# Is 5kW enough...?



## stealthhack (Aug 18, 2011)

Hi guys, from some time i'm collecting parts for EV conversion and i'm wondering can 5kW of constant power drive a car in a city driving with 45kmph(25mph)?
I'm asking people that have EV's to tell some information about power usage under various conditions(hill climbing, flat roads, etc.).
I still don't have donor car but will be in the small/compact class(2000-2900lbs(900-1300kg) weight converted).
Probbably if our Chevrolet dealers deliver Spark EV in my country will discontinue work on this project, but for now i'm on it.
My parts are: motor is AD AC24LS controller is [email protected](peak) Sevcon size 6, some very thick cables for the 3ph system, contactor for the main power line.
I also have some pretty decent quantity of 18650 cells (arround 200 different in capacities) from used laptop batteries, and counting(will have problems with managing so many cells), i hope they can sustain 3C at discharge.
Still collecting cash for BCAP3000 capacitors(40 as minimum + CMS), and still thinking for Hybrid drive.
Hope that will help with the main question.
Thanks in advance.


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## Ziggythewiz (May 16, 2010)

probably enough for a very light car...my bug uses a little more.

18650 cells and capacitors...sounds like a good way to spend the most time possible to get down the driveway.


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## palmer_md (Jul 22, 2011)

5kW is enough to maintain that speed, but going to be hard to get up to that speed with on 5kW available.

edit: ok I see you have 40kW on tap, that should work for city driving.


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## stealthhack (Aug 18, 2011)

palmer_md said:


> 5kW is enough to maintain that speed, but going to be hard to get up to that speed with on 5kW available.
> 
> edit: ok I see you have 40kW on tap, that should work for city driving.


Yea but thinking of putting 5kW generator onboard with Maxwell caps for a nice takeoff from standstill! 
But will do the work for city driving, maybe later will add some LiPo's to the car so to be valid for highway driving. 
The good news is that caps don't influence from sub-zero temps in winter, so the generator.


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## ga2500ev (Apr 20, 2008)

stealthhack said:


> Yea but thinking of putting 5kW generator onboard with Maxwell caps for a nice takeoff from standstill!
> But will do the work for city driving, maybe later will add some LiPo's to the car so to be valid for highway driving.
> The good news is that caps don't influence from sub-zero temps in winter, so the generator.


It's really not an EV if you need a generator to run it, no?

ga2500ev


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## stealthhack (Aug 18, 2011)

ga2500ev said:


> It's really not an EV if you need a generator to run it, no?
> 
> ga2500ev


It's probably a budget HEV but the energy needed for sustaining a road trip is in Hydrogen and Ultracaps.
Both are EXPENSIVE both are with future of developing.
LiFePo's can't deliver that Power to weight ratio, but a generator working on Natural Gas for example can!


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## Ziggythewiz (May 16, 2010)

stealthhack said:


> It's probably a budget HEV but the energy needed for sustaining a road trip is in Hydrogen and Ultracaps.


Ultracaps won't store any power though, they just buffer it. Spend the $$$ and weight on lithium instead.


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## CKidder (Dec 12, 2009)

Ziggythewiz said:


> Ultracaps won't store any power though, they just buffer it. Spend the $$$ and weight on lithium instead.


Not strictly true. Capacitors do store charge. If you do not believe an ultracap stores power then try this: connect one up to the proper max voltage. Now, disconnect it and put your hands on the terminals? Out of the hospital yet? How many fingers do you still have? I know what you mean though and your advice is sound. Spend the money on batteries not ultra caps. For the money batteries get you orders of magnitude more range.


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## ga2500ev (Apr 20, 2008)

stealthhack said:


> It's probably a budget HEV but the energy needed for sustaining a road trip is in Hydrogen and Ultracaps.
> Both are EXPENSIVE both are with future of developing.
> LiFePo's can't deliver that Power to weight ratio, but a generator working on Natural Gas for example can!


The problem is that once you start hybridtizing an EV, then very quickly it gets to the point where it doesn't function without the non EV part of the hybrid.

Just for a moment step back and describe your needs. Often when a poster starts throwing out terms like "sustaining a road trip" the specs very quickly start to balloon.

