# Relationship between Volts, Amps, and Acceleration



## 3dplane (Feb 27, 2008)

Hi ronin! If you want to look at it this way:volts=top speed ,amps= acceleration,amp/hours(capacity/batt size).=range.You also need big enough batteries so they are able to hold their voltage under load(accel.)In your case of 18 miles range you probably get away with 85-115 A/H batts.(marine batteries or so)I'm not sure about 80 MPH unless your car is really aerodynamic.See what others have to say


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## ronin4sale (Jan 29, 2008)

So if I went with 13 12v 100 A/H batts using a WarP 9" motor and a Curtis controller, I should be in good shape? Would there be any advantage to using a Zilla 1k? I have heard these are the only ones that can handle a high amperage for quicker acceleration which I am hoping to achieve, but I wouldn't want to spend the money if the batteries couldn't produce much more than the Curtis could handle. I cant seem to find information on batteries detailed enough to tell me what I want to know. 100 A/H means it can hold 100 amps for 1 hour right? To use the zilla I would want batteries that could produce close to 1000amps right?

Whats does CCA mean? http://www.eastpennunigy.com/assets/base/0909.pdf the 8GU1HM for instance...

If I had 13 of these, I would have a 156V system, but what would be the max amps it could output instantaneously? is that the CCA number? 

The Warp9 motor says its recommended for voltage between 150-160, but whats the recommended max amps? With 8GU1HM would my AH be only 21? Or how does that work? This battery thing is by far the most confusing thing with building an EV (to me at least). I thought batteries in series you add Volts and in parallel you add Amps right? So if you think I would need 100ah I would have to go with the 8GGC2M? That seems like a lot more than I was thinking I would need.


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## Dennis (Feb 25, 2008)

> So after looking over everything I want from the car. I have decided that since my work is only 9 miles from my house, I would rather increase performance than range. *I know that Volts = top speed, and Amps = range? How does it all relate to acceleration though? Would I want a high amp battery with a lower Amp hrs? High voltage gives me a higher top speed i know.*
> 
> My commute will be about 25% back roads and 75% highway. So I'd like to get a top speed of around 80, but be able to drive it comfortably at 70. I'd also like to get under 10 seconds 0-60 time. I think with only requiring a 20 mile range on the vehicle with a mixture of hwy speeds and back roads it shouldnt be too hard if i choose the right battery.



The voltage can also play a role in acceleration, because it effects the current that flows through the motor when under load. For example when you floor the accelerator, a motor at 48 volts with a 300 AMP limit on the controller may develop the 40 ft*lbs from 0 to 2000 RPM while at 72 volts it could be 40 ft*lbs at 0 to 3500 RPM. 

Now you may stop me right here and say WAIT A MINUTE! How is a series wound motor developing the same torque from 0 to 2000 RPM when the dyno sheet shows the curve decreasing rapidly as RPM's increase??? Well to answer that question it has to do with the controller. What the controller does is feed the motor voltage until the current limit is reached *if you have the accelerator pedal floored( keep that in mind)*. The controller if programmable can be set to feed the voltage rate to the motor really fast at full pedal conditions until the current is 300 amps, then it slows down the voltage feed rate to keep it at 300 amps. Eventually though the motor's back voltage that opposes the voltage feeding it will get to the point that the voltage difference is not enough for 300 AMPS to flow through the motor's electrical resistance so the torque will begin to fall off as its RPM's increase.


Here is a picture to help convey my point:


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## theboy16 (Feb 28, 2008)

13 12 volt batteries would give a total pack voltage of 156volts, correct me if i'm wrong, but the curtis only goes up to 144?

anyways, what makes you go is POWER. you can deliver power in two ways:

with high voltage, low amps or, with low voltage, high amps 

power = voltage x amps. 

high currents do, however, generate high magnetic fields in the motor, which makes high torque, which will accelerate you quicker.

for range, you want a high storage capacity, measured in amp/hours. that's how many amps you could pull from a battery for one hour before it gets to 80% depth of discharge (DOD)

as for the controller, keep this in mind: on the battery side, there is always a 156v input (for your 13 batt set up) and a varing current, depending on speed/acceleration/other factors
on the motor side of the controller, its varying the voltage and current to control the speed of the motor. when you accelerate from a stop, the motor side of the controller is set to a low voltage, very very high amperage, to produce torque. you'll see in a sec why this is important

controllers are rated at a max amperage. that amperage is not only the battery amps, but the motor amps, too. so even if you pack can't put out more than 400-500 amps at any given time, the controller might put out 1000amps to your motor during accelerating (by lowering the voltage).

1000 motor amps would be quite peppy, but if your curtis only handles 500-550 amps, then we limit ourselves in terms of performance and/or fry the curtis controller

remember, battery amps and motor amps are two different numbers, which can't exceed the controller limit

ie batt input to the controller: 156 volts, 200 amps 
contorller output to the motor:40 volts, 780 amps
same power, different amp/voltage combinations
the curtis couldn't handle this above examples, the zilla could

hope this helps!

