Electric Vehicle Battery specification (very basic)

[Mech_Engineer]

I take it i would have to have a specialised motor for this kind of application, i.e. not the one i am using.

Not true, any electric motor can be used as a generator (brushed, brushless, etc). You would have to design your motor controller to be capable of acting as a charging circuit in certain situations though, like when throttle input is zero and brakes are applied.

 

[robinfisichel]

Not true, any electric motor can be used as a generator (brushed, brushless, etc). You would have to design your motor controller to be capable of acting as a charging circuit in certain situations though, like when throttle input is zero and brakes are applied.

Great!

 

[mheslep]

A regenerative braking system doesn't completely replace the conventional brakes.

Yes I know, I argue in part because of the reasons I outlined above.

 

[robinfisichel]

I have another problem!

I was calculating how much extra capacity i would need to run the standard onboard systems, (radio and headlamps) and calculated:

Radio Power = 10w
Light power = (2x50w) + (2x35w) for 4 lights breaking and headlamps.

If the lights run on 12v then the current drawn is 170/12 = 14.16 A

and to run them for 4 hours continuously = 56 Ah capacity needed!

Surely I am doing something wrong, i wanted a 100Ah battery which now looks impossible.
For this car i can do 40mph constant for 4 hours on 100Ah and the lights are going to use half of that!

 

[mheslep]

I have another problem!

I was calculating how much extra capacity i would need to run the standard onboard systems, (radio and headlamps) and calculated:

Radio Power = 10w
Light power = (2x50w) + (2x35w) for 4 lights breaking and headlamps.

If the lights run on 12v then the current drawn is 170/12 = 14.16 A

and to run them for 4 hours continuously = 56 Ah capacity needed!

Surely I am doing something wrong, i wanted a 100Ah battery which now looks impossible.
For this car i can do 40mph constant for 4 hours on 100Ah and the lights are going to use half of that!

No, convert to watt-hours. The lighting is small compared to any heating and A/C loads.

 

[xxChrisxx]

No, convert to watt-hours. The lighting is small compared to any heating and A/C loads.

This is what confused me as to the way he was doing it.

Why isn't everything specced in Watt hours (power-time)? He only needs Ah if he knows specifically the voltage of the battery and that that voltage will run the motor.OP: That motor you specced.

Vnominal = 156
A nominal to reach power input= 96 A
Power input/output = 15 kW

Using 12V batteries. You need 13 of them to reach the required nominal voltage.
If EACH is a 100Ah battery. You will be drawing 156 Volts for 1.04hours. This is 15 kWh.Adding lights: 100 watt headlights. + Whatever to rach your 170W
Say they are 12V lights. As you say they draw a current of: 170/12 = 14 (ish)

However your 100Ah comes from a voltage draw of 156Volts. For the lights you arent drawing that. They are drawing 12V from 13 batteries (0.92V). You know their rating is 170W. So to run them for the 4 hours you need 680Wh of power. From your batteries to run lights.

The lights use 1% of the power needed to run the motor.

 

[robinfisichel]

However your 100Ah comes from a voltage draw of 156Volts. For the lights you arent drawing that. They are drawing 12V from 13 batteries (0.92V). You know their rating is 170W. So to run them for the 4 hours you need 680Wh of power. From your batteries to run lights.

The lights use 1% of the power needed to run the motor.

Thanks, makes more sense now, though I am using 3.3v batteries so i guess its 0.25v per battery instead. I am confused about the difference between Wh and Ah..

 

[xxChrisxx]

Thanks, makes more sense now, though I am using 3.3v batteries so i guess its 0.25v per battery instead. I am confused about the difference between Wh and Ah..

Watt hours is the total 'juice' you have available.

Amp hours is only useful if you know the voltage that is being drawn. If the voltages are different to the battery rating the Amp hours will alter.

Eg. 12 V battery with 100 Ah = 1200 Wh total juice avialble.
3.3 V battery rated at 100 Ah = 330 Wh available.