Let's start with a simple breakdown. Categorize the percentages of daily trips you will make with the proposed vehicle in the following mileage ranges:

1-10 miles
11-25 miles
26-50 miles
51-100 miles
100+ miles

The reason that I ask for percentages is that much of the focus gravitates to the last category. But the percentage of trips in that category is minuscule. For my needs for example this is probably about right:

1-10 miles: 25%
11-25 miles: 60%
26-50 miles: 10%
51-100 miles: 4%
101+ miles: <1%

I would like to say for the record that even with my current gas vehicle that typically I will rent a car for along road trip. Why? Because the number of trips that qualify is so few and I don't have to worry if my car has some issue on the trip.

A moderate lithium pack will handle 85% of my driving needs and a really good one brings that up to 95-99%. So it doesn't seem to make a lot of sense to spend significant resources and weight just to support the what if of "I need to jump in my vehicle right now and drive 300 miles."

If it turns out that supporting that last 5% is crucial, and there is even a minimum opportunity to plan, then the addition of a generator trailer would be a better bet. You can attach it when you need it, and detach it when you don't.

BTW Hydrogen doesn't produce energy. It only carries it. And all large scale hydrogen production is done by cracking hydrocarbons. So considering the effort and the expense, a fuel efficient gas vehicle is actually a better bet.

Might I suggest that you start with a clean sheet and go with just batteries. Design in as much lithium that you can fit/afford, then run the range numbers. Then and only then when you cannot fulfill your usual driving patterns should you look to alternative methods.

ga2500ev


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## stealthhack (Aug 18, 2011)

ga2500ev said:


> The problem is that once you start hybridtizing an EV, then very quickly it gets to the point where it doesn't function without the non EV part of the hybrid.
> 
> Just for a moment step back and describe your needs. Often when a poster starts throwing out terms like "sustaining a road trip" the specs very quickly start to balloon.
> 
> ...


In my EV country forum some guys sell cells from Nissan LEAF 22kW( 48 modules 7,6 Volts 60Ah each) for 6000$ without taxes(125$ each cell). 
I have 80 volt system so about 10-12 cells is the minimum to move the vehicle arround AND even after that there's low mileage with these cells, wich is sufficient for my daily trip to work and back(5-10 miles in each way) but nothing more. the cells are these:








Is it good investment 12 cells for beginning and maybe year or 2 later more advanced cells?
And to ask, how to make the BMS system for these cells(7,6V)?


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## Karter2 (Nov 17, 2011)

The Leaf packs are good choice for EV us ( obviously !) ..but you will be really stretching their performance with only 12 modules (<5kWhr).
You will need to limit the amp draw to avoid damage to the cells , and to make your 20 mile range.
EVTV are testing those packs currently..
http://www.youtube.com/watch?v=oZuu4...tu.be&t=24m46s

You will see each 7.6 volt "module" is made up of 4 3.8 volt cells in a 2s,2p config.
Also check the other threads on this forum that have discussed these packs.
http://www.diyelectriccar.com/forums/showthread.php/place-selling-nissan-leaf-batteries-92122.html


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## IamIan (Mar 29, 2009)

stealthhack said:


> LiFePo's can't deliver that Power to weight ratio, but a generator working on Natural Gas for example can!


The only relevant amount of power is the weakest link.

The 80V 550A Peak you listed is ~44kw.

Your other listed components aren't able to use the full ~120kw usable power from the ~45 Lbs of capacitors ... you'll only ever use a max peak of that ~44kw... and that is entirely different amount of power to weight ratio.

Capacitor Spec sheet lists 3.04wh each x40 = ~122 wh of stored energy ... from ~45+ Lbs

~44kw will eat through that ~122 wh of capacitor energy in about ~10 seconds... while the Voltage of that capacitor stack drops like a rock... which will make it very difficult to keep that ~44kw long before 10 seconds.

The same ~45 Lbs of Good quality LiFePO4 can also do that ~44kw.

For example ... Going by the spec sheet for the A123 20Ah pouch cell ... that same ~45 Lbs could do up to 41 cells ... if we do 40 cells ... 2p20s ... you could see up to a max of ~60kw @ 90%SoC and down to a max ~32kw @10% SoC... but also worth considering is that in the same ~45 Lbs instead of just ~122wh of energy storage you'd have almost 20x as much stored energy in such a battery pack.

Although a ~45 Lbs ~44 kw Natural Gas Generator is also a fairly tall order... I don't know of any with that high of a power to weight ratio... maybe up to ~20kw in ~45 Lbs ... but I don't see ~44kw power to weight ratio for a natural gas generator.


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## stealthhack (Aug 18, 2011)

IamIan said:


> The only relevant amount of power is the weakest link.
> 
> The 80V 550A Peak you listed is ~44kw.
> 
> ...


