# Max Continuous Discharge and Peak Current..



## ev_geek (Mar 25, 2009)

Excuse the newbie question. I was trying to spec the LiFEPOS4 batteries and the vendor is asking Max Continuous Discharge and Peak Current for the vehicle so he can recommend appropriate protection boards...

Now, while I principally understand the terms...are the related to the motor controller we would use? And how does one arrive at the appropriate numbers for these?


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## dimitri (May 16, 2008)

Its related to how much power you expect to use to accelerate and sustain cruising speeds. Its limited by controller and motor choice and drives a decision of how many cells to get ( system voltage ) and the size of the cells ( AH size ). You'd have to give us more details about your EV specs to get any specific advise.

Hope this helps.


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## atzi (Jun 26, 2008)

ev_geek said:


> Max Continuous Discharge and Peak Current for the vehicle so he can recommend appropriate protection boards...


My example I believe would be a 144 volt battery pack and a 500 amp maximum controller. So peak current watts would be 72,000 watts or 72kw.
Maybe "max continuous discharge" would be about 250 amp at 144 volts or 
36,000 watts or 36kw. I say 250 amp from watching my amp meter and 250 amp is what I draw on a long hill running 55-65 mph.
Hope that helps some.


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## ev_geek (Mar 25, 2009)

I am planning to use a 48V 100Ah battery pack. The motor is a 3kW motor.
Based on this I figured the max continous discharge current would be 3000/48 or roughly 60Amps(considering the motor is running at max rating). Peak current I am assuming is related to acceleration...For the best discharge characteristic of the battery, limiting it to 100A would be the safest, isn't it? Ofcourse this may not give the best response...so, limiting it to 200A would be more appropriate?

The vehicle i am planning for has a 300-350kg laden wt and am not expecting it to go beyond 50kms/hour.



dimitri said:


> Its related to how much power you expect to use to accelerate and sustain cruising speeds. Its limited by controller and motor choice and drives a decision of how many cells to get ( system voltage ) and the size of the cells ( AH size ). You'd have to give us more details about your EV specs to get any specific advise.
> 
> Hope this helps.


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## Lordwacky (Jan 28, 2009)

ev_geek said:


> I am planning to use a 48V 100Ah battery pack. The motor is a 3kW motor.
> Based on this I figured the max continous discharge current would be 3000/48 or roughly 60Amps(considering the motor is running at max rating). Peak current I am assuming is related to acceleration...For the best discharge characteristic of the battery, limiting it to 100A would be the safest, isn't it? Ofcourse this may not give the best response...so, limiting it to 200A would be more appropriate?
> 
> The vehicle i am planning for has a 300-350kg laden wt and am not expecting it to go beyond 50kms/hour.


Most LiFePO batteries that i have seen are rated for 3C or greater continuous and 10 C or great Peak (surge) discharge. so if you have a 100ahr pack you should (depending on your C rating) be able to pull up to 3(100) = 300 amps from your pack continuously and 10 (100) = 1000 amps surge with out damage. Your controller however is most likely going to be your limiting factor, since your build is small. 

In general the Battery (depending on your C rating) is rated for up to 300 Amps continuous draw so I see no reason to limit the current to less then 300Amps as far as the battery goes. Safety, however is another question. you may to limit your current so you don't give yourself whiplash every-time you set off from a stop light.


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## dimitri (May 16, 2008)

ev_geek said:


> I am planning to use a 48V 100Ah battery pack. The motor is a 3kW motor.
> Based on this I figured the max continous discharge current would be 3000/48 or roughly 60Amps(considering the motor is running at max rating). Peak current I am assuming is related to acceleration...For the best discharge characteristic of the battery, limiting it to 100A would be the safest, isn't it? Ofcourse this may not give the best response...so, limiting it to 200A would be more appropriate?
> 
> The vehicle i am planning for has a 300-350kg laden wt and am not expecting it to go beyond 50kms/hour.


Clearly your battery is more powerful than your motor, so motor and/or controller will be your bottleneck. Even at full load your motor will draw less than 1C from the battery.

I'm not an expert, but I'm not sure that 3kW motor is enough to accelerate 350kg vehicle. I have a 2kW motor on my electric push mower and it hardly pushes itself...

You might want to look closer into your motor selection. Check some projects on EVAlbum and see what size motors people put on similar size EVs.


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## ev_geek (Mar 25, 2009)

Dimitri,
I hope you are wrong! Reva, an electic vehicle from India uses a 4.4kW motor and can drive upto 80kms/hr! I think the motor has to be tuned for higher torque.

http://en.wikipedia.org/wiki/Reva

The unladen wt of Reva is about 600kg and can take an extra 270kgs.





dimitri said:


> Clearly your battery is more powerful than your motor, so motor and/or controller will be your bottleneck. Even at full load your motor will draw less than 1C from the battery.
> 
> I'm not an expert, but I'm not sure that 3kW motor is enough to accelerate 350kg vehicle. I have a 2kW motor on my electric push mower and it hardly pushes itself...
> 
> You might want to look closer into your motor selection. Check some projects on EVAlbum and see what size motors people put on similar size EVs.


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## blackpanther-st (Apr 4, 2009)

ev_geek said:


> I am planning to use a 48V 100Ah battery pack. The motor is a 3kW motor.
> Based on this I figured the max continous discharge current would be 3000/48 or roughly 60Amps(considering the motor is running at max rating). Peak current I am assuming is related to acceleration...For the best discharge characteristic of the battery, limiting it to 100A would be the safest, isn't it? Ofcourse this may not give the best response...so, limiting it to 200A would be more appropriate?
> 
> The vehicle i am planning for has a 300-350kg laden wt and am not expecting it to go beyond 50kms/hour.


