# How to select what voltage/batteries to use?



## SCEV (Apr 10, 2012)

I'm new to the EV game, and planning to convert a Porsche 944 in the coming months (still on the hunt for the right donor). I'm trying to select my components (batteries, controller, motor, etc.), and just seem to be getting more confused the deeper I dig.

I'm trying to achieve a range of 70-80 miles, top speed of at least 90, and decent acceleration (0-60 in 7 ish seconds).

How do I determine my battery voltage and/or type of battery to go with? Do I start with selecting batteries, and then choose motor and controller, or is it the other way around?

Thanks,
Scott


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## rwaudio (May 22, 2008)

My logic was Motor - Controller - Batteries

Choose a motor that is large enough to maintain the continuous power you need (enough thermal mass to not overheat) and has an RPM range to work with your transmission/gearbox.

Choose a Controller with the correct voltage and current to work with your motor and will give you the desired power and torque.

Choose a Battery with high enough continuous and peak current to meet your power requirements, as well as range requirements.

You can always add more battery later to increase range but you should already have enough "power".

That was my logic, but there are others. The biggest thing to get your head around is the Battery is what determines the power for the most part, not the motor and controller. You simply choose the appropriate size motor/controller to allow the battery to produce the power you are looking for.

Using a Soliton1 + Warp 11HV with a 120v string of 60Ah prismatic cells won't give you much. (CALB/TS/Winston etc.)
Using the same Soliton1 + Warp 11HV with a 310v string of 60Ah A123 pouches will take you to an entirely different power level.

The trick is finding the right combination for YOUR BUILD. I (or anyone) can suggest what I am using but that doesn't mean it is the best thing for your car.


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## Yabert (Feb 7, 2010)

Hi SCEV

Range is related battery pack capacity (and speed).
Acceleration is related to peak power (and weight).
Top speed is related..... who care, with this acceleration requirement, you will probably achieve over 100 mph.



PThompson509 said:


> Assumption: 300 wh/mile.
> 
> 80 mi * 300 wh/mi = 24000 wh = 24kw.
> 
> ...


So, a 30Kwh battery capacity can be good.
30Kwh is a battery pack of 144v 208Ah or a 205v 146Ah or any other combination of voltage x Ah.

About acceleration, I thinks a good way is to compared peak power with existing 944. If 200 hp is enough to achieve you 0-60 goal, design your conversion around this peak power.


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## SCEV (Apr 10, 2012)

rwaudio said:


> Using the same Soliton1 + Warp 11HV with a 310v string of 60Ah A123 pouches will take you to an entirely different power level.


Hmmm...I noticed you mentioned 310V with a motor rated for 288V. I've also seen a lot of stats from EV builds where there battery pack voltage is higher than the rated motor voltage (I think most Warp motors are 170V). Does the controller drop the voltage going to the motor? I thought it just limited the amperage...


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## somanywelps (Jan 25, 2012)

SCEV said:


> Hmmm...I noticed you mentioned 310V with a motor rated for 288V. I've also seen a lot of stats from EV builds where there battery pack voltage is higher than the rated motor voltage (I think most Warp motors are 170V). Does the controller drop the voltage going to the motor? I thought it just limited the amperage...


No, it controls the voltage.

The Amperage is controlled by PWM from the controller.

Motor Amps >= Battery Amps

Motor Voltage <= Battery pack voltage


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## rwaudio (May 22, 2008)

SCEV said:


> Hmmm...I noticed you mentioned 310V with a motor rated for 288V. I've also seen a lot of stats from EV builds where there battery pack voltage is higher than the rated motor voltage (I think most Warp motors are 170V). Does the controller drop the voltage going to the motor? I thought it just limited the amperage...


The controller varies the PWM duty cycle of the battery voltage across the motor, even though they are 310v pulses the motor is a big inductor so the effective voltage across the motor is a product of the battery voltage and duty cycle. This means the effective voltage across the motor is lower or equal to battery voltage (not getting into back emf for simplicity)

There are some motor and controller experts here that could get much further in-depth on the subject but that's the very basics of it.

So in simple terms you can safely have a 310v pack using a soliton1 on a 288v motor or even a 170v motor with the proper settings in the controller. Others could go into more detail about efficiency/losses and other stuff at high battery voltage vs low voltage but it gets complicated.


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## SCEV (Apr 10, 2012)

somanywelps said:


> No, it controls the voltage.
> 
> The Amperage is controlled by PWM from the controller.
> 
> ...


So if I go with a standard Warp motor rated for 170V, I want my battery pack to be at least 170V, but higher is okay? What happens if it is lower?


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## ga2500ev (Apr 20, 2008)

SCEV said:


> So if I go with a standard Warp motor rated for 170V, I want my battery pack to be at least 170V, but higher is okay? What happens if it is lower?


Voltage and motor RPM are directly related. Lowering the voltage lowers the top RPM.

However, this point may be moot if the RPM associated with that voltage is above the normal operating range of the motor, i.e. past its redline.

A very good FAQ on all the issues you are asking about is the Warp/Transwarp owners manual. It discusses all of this and more. You can find a copy here.

As for the voltage, it can be higher because the motor acts as an inductor and integrates the PWM pulses into an average voltage. However, that average voltage cannot exceed the maximum rating of the motor, or it'll burn up. The advantage you get from the higher voltage is more overall capacity in Wh.