The capacitors may not discharge entirely but will be sufficient to make soft discharge curve of Amperes from the battery, that will prolongs their life by 1-2 years i supose(even useful for LiFePo4, but mostly for Lead battery's). 
Naturally this is true in city driving with lots of start-stop situations.
44kW is sufficient for good accelerating of vehicles up to 2800-3000lbs i supose. 
Weight of sufficient power generator starts from 100lbs and above, speaking about i have 6,5kw DC 75V S3-15% motor(Forklift motor pump) in my basement that can put in work with a good natural gas motor(or even 1 piston diesel engine for good MPG). 



IamIan said:


> The same ~45 Lbs of Good quality LiFePO4 can also do that ~44kw.


Yea but not without damaging the cells(depends on how long you pressing full throttle).



IamIan said:


> For example ... Going by the spec sheet for the A123 20Ah pouch cell ... that same ~45 Lbs could do up to 41 cells ... if we do 40 cells ... 2p20s ... you could see up to a max of ~60kw @ 90%SoC and down to a max ~32kw @10% SoC... but also worth considering is that in the same ~45 Lbs instead of just ~122wh of energy storage you'd have almost 20x as much stored energy in such a battery pack.


Mate you're absolutely right about these pouch cells but did you consider that caps will outlast maybe 5-10 times longer(yea they can't store that much energy but will make the car with a good start at the green light even with less efficient batteries that are even cheaper and less complicated to charge).
+ At winter performance of caps and LiFePo's start to differ considerably.
I see everything is a tradeoff cons for pros and vice versa.


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## IamIan (Mar 29, 2009)

stealthhack said:


> Yea but not without damaging the cells(depends on how long you pressing full throttle).


Going by the spec sheet The same weight ~45Lbs 2p20s A120 20Ah pouch cells I referenced will be able to discharge at up to that ~44kw power rate for about ~1/2 of their entire Ah Capacity ... 44kw/40cells=1.1kw per cell the max discharge power graph per cell doesn't dip down that low until under 50%SoC.

So they could do that ~44kw discharge for about ~90Seconds straight compared to the less than ~10 Seconds of the capacitors ... and ~50% SoC is perfectly safe and fine for them without damaging the cells... 44kw of 'full throttle' is still just 44kw... AFAIK ~90Seconds exceeds your desired peak power usage pulse , and it's the same ~45 Lbs.



stealthhack said:


> I see everything is a tradeoff cons for pros and vice versa.


Very True.
Also you are true there are other potential life time operating effects differences to consider ... and the Capacitors do have their strengths ... no doubt about that at all.

All I intended was to make sure you are aware there are batteries that can do the power rates you are asking for ... and that you are aware and ok with the trade offs you are making.

Best of luck ... be sure to post pictures as you progress.


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## stealthhack (Aug 18, 2011)

IamIan said:


> Going by the spec sheet The same weight ~45Lbs 2p20s A120 20Ah pouch cells I referenced will be able to discharge at up to that ~44kw power rate for about ~1/2 of their entire Ah Capacity ... 44kw/40cells=1.1kw per cell the max discharge power graph per cell doesn't dip down that low until under 50%SoC.


Did the Leaf cells can do the same discharge or their sag is bigger?


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## Duncan (Dec 8, 2008)

About the caps

_Capacitor Spec sheet lists 3.04wh each x40 = ~122 wh of stored energy ... from ~45+ Lbs

~44kw will eat through that ~122 wh of capacitor energy in about ~10 seconds... while the Voltage of that capacitor stack drops like a rock... which will make it very difficult to keep that ~44kw long before 10 seconds._

Unfortunately you need to drop your entire voltage to get your 10 seconds of power,
If you still have the battery connected the voltage drop will be limited by the battery,

You may get a 10% voltage drop - and get your extra burst for a whole 1 second, more likely less than half that

The only way to get a decent amount of that 122wh back out is to have a fancy high power variable voltage booster (buck converter) on the capacitors boosting the dropping voltage to feed to the main controller - that is doable but very expensive


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## IamIan (Mar 29, 2009)

stealthhack said:


> Did the Leaf cells can do the same discharge or their sag is bigger?


I've been told the Leaf Cells can handel sustained rates of 4C without issue or damage ... which is less than the A123 Spec ... but sorry, I don't have a spec sheet for the Leaf cells to see how far they can be pushed ... and for what % of the SoC window.


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