 20Amps continuous each, if I'm not mistaken I should be able to pump 3200W out of them without even bashing the cells. Not that I want to burst all day, but just be able to draw out the power from times to times.

When I estimated the ratings with the fixed 3200W power, i came to the following results :

10S6P - 37V nominal needs 86A, surcharge at 42V and you go 76A

12S5P - 44.4V nominal needs 72A, surcharge at 50.4V and you go 63A

Now if you raise up to 15S4P - 55.5V nominal needs only 58A, and I won't ever surcharge this.

Going from 86A down to 58A is a HUGE difference in heat production, for the same amount of power (and I guess more torque with Voltage) in the motor. Plus the total amps drawed from the batteries make useless to go as high as 3.4C discharge rate.

The motors will power wheels from sizes varying between 80mm, 110mm and 125mm, which can be altered with gear reduction. The load is my own body weight + let's say 7Kg? Around 90kg total load. I can help with my legs, which means at least 200W available to help from stand start.

Since the ESc is rated for 60V 80A continuous I could always stay under the continuous limits and never warm up the controller. But I'm afraid of voltage spikes since I don't want to see them go above the 60V limit! What do you think?

 

[arkmundi]

I'd go for the 15s4p = 55.5V 10Ah design. The lesser 3P design is pressing on the discharge limits of the cells at 80amps. And the higher voltage for your motor I believe is preferred. Understand, however, that heading towards max-discharge rates for battery cells means shortening their usable lifetime. If it were me, I'd run at 15s4p and then double the capacity with a second 15s4p pack as soon as affordable.

We're not the only ones who have struggled with the question. After some methodical searching on the forum, I liked this answer: e-motorcycle built from scratch!

As you might know, electrical power equals to:
P=V*I

And Joules losses are given by:
Pj=R*I^2

Hence, reducing the current to reduce losses and increase the voltage to maintain power is only common sense.

Since most of the losses occur in the battery pack, due to internal resistance of the cells, it is easier to design a high voltage battery pack that respects the maximum C-rate of the cells and fits inside a motorcycle.

 

[riba2233]

Again, you are on a wrong train of thoughts.

If you have the same number of cells, and use them at same power level, each cell will see the same load regardless of cell configuration. (S and P canfiguration)

Also, this:

Hence, reducing the current to reduce losses and increase the voltage to maintain power is only common sense.

Is bypassed by simply using proper cable gauge.

 

[Vanarian]

Again, you are on a wrong train of thoughts.

If you have the same number of cells, and use them at same power level, each cell will see the same load regardless of cell configuration. (S and P canfiguration)

Also, this:

Hence, reducing the current to reduce losses and increase the voltage to maintain power is only common sense.

Is bypassed by simply using proper cable gauge.

Common sense would make me think that you're right about the cell load because cell number doesn't change but actually the voltage is a constant when you series the cells, it can only drop over time of battery use where you actually regulate how much amps you want the batteries to output right ?

So if you series 55V and only ask 58A of the 10Ah of cells I don't understand how you stress as much the cells as when you ask 86A once serried to 37V. Of course the available Ah is 10 and not 15 but you ask way less amps, the cells should not be as much discharged ?
Unless for the asked 58A cells will have to give again the same 3.4 C of discharge on 10Ah than 86A on 15Ah.

Though I disagree about your statement on the heat loss. Heat loss is a proper power and efficiency loss factor in almost everything that includes movement. Copper losses rise way faster than iron losses so there is no contest there :wink:

I'd go for the 15s4p = 55.5V 10Ah design. The lesser 3P design is pressing on the discharge limits of the cells at 80amps. And the higher voltage for your motor I believe is preferred. Understand, however, that heading towards max-discharge rates for battery cells means shortening their usable lifetime. If it were me, I'd run at 15s4p and then double the capacity with a second 15s4p pack as soon as affordable.

We're not the only ones who have struggled with the question. After some methodical searching on the forum, I liked this answer: e-motorcycle built from scratch!

As you might know, electrical power equals to:
P=V*I

And Joules losses are given by:
Pj=R*I^2

Hence, reducing the current to reduce losses and increase the voltage to maintain power is only common sense.

Since most of the losses occur in the battery pack, due to internal resistance of the cells, it is easier to design a high voltage battery pack that respects the maximum C-rate of the cells and fits inside a motorcycle.

Thank you for the link!

Electrics are hard on the mind.

 

[arkmundi]

There is always room for improving our understanding of the electrics behind batteries and their configuration in parallel and series. While partially correct when considering total load and discharge that it does not matter how you configure the connections, its essential to understand that batteries are part of the wiring. Current flows through them. They have special impedance characteristics because of the electrolytes, that change over time. Batteries ultimately fail because of increasing internal resistance. Limiting electrolyte degradation and lithium plating, reducing heat loss, extending cell cycle life are also useful considerations, apart from total watt-hour capacity. I have undertaken to improve my own understanding, through additional reading so as to assure the accuracy of my posts in this thread, and have in fact edited my posts so as to reduce any confusion.

So, while the answer to the original question....

Do you guys know how to determine the sweet spot between V and A for a fixed power limit, to obtain the best range possible?

... can be correctly answered by it doesn't matter, it is, I believe more correct to respond maximize voltage in series, respecting the C-rate discharge limits of the cells involved, and not exceeding the voltage input limits of either controller or motor Thanks!

 

[riba2233]

So if you series 55V and only ask 58A of the 10Ah of cells I don't understand how you stress as much the cells as when you ask 86A once serried to 37V.

Because you have more cells in parallel now. So each cell will see same current as in another example (55V, 10 Ah)

Though I disagree about your statement on the heat loss. Heat loss is a proper power and efficiency loss factor in almost everything that includes movement. Copper losses rise way faster than iron losses so there is no contest there :wink:

They do, and that's why you use thicker cable on lower voltage for same power.

... can be correctly answered by it doesn't matter, it is, I believe more correct to respond maximize voltage in series, respecting the C-rate discharge limits of the cells involved, and not exceeding the voltage input limits of either controller or motor Thanks!

Yes, that's the situation if you already have your controller and motor rated for that voltage. But you can always chose motor and controller that work on lower voltage and higher current, use thicker cables, and you don't loose anything, you have the same performance. Cons of lower voltage is that you have to use thicker cables and bigger connectors, and the pros is that you have a smaller (cheaper, simpler) BMS that is easier to wire, and your system is safer for you.

 

[Vanarian]

Ok I think I understand a bit better

Thank you again for the explanations ! I'll try to size my compnents to determine the package ; I'll try the 15S setting and hope that I won't fry my stuff :lol: And if the cells are on the extreme discharge side, I'll try adding some cells.

 

[scoot-e]

Correct me if I'm wrong, and yes, the watt hours are equal in all the scenarios you listed, but with the higher voltage (assuming you don't limit the speed specifically through a CA or controller setting) you have the potential for higher speed than at a lower voltage and higher speed translates to quicker loss of range than at lower speeds, so assuming you only need to go, say 20mph max ever, then you'd want to match it to your controller/motor/kv to get the most distance out of the setup imho...if your voltage will allow your setup to go 30mph say, but you never need or want to go that fast but don't limit the motor somehow then you may get speed at the expense of range unless you were to very carefully manage throttle (which I find hard to do personally) even if the scenarios you listed are technically the same it doesn't seem to work out that way depending on the setup you are using and what's most efficient for it...

Depends what your requirements for speed (higher volts), range (higher Ah capacity) and torque/power (hill climbing - amps) are...generally speaking...