Focusing on voltage is probably not the correct approach. Your biggest problem is range. You need to figure out how to get the capacity you need to get the distance you require. Configure that capacity at 156V-170V and pick a motor that gives you sufficient RPMs to get the speed that you need. It's really as simple as that.

ga2500ev


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## Yabert (Feb 7, 2010)

SCEV said:


> So if I go with a standard Warp motor rated for 170V, I want my battery pack to be at least 170V, but higher is okay? What happens if it is lower?


With same amperage controller (let say 1000A) your peak power will be lower.
Kw = Volt x Amps


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## messki678 (Apr 21, 2012)

hi,
rwaudio. excellent answer. Thanks for this information. its helping


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## somanywelps (Jan 25, 2012)

Yabert said:


> With same amperage controller (let say 1000A) your peak power will be lower.
> Kw = Volt x Amps


What happens to power if you halve the amperage and double the voltage. 
(Assuming constant RPM from load.)


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## MN Driver (Sep 29, 2009)

ga2500ev said:


> Voltage and motor RPM are directly related. Lowering the voltage lowers the top RPM.
> 
> However, this point may be moot if the RPM associated with that voltage is above the normal operating range of the motor, i.e. past its redline.


That is leaving a good piece of information out though. It doesn't lower the top RPM, it lowers the performance at higher RPM.

For great acceleration performance you need to actually push to the max voltage rating of that motor at battery load, this requires that voltage sag is taken into account. For example a 192v 180Ah CALB(or most single unit prismatics of a similar Ah capacity) pack with a 1000 amp load will sag to about 150v or possibly lower and then back EMF will take over a bit. Should be fine for performance but you take a look at Rickard's chart of 57 (182.4v nominal) 180Ah CALB cells at a 1000 amp load peak out at about a 3200RPM speed with a Warp 9 at 145 volt under load. If the Warp 9 can take 170v to the motor, it would be better there. If we believe that it can take 192v to the motor, we need a battery that is beyond that voltage nominally to be there under load.

http://2.bp.blogspot.com/-iTPyxw58rqI/TaQ05v-hsBI/AAAAAAAACIc/zY-XNPj5myk/s1600/redux3rdgear-2.jpg



somanywelps said:


> What happens to power if you halve the amperage and double the voltage.
> (Assuming constant RPM from load.)


Let's put three different 20kwh battery combinations of the same capacity into comparison. These numbers are based on chart Jack Rickard posted using CALB cells(posted above), from a poster using 100Ah CALB cells and commented on the voltage sag on this site and from the chart that Sinopoly provided for their 100Ah cell and from CALB's published performance specs.

Low voltage 180Ah 20kwh pack
35 cells at 180Ah is 20160wh and 112v nominal
At 1000 amps it will sag to 89 volts and provide about 89kw

Mid voltage 100Ah 20kwh pack
63 cells at 100Ah is 20160wh and 201.6v nominal
At 800 amps it will sag to about 155volts and provide about 124kw

High voltage 20kwh pack
100 cells at 60Ah is 19200wh and 320v nominal
At 600 amps it will sag to about 250 volts and provide about 150kw

You need a motor that can handle 89 volts, 155 volts, and 250 volts under those loads, aka not arc to death, for this to make sense and it also depends on the torque per amp and whether or not your motor would arc under these circumstances. In my application, I'm going with a Soliton Jr and a Kostov 9" with a nominal(not max) voltage of 220v and can only need 20kwh in my car, the Kostov won't swallow 1000 amps and produces lots of torque per amp for a 9", so a Sol Jr. is perfect. You can take a guess which pack I'm using. If I was going with a Warp 9 motor, I'd go slightly larger on the pack and probably go with a Sol 1 if it could fit. Problem is that a Warp 9 is too long for my tiny car and I'm not going to compromise with an Impulse. I also like the idea that the Sol Jr is smaller and lighter so it's an easier fit and cheaper for me to buy. It all depends on the application, decide on the motor and controller you want and my suggestion is to aim higher rather than lower with the voltage so you can get great performance on the highway and not be pokey like the 144v lead-acid cars because they can't fit or handle the extra weight of a higher voltage pack and sag under the high amperage needed to try to meet highway speeds and actually perform at those speeds. In my case, I'd like to be able to go as fast as possible with the least battery stress as long as everything can take it.

My 2 cents, HTH!

Edit: I don't want to give Tesseract a heart attack so I'll post this before he need to post about this again in another thread. 350v is the absolute max so 100 cells is a bit high, be sure your fully charged voltage is safely under this with some buffer. Preferably 340v or the controller won't turn on. [Read: http://www.diyelectriccar.com/forums/showpost.php?p=296938&postcount=4 if you don't know what I'm talking about]

Also, those levels of current I posted above are pushing those batteries really hard but IMO they could do it for 10 seconds at a time if they aren't already hot and you aren't about to push them into a long haul up a hill or with a heavy non-aero car that takes a high load all the time. Take mind of your cruising load too. I'm not someone who does massive amounts of 0-60 runs with my foot to the floor and I'm aiming for a lightweight aero conversion this information wasn't meant for those people opposite to this or in any form of severe service. Monitor the temps of your battery terminals to be sure what I'm saying isn't getting your batteries toasty and do your own due diligence.


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## drgrieve (Apr 14, 2011)

You've left out the most important requirements.

Number 1 is budget.
Number 2 is time available, skill in electronics and mechanics and access to fabrication equipment.

The amount of options available differs vastly depending on the answer to those two questions.


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