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Old 02-28-2012, 12:32 AM
fixallthings fixallthings is offline
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Default Should I go for ah or voltage?

I'm starting to look at doing an electric because the cost of gas is going up (again) and the cost if lithium is coming down (finally). I've been reading all I can on the subject and watching as many videos as I can find and have gotten myself totally confused. (Talk about trying to take a sip from a firehose.) Anyway, I'm looking at a 91 Cadillac Seville with an electronic automatic transmission so I can use an idle function on the motor to provide A/C and control the shift points electronically rather than mechanically. I'm hoping to be able to switch this off when not needed to increase the range slightly. I figure that a Warp9 and Soliton should be enough for the motive power, now I just need to figure out the battery pack. I'm leaning towards A123 cells because of the flexibility they would provide in pack construction. Mr. Rickard has me very interested in his built-up 120ah batteries and I calculate that a 140V/120ah battery would require approximately 6ft or so of length. Thinking that I could fit one 6ft string down the center console and two 3ft sections behind the rear seats I'm wondering if it would be better to have them all connected in series and have the controller step the voltage down or connect them in parallel to double the ah and have the pack voltage be self-limiting? Would the higher voltage of the series connection prevent the sag during accelleration or would the 20C capacity of the cells do that with the parallel connection? Ideally I'd like to have a 50mi range at about 60mph because I do need to travel on the highway to get to a few places I have to go, but most of my driving is short in-town hops. Am I crazy to think this will work? Am I trying to push a rope uphill? Should I forget this and just throw myself into building a stripped Geo with a 500A controller?
Part of the reason for this build is transportation, but another part is to show what EVs can be. Nobody is going to think about a car that is small, uncomfortable, and costs $100k+, because as nice as those cars might be, they're not practical for everyone. If something like this could be put together for around $15-$18K then more people might notice. That's about the end of my rant. Any information will be greatly appreciated. Thanks in advance.
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Old 03-03-2012, 08:49 PM
fixallthings fixallthings is offline
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Default Re: Should I go for ah or voltage?

I guess this was a stupid question.
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Old 03-03-2012, 09:45 PM
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Default Re: Should I go for ah or voltage?

It helps if you can break your request into discrete questions. One huge block with a bunch of questions mixed in can be difficult to follow and/or respond to.

For the range and speed you mention, the 140V/120ah pack would definitely work better than splitting the voltage and doubling up on the ah. 70 volt would have a difficult time running highway speeds and higher voltage is generally preferred to minimize losses.
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Old 03-03-2012, 10:04 PM
fixallthings fixallthings is offline
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Default Re: Should I go for ah or voltage?

Thanks for the reply. I was wondering if 280V at 120ah would be better than 140V at 240 ah. There's a lot of room in that car if I do a bit of cutting.
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Old 03-03-2012, 11:19 PM
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Default Re: Should I go for ah or voltage?

I think most would say that as long as you're within the limits of your controller, more voltage is better.
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Old 03-03-2012, 11:52 PM
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Default Re: Should I go for ah or voltage?

The thing is, power is measured in watts, and watts equals volts times amps. 746 watts is 1 horsepower, but since motor efficiency will always be less than 100% it won't quite be 1 horsepower at the shaft.

So... better will come down to what you need for the parts you choose. Can the selected motor take the voltage? If not the controller can limit motor voltage (better controllers can) but that will limit power. If the chosen motor doesn't make a lot of torque but is good at revving up then more amps and less voltage will help you take advantage of that. Torque is set by amps and voltage pushes the rpm range up. Motors may make anywhere from 50 to about 110 ft-lb of torque at 400 amps (to pick a point) and the more torque they make the lower the rpm that will be at for any given voltage. What voltage can the controller take and how many amps can it dish out? How many amps can the cells you want to use put out?

I would recommend starting with a set of goals and then selecting a motor to suit that. From there the other parts choices begin to logically follow. Since batteries are still pricey and a conversion will take a good amount of your time I recommend starting with the smallest and lightest car you like -- be sure to start with something you really like.
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Old 03-04-2012, 05:46 PM
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Cool Re: Should I go for ah or voltage?

Okay, these are your options:

Massive torque (amps) with moderate to high voltage, car must be geared for down shifting to utilize your torque, low rpm. This style car is for short runs, drag racing. Rpm band 0 to 5000 rpm.

Massive Hp by higher rpms requires high voltage with moderate torque. The rpms increase with voltage. This style car is used for road racing, circle track. Rpm band 0 to 9000 rpm.
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Old 03-04-2012, 05:50 PM
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Cool Re: Should I go for ah or voltage?

Quote:
Originally Posted by Ziggythewiz View Post
I think most would say that as long as you're within the limits of your controller, more voltage is better.
Bad advice, you can have all the voltage in the World, but if the amperage is low then the pick-up will be very sluggish.

Famous quote by Enzo Ferrari:

"Horsepower sells cars, but torque wins racers"
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Old 03-04-2012, 06:30 PM
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Default Re: Should I go for ah or voltage?

in my humble opinion building a pack with 240AH of A123 20AH cells is ridiculously over building amperage of the battery pack...@ 20C (continuous) thats 4,800A!

You need to match the battery voltage and amperage with the motor and controller, the three are related...

you mentioned Warp9 and Soliton1 so lets work off of those for the moment...

240V pack with 120AH fits a lot better with those two components...

The 20AH A123 cells have been tested at 7C and only sag to 2.85V, all you need is 8.5C to make 120AH = 1020A, so lets assume 2.75V sagged @ 1000A draw

with 87cells in series (280V nom) this means it sags to 240V, thats more than you need to apply to your motor that is limited to 200V max....

you could tailor your pack down a bit to save money...237V 120AH is all you need of the A123 20AH cells....

237V sagged down to 2.75V per cell @ 1000A draw = 200V, the perfect amount for the Warp9, no more no less....8.5C is not too much draw for a 120AH pack, so you're covered there too.

73s6p = 459lbs. = 28.4kwh (@284wh/mile = 100 miles range) = @20$/cell = $8,760
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Old 03-04-2012, 07:57 PM
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Default Re: Should I go for ah or voltage?

Quote:
Originally Posted by LithiumaniacsEVRacing View Post
Bad advice, you can have all the voltage in the World, but if the amperage is low then the pick-up will be very sluggish.
The controller tends to convert volts into amps at lower speeds.
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