So if you are using 3.3V batteries. You need 156/3.3 = 47.27 = 48 batteries to provide the necessary vltage to run the motor.

If they are 100Ah batteries. And the current draw is 96A then the batteries will last for 1.04 hours.

Basically you can run your motor at 15kW power output for just over an hour using the batteries you have specified. At cruise you said that you will be using something like 9kW. So you have a total of 1.6 hours at cruising speed.

Giving a total range of 112 miles. (Not including increased power for accelerating to that speed).All you care about for this project is the power usage. So put everything in Wh.Also do a search for batteries, as I'm very sure you'll be hard pressed to find 100Ah betteries avaiable in the sizes you stated. This also may be only a technical project, but it's always good to show you've thought of commerical aspects.

ie. If you specify Li-ion batteries you are probably going to be looking at £35000 in batteries alone. But it will be light.
If you specify lead acid. You are looking at at least 4x the weight of li-ion but £1500 in batteries.
(NUMBERS ARE APPROXIMATE)

 

[robinfisichel]

Thanks i think i get it now, there's a company called thunderstruck motors who do batteries for 100 Ah 3.3v for about $150 dollars a piece or something, so a lot of cash for sure, but that's my least concern.

Tesla uses something like 6000 batteries, and has a tonne of power and torque, but at the same time costs £100,000 nearly! The thing with the Tesla is that they have some arranged in series and some in paralell, it must be some tradeoff between weight, voltage and capacity or something.

 

[mheslep]

This is what confused me as to the way he was doing it.

Why isn't everything specced in Watt hours (power-time)?

Answered above:
https://www.physicsforums.com/showpost.php?p=2659483&postcount=20

 

[mheslep]

All you care about for this project is the power usage. So put everything in Wh.

That's energy. An EV design needs to consider both power and energy.

Also do a search for batteries, as I'm very sure you'll be hard pressed to find 100Ah betteries avaiable in the sizes you stated. This also may be only a technical project, but it's always good to show you've thought of commerical aspects.

ie. If you specify Li-ion batteries you are probably going to be looking at £35000 in batteries alone. But it will be light.
If you specify lead acid. You are looking at at least 4x the weight of li-ion but £1500 in batteries.
(NUMBERS ARE APPROXIMATE)

xxChrisxx, those claims need references in this engineering forum. 15kWh of Li Ion batteries can be purchased for less than $7000 (£4600).
http://evcomponents.com/cscart/index.php?dispatch=categories.view&category_id=171

 

[xxChrisxx]

That's energy. An EV design needs to consider both power and energy.

Of course, but to spec your batteries a sensible starting place is to make sure you have all the energy you need to complete the task.

I tihnk the post made it clear the point I was trying to convey this. As the OP was clearly getting confused between watt-hours and amp-hours and how to use them.

xxChrisxx, those claims need references in this engineering forum. 15kWh of Li Ion batteries can be purchased for less than $7000 (£4600).
http://evcomponents.com/cscart/index.php?dispatch=categories.view&category_id=171

So it seems battery technology has moved on quite a bit from when I last looked at this, I seem to remember figures in the region of £2 per watt hour for Li-ion and about 10p for lead acid. I was at work and couldn't sit searching for new figures (I should have done but didn't).

That's fair enough, my numbers are way out. The principle of looking from a commercial aspect is sound though.

 

[Topher925]

I noticed one that thing hasn't been mentioned yet is battery life. If the project will be using lithium anything, take all those specs you have and double them if you want your packs to last longer than 100 cycles. Li-Ion batts by their nature have short lifetimes, and in order to significantly extend their life they have to be only partially discharged. Theres a lot of new battery start-ups out there that say their product can be fulling discharged or 80% discharge, but I have yet to see one of them provide the field test data to back those claims up.