The question I would ask here is whether that 3K rating for your motor is the factory continuous rating or it's (EV peak) rating? most motors can handle up to 10X their rating for a few seconds to get off the starting line, and then something like 3 - 5X for a short duration while you accelerate up to speed. the size and weight of the motor determine how long you can sustain that acceleration over current for. 

Your 3K is about 4 HP; should be enough to sustain speed for your light weight, but you will probably be wanting more for acceleration. If that 3K is the continuous rating then you should be able to get it. 

Probably something in the order of 450A peak, and 200A for a 1 to 2 minute rating would be safe, but without seeing the motor or knowing its physical size and weight I can't say for sure. These ratings would give you a just over 12 HP for acceleration and a peak start up power of a little more than 28 HP.  It would be best to show your motor to someone with more experience, and get a good recommendation for it.


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## Hadleigh Reid (Jul 22, 2008)

How about a Warp ( with a 155v 1000amp LogiSystems controller?

I want to be able to deliver that from the battery after sag and all that. Any recommendations for a lithium pack?

Thanks


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## Tesseract (Sep 27, 2008)

Hadleigh Reid said:


> How about a Warp


You already asked this question in another thread. Please wait for someone to respond to your original post.


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## dimitri (May 16, 2008)

Hadleigh Reid said:


> How about a Warp 9 with a 155v 1000amp LogiSystems controller?
> 
> I want to be able to deliver that from the battery after sag and all that. Any recommendations for a lithium pack?
> 
> Thanks


I will answer in this thread since it seems more appropriate to the subject.

When planning LiFePo4 pack I recommend to go for max voltage first, then divide by 3.2V nominal to see how many cells you get. Then for given number of cells, pick the largest size you can afford / fit into the car / support the weight of / etc. Also, figure out approximate typical discharge current of the car of your size by looking at EV Album, and don't buy cells of smaller size than that, so you don't go over 1C during cruising speeds.

So, lets apply this to your example. 155V controller is your voltage limit, divide by 3.2V is 48 cells for 153V nominal pack voltage. However, when fully charged it may get to 3.8V per cell ( depends on your charger profile ), so controller will see 182V for a minute after full charge. Check with controller vendor to see if this is OK. If not, I recommend to lower to a more popular 45 cells config for 144V nominal.

Now, we don't know how big your car is, but since you mentioned Warp9 I would recommend bare minimum of 100AH cells, preferrably 160AH or more popular lately 180AH or even 200AH if you can afford and fit them. New blue cells use same form factor for all these 3 sizes, so its a matter of weight and cost.

Hope this helps.


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## tomofreno (Mar 3, 2009)

Say you want to accelerate from zero mph to 60 km/hr, or 13.9 m/s and your energy conversion system is 100% efficient and the vehicle is in a vacuum with no friction. The kinetic energy of a 350 kg vehicle at 60 km/hr (36 mph) is 97,222 Joule or about 27 Wh. To accelerate to this velocity in a modest 8 seconds or 2.2 e-3 h would require a power of 12,272 W or about 16.5 H.P. At 3kW it will take about 4 times this long or 32 seconds. 

In real life you of course have to consider not only the above inertia of the vehicle but also rolling and drag resistances and efficiencies of the drive train, motor, and controller, as well as energy dissipation in the batteries themselves as described by the Peukert exponent, so I would recommend a considerably more powerful motor.

Tom


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## blackpanther-st (Apr 4, 2009)

tomofreno said:


> In real life you of course have to consider not only the above inertia of the vehicle but also rolling and drag resistances and efficiencies of the drive train, motor, and controller, as well as energy dissipation in the batteries themselves as described by the Peukert exponent, so I would recommend a considerably more powerful motor.
> 
> Tom


I depends on whether that 3K watt rating is continuous or peek as I mentioned in my last post. If it is peek then you would be correct and this motor would be insufficient, but if that is the continuous rating (which is how most electric motors are rated) then this motor is right about the ideal size for his project according to your calculations and the expectations of normal driving.  
Performance or drag racing would be a different story.


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## tomofreno (Mar 3, 2009)

Yes, if peak power is about 3x or 4x 3kW then it should have sufficient power for acceleration. It should easily have enough power to push the car at lower velocities. For example, say the rolling resistance is 0.015 and drag force is about half the rolling resistance force at lower speeds. Then the rolling resistance force on level ground is 0.015*350*9.8 = 51.5 N. and drag force is about 26 N. The power (force times velocity) required to move at say 80 km/hr (~50 mph) is (51.5+26)*22.2m/s = 1720 W.

Tom


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## Bowser330 (Jun 15, 2008)

@Tomofreno

I was looking at your calcs and it got me interested in my own...sorry to sidetrack...but....

1200kg wanting to go to 100km/h(60mph) in 4 seconds....

1200kg x 27.78m/s = 33,333NT

33,333NT x 27.78 = 926,000J = 257.22WH

257.22WH x .0011Hr (4seconds) = 234kw

If I had a system of 300V and 800A = 240kw this would be possible....

TESS WE NEED YOUR 800A CONTROLLER!!!!


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## Nomad (May 8, 2009)

tomofreno said:


> Yes, if peak power is about 3x or 4x 3kW then it should have sufficient power for acceleration. It should easily have enough power to push the car at lower velocities. For example, say the rolling resistance is 0.015 and drag force is about half the rolling resistance force at lower speeds. Then the rolling resistance force on level ground is 0.015*350*9.8 = 51.5 N. and drag force is about 26 N. The power (force times velocity) required to move at say 80 km/hr (~50 mph) is (51.5+26)*22.2m/s = 1720 W.
> 
> Tom


Tom could you explain this a bit better? What each number means and stands for?


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