PS, can we get others to contribute their real life motor amperage readings under different load conditions? thanks


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## theboy16 (Feb 28, 2008)

CCA stands for cold cranking amps, a designation used on starting batteries.

starting batteries put out alot of amps for a short period of time, and are never discharged very much. doing so dramatically shortens their life.

starting batteries are meant to be used for a short period of time (start the vehicle) then be charged by the alternator immediatlely after. we can't use starting batteries in EVs, because of their physical construction. you might get 5 runs out of them before they're toast.

anyways, CCA is the current a battery can deliver continuously, for 30 seconds, at a temperature of 0 degrees farenheit


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## ronin4sale (Jan 29, 2008)

Could someone pick out a few batteries for me to choose from and explain their performance and why they would be good / bad? All this theory stuff is becoming confusing and not applying it to real products. Maybe show me one battery setup that would be idea for range and one that would be used in a race car / sports car?


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## theboy16 (Feb 28, 2008)

for range, you want the biggest capacity possible; measured in kilowatt-hours

13, 12 volt 21ah batteries give you a total of 13x12x21 = 3.276 Kwh, that's not a whole lot.

13, 12 volt, 100 am batteries gives you a total of 13x12x100 = 15.6 Kwh

most evs use 0.3-0.4 kwh/mile your 18 mile round trip would need 18x0.4 = 7.2kwh of batteries.

race cars, like the white zombie, have packs just big enough for a couple runs, otherwise the weight of a large pack would slow them down.

if you want to put 7.2kwh of batteries in your car, it would *just* be enough. if you want some power to spare run extra errands, or play with the power available to you, then you want a large pack, in the 80-100 ah/battery so that you have

13*12*80 = 12.48Kwh to 13*12*100 15.6 Kwh

as far as proposing actually batteries, that becomes kinda personal. for the same capacity, do you want to go with fewer, larger batteries? or many smaller batteries? it depends on the space you have to play with in your car.

also, when looking at batteries, not that the amp-hour rating is rated over 1, 5, or 20 hours. the 20 hour rate is higher than the 1 hour rate, this is because of the battery's chemical reaction rate. basically. the lower hour rating (usually 5 is marked, sometimes 1) is the most accurate for EVs


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## ronin4sale (Jan 29, 2008)

Thanks theboy16. That last post cleared up a lot of my questions! You should consider doing a wiki combining a lot of the information you shared with me. I would have to imagine a lot of people have similar questions and need explanations like that.


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## Dennis (Feb 25, 2008)

theboy16 said:


> 13 12 volt batteries would give a total pack voltage of 156volts, correct me if i'm wrong, but the curtis only goes up to 144?
> 
> anyways, what makes you go is POWER. you can deliver power in two ways:
> 
> ...


The current is limited at first start off from a standstill of the vehicle by aggressive pulse width modulation duty cycle change of the voltage ON time to OFF time ratio (almost 0% duty cycle, voltage average is near 0 volts) and gradually the ratio of the ON time to OFF time gets larger until the CURRENT LIMIT of the motor that the user has programmed is reached, also known as soft start. Have a look at my graph....... No controller is going to just blast out voltage of unknown magnitude because of the risk of destroying the controller even if the current limit circuit was monitoring the current levels. The current limit circuit can only respond so fast to a change in current. I hope you realize that a series-wound motor is basically a dead short circuit when it is initially at standstill.......


Actually it's not simply lowering the voltage that increases the current. However, the inductance of the motor and the capacitor bank of the controller is what allows this high current provided the voltage is changing rapidly which its is since it's pulse width modulation. It's similar to a DC-DC converter topology. Just realize that if I got a resistor and drop the voltage that the current will not increase. That is only possible with inductance/capacitance network and a rapidly changing voltage by switching the DC using transistors or SCR's. Also transformers can do what you are talking about (they require AC), but not raw DC from a battery going through a resistor to drop voltage as I have stated...



theboy16 said:


> 1000 motor amps would be quite peppy, but if your Curtis only handles 500-550 amps, then we limit ourselves in terms of performance and/or* fry the curtis controller remember*, battery amps and motor amps are two different numbers, which can't exceed the controller limit.
> 
> 
> 
> ...


 If the motor were stalled it would not damage the controller because the current limit circuits will change the duty cycle of the ON to OFF times of the voltage to a very low % such that the current will not damage the controller. If high currents are endured for a long duration then the controller will cut the current way back if it gets hot.... 




theboy16 said:


> PS, can we get others to contribute their real life motor amperage readings under different load conditions? thanks


It can be done by wiring a shunt in series of the motor circuit. Just measure the millivolts that are dropped across it with a high end voltmeter and apply the ratio specs they used, like 50mV = 500 amps, voltmeter displays 10mV*(500A/50mV)= 100 amps for example.


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## theboy16 (Feb 28, 2008)

awsome! thanks for clearing that up. i'm glad one of us is an expert on these things


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