Another thing not mentioned yet, if using Li-ion, the cooling system is very very very important. Like, extremely important. Did I mentioned the design of the cooling system is important? Li based chemistries in general have a higher internal resistance than others such as NiMH and have a higher volumetric power density as well. This means in each cell you are generating more heat with less surface area to remove that heat. In order for your batts to last, and for you to not kill yourself, you need proper thermal management. This is the hard part. You can spec out your battery packs with a pen and the back of a napkin, but for thermal management you need to do some real analysis.

Remember what happened to all those Dell laptops? Poor thermal management, that's what happened.

http://gizmodo.com/182257/dell-laptop-explodes-in-flames

 

[mheslep]

Common Li Ion chemistry now is LiFePO in the form of nano particles.
http://www.a123systems.com/a123/technology/life
http://www.a123systems.com/a123/img/technology/life-graph1.jpg

 

[Topher925]

Common Li Ion chemistry now is LiFePO in the form of nano particles.
http://www.a123systems.com/a123/technology/life
http://www.a123systems.com/a123/img/technology/life-graph1.jpg

 

[OmCheeto]

top speed = 122 mph based on the AC24 specification

I think this is a bit optimistic. The torque curve for the system is flat from 0 to 4000 rpm, but from there it drops linearly in half at only 6000 rpm. Hp also drops from 40hp to 16hp from 4000 to 8000 rpm. I think the top speed will be in the 80 to 90 mph range.

 

[xxChrisxx]

I think this is a bit optimistic. The torque curve for the system is flat from 0 to 4000 rpm, but from there it drops linearly in half at only 6000 rpm. Hp also drops from 40hp to 16hp from 4000 to 8000 rpm. I think the top speed will be in the 80 to 90 mph range.

Torque makes no difference (well... kind of). He has the power to reach and sustain a speed around 122. It just depends if he's going to use a gearbox, or just leave the electric motor as it is. Also depends on the size of the wheels too. If it's electric motor + no gearing then 122 is unachievable.

122 mph requires 31kW assuming 85% efficiency of the motor. It's a bit of a stretch but not totally out of the question.

 

[mheslep]

Meaning what, you have better information?
http://www.ipd.anl.gov/anlpubs/2008/02/60978.pdf, tables 3,4
http://pubs.its.ucdavis.edu/publication_detail.php?id=1292, table 1

 

[Topher925]

Meaning what, you have better information?
http://www.ipd.anl.gov/anlpubs/2008/02/60978.pdf, tables 3,4
http://pubs.its.ucdavis.edu/publication_detail.php?id=1292, table 1

Meaning that I've heard the same argument and seen that same graph close to probably 100 times by now. It seems that just because a battery chemistry has the word "nano particles" in it people seem to think its some revolutionary battery technology. While A123 (and others) have made some big leaps in battery technology there is still NOTHING in terms of high energy density battery tech out there that doesn't suffer from significant degradation and performance loss which makes it difficult to use in automotive applications. Even the link you posted shows that A123 batteries have a poor cycle life (shallow discharge even) compared to other "non-nano" chemistries.

Cycle Life (shallow cycles) ~240k

http://www.ipd.anl.gov/anlpubs/2008/02/60978.pdf

My point is, thermal and system management are key to getting the most out of any battery. If you're planning to just plug the packs into your motor controller and cycle as if they were lead acid, you're going to have problems. It doesn't matter how fancy your battery is or how many "nano-particles" each cell contains, if you don't know what you're doing with lithium based batts the performance and life will be short lived. Not to mention the high probability of having a major fire hazard on your hands.

 

[mheslep]

Meaning that I've heard the same argument and seen that same graph close to probably 100 times by now.

You were aware, and yet you still posted in this forum https://www.physicsforums.com/showpost.php?p=2666688&postcount=43" that Li-Ion had '100 cycles' (unless one derates it)?

It seems that just because a battery chemistry has the word "nano particles" in it people seem to think its some revolutionary battery technology. While A123 (and others) have made some big leaps in battery technology there is still NOTHING in terms of high energy density battery tech out there that doesn't suffer from significant degradation and performance loss which makes it difficult to use in automotive applications.

What constitutes 'significant degradation and performance' loss? Reference?

Even the link you posted shows that A123 batteries have a poor cycle life (shallow discharge even) compared to other "non-nano" chemistries.

I think you missed the 'k'. That is 240,000 shallow cycles.

My point is, thermal and system management are key to getting the most out of any battery.

Agreed, though 'key' is subjective.

If you're planning to just plug the packs into your motor controller and cycle as if they were lead acid, you're going to have problems. It doesn't matter how fancy your battery is or how many "nano-particles" each cell contains, if you don't know what you're doing with lithium based batts the performance and life will be short lived.

Nanotech for batteries is not all about energy density. A significant source of failure over charge cycles is the stress induced from repeated physical contraction and expansion of the electrode crystal lattice as it accepts or depletes charge, causing the electrodes to crack and disintegrate. Nano structures have in some cases greatly reduced this effect.

Not to mention the high probability of having a major fire hazard on your hands.

Dell laptop batteries are not really a relevant to the topic of this thread which is 'Electric Vehicle Battery' specs.

 

[Topher925]

You were aware, and yet you still posted in this forum https://www.physicsforums.com/showpost.php?p=2666688&postcount=43" that Li-Ion had '100 cycles' (unless one derates it)?

You're taking what I said out of context. The specs that were determined in previous posts were assuming deep cycling along with heavy transient operation with discharge rates greater than 1C. That will kill a cell fast, especially if the cell isn't adequately cooled. The graph which you posted, and I always I have to explain, is for a steady state cycling at a charge/discharge rate of 1C. This will never happen in a real automotive application. Thats like saying someone who can run 1 mile in 5 minutes, can run 10 miles, up and down hills, in 50 minutes. Graphs such as the one you posted provide little information on the actual lifetime performance of a battery. When you start to consider harsh conditions, plus transient operation, plus actual time, the slope of that curve gets a lot steeper.

What constitutes 'significant degradation and performance' loss? Reference?

Depends on the application. For cars, 5,000 hours of discharge operation with less than 30% loss in capacity is a good bar.

Dell laptop batteries are not really a relevant to the topic of this thread which is 'Electric Vehicle Battery' specs.

Yeah, but they are. There's not a whole lot of difference between batteries used in laptops and batteries used in cars. The Tesla Roadster for example uses glorified laptop batteries.

http://www.teslamotors.com/display_data/TeslaRoadsterBatterySystem.pdf

I'm not saying you're going to find laptop batteries in every hybrid or electric car, but there isn't really that big of a difference between the designs in most cases except for the packaging and maybe slight modification of the materials. Battery manufacturers like to make lots of small cells and likes to make them cheaply. This revolves around reliability standards. Its not to often you find a manufacturer making one large specific cell for one specific customer.

 

[mheslep]

...Yeah, but they are. There's not a whole lot of difference between batteries used in laptops and batteries used in cars. The Tesla Roadster for example uses glorified laptop batteries.

http://www.teslamotors.com/display_data/TeslaRoadsterBatterySystem.pdf

I'm not saying you're going to find laptop batteries in every hybrid or electric car, but there isn't really that big of a difference between the designs in most cases except for the packaging and maybe slight modification of the materials. [...]

Not so. The difference is large as a little background reading would quickly show. Laptop batteries use Co chemistries which have a fairly weak bond with the Oxygen atoms, worsening at high temperatures. It is the unbinding of O that contributes to the thermal runaway and subsequent fires. The newer Li Ion design uses FePO; P binds tightly to O, won't let it go. Commercial EV's do not use laptop chemistry, the Tesla being the only exception as it was early out of the gate, and Tesla used a sophisticated thermal system to compensate.

 

[mheslep]

You're taking what I said out of context. The specs that were determined in previous posts were assuming deep cycling along with heavy transient operation with discharge rates greater than 1C. That will kill a cell fast, especially if the cell isn't adequately cooled. The graph which you posted, and I always I have to explain, is for a steady state cycling at a charge/discharge rate of 1C. This will never happen in a real automotive application.

In this thread the https://www.physicsforums.com/showpost.php?p=2658594&postcount=6" a 15kWh pack with a 40kW motor load at max power/acceleration (2.7C), and with an average discharge over several hours at highway speeds, thus ~0.5C average. With an extremely heavy accelerator foot and poor heat dissipation on the battery pack, we can expect Li Ion to lifecycle to drop by perhaps half down to maybe 1500, but '100 cycles' is off by an order of magnitude.

 

[xxChrisxx]

No offence to anyone in the profession. But the above few posts outlines quite nicely why mechanical engineering is more cool than electrical engineering.

 

[pchoopanya]

Hello,

I know this is rude but I do have the same kind of project which is to propose the powertrain design of an electric bus.

I am kind of lost, do not where to start. Of course, I have read so many articles though. So I think I have some background knowledge, but I just want someone to point out how I can put these jigsaw pieces together.

Is it possible to tell me how I can calculate how big the motor will need to be? Are we talking about its delivered power in kW? Is rated power simply a maximum power?

All so, how do I do the same for a battery and a fuel cell?


Thank you very much

 

[mgb_phys]

I assume this is just a school project and somebody isn't trying to design an electric bus based off advice on the internet...

Find out how many HP a bus engine has.
Convert that to kw
Find out what range you need - how many hours.
Work out how many batteries you need for that number of KW-hours.
Realise why there aren't (m)any battery powered busses.

 

[pchoopanya]

1. The original bus (diesel engine) deliveres 170 kW of maximum power

2. But this whole powertrain is to be replaced by a hybrid fuel cell/battery electric powertrain

3. The bus is to be operating in Bangkok, Thailand. So, for each one round trip, without re-fuelling between the rout, it will be on the road for 3 hours and only covers about 50 km before returning to the start point where it can recharge its battery.

 

[mheslep]

Hello,

I know this is rude but I do have the same kind of project which is to propose the powertrain design of an electric bus.

...

China built a small E-bus fleet for the Olympics. You might try to get information from the manufacturer.
http://www.sinautecus.com/products.html#hybrid

 

[Ken Freeman]

I think one thing that's missing in both design exercises is a good analysis of the bus/other vehicle mission profile: speeds, accel/decel, climbing delta-H and climb speed, and find out what is actualy the reason for the necessary propulsion torque and power, and at what road speed. The comfort, convenience, and safety (lighting, electric braking, electric steering) and other vehicle controls subsystems need the same analysis to deternine how much to add into provide those services, and whether diversity of load can be used to reduce the actual system rating. The mission profile should include service/battery condition assessment intervals, as well as customer total mission duration (10years, 1.2M miles is not out of line for a heavy commercial vehicle, and buses might be in the same megamile ballpark).

Clearly, battery recharge is a key requirement that also needs to be understood.

 

[mheslep]

I think one thing that's missing in both design exercises is a good analysis of the bus/other vehicle mission profile: speeds, accel/decel, climbing delta-H and climb speed, and find out what is actualy the reason for the necessary propulsion torque and power, and at what road speed. The comfort, convenience, and safety (lighting, electric braking, electric steering) and other vehicle controls subsystems need the same analysis to deternine how much to add into provide those services, and whether diversity of load can be used to reduce the actual system rating. The mission profile should include service/battery condition assessment intervals, as well as customer total mission duration (10years, 1.2M miles is not out of line for a heavy commercial vehicle, and buses might be in the same megamile ballpark).

Clearly, battery recharge is a key requirement that also needs to be understood.

Carnegie Mellon has an online research project that provides much of that information.