# Should I go for ah or voltage?



## fixallthings (Sep 4, 2009)

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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## fixallthings (Sep 4, 2009)

I guess this was a stupid question.


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## Ziggythewiz (May 16, 2010)

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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## fixallthings (Sep 4, 2009)

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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## Ziggythewiz (May 16, 2010)

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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## EVfun (Mar 14, 2010)

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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## LithiumaniacsEVRacing (Oct 9, 2010)

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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## LithiumaniacsEVRacing (Oct 9, 2010)

Ziggythewiz said:


> 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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## Bowser330 (Jun 15, 2008)

in my humble opinion building a pack with 240AH of A123 20AH cells is ridiculously over building amperage of the battery [email protected] 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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## Ziggythewiz (May 16, 2010)

LithiumaniacsEVRacing said:


> 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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## LithiumaniacsEVRacing (Oct 9, 2010)

Ziggythewiz said:


> The controller tends to convert volts into amps at lower speeds.


Only if the battery pack can produce the amps needed, and the controller settings are correct. All I am pointing out is if you build a car with low amperage (500 MA) and increase the voltage (400 Volts), your take-off will be very sluggish but after you get moving the voltage will speed you up. Now, if you build a car with high amperage (3000 MA) and stay with moderate voltage (290 volts), you will launch like a rocket, to maintain your quick "take-off" you can keep down shifting and utilize them amps. You can also have it both ways if you buy a Shiva!


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## Ziggythewiz (May 16, 2010)

So 500 megaamps is low amperage now?


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## steven4601 (Nov 11, 2010)

Power is power, how sad it appears some keep on pondering about torque and more torque. For a drag racer, start-torque translates to take-off acceleration. Very nice, that makes wheelies pop in the air.

Once the vehicle is moving you need power! And power is what generally wins the race. Ron is confusing that he is actually building a high power vehicle, the torque follows automagically because of the power input OR if you like to see in the other perspective, Ron is building a very high torque & high rpm vehicle that produces a lot of power. 


Back to the question of the thread-starter

In an ideal world, it doesn't matter, A battery pack that is 420V 36Ah or 144V 100A. But in practice higher voltage does have some advantages regarding total system efficiency. Resistive losses go up by current * current. Also denoted as I²R losses.

If you want 100kW battery power, 
420V pack
total resistance 0.3 Ohm
@ 300A discharge current provides:
420 - 300*.3 = 320V
Power = 320 * 300 = 96kW output power, 27kw losses.

144V pack
total resistance 0.06 Ohm
@ 1000 Amps
144 - 1000*.06 = 84V
Power = 84V * 1000 = 84kW output power, 60kw lossses.

84Volts will seriously hamper the torque @ higher rpms for any motor.


That is the same reason power lines do not use 110V/230V but 50 to 400kV for power transmission across larger distances.

Note: 420V 0.3 Ohm is not random data, but data from my little traction pack in the Z3. The 144V pack data is what I could calculate as accurate for 100Ah LiFePo4 cells, correct me if im wrong.


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## major (Apr 4, 2008)

steven4601 said:


> But in practice higher voltage does have some advantages regarding total system efficiency. Resistive losses go up by current * current. Also denoted as I²R losses.......correct me if im wrong.


Hi steven,

You are correct with the example, however wrong to apply the result universally. A fair comparison would be to use the same cells for both the high and low voltage pack. For instance, take your 3P130S 12Ah Headways and reconfigure to 9P43S. Calculate the power at 8.33C (300A and 900A respectively) and you will get equal results.

Nice Z3 conversion BTW.

major


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## steven4601 (Nov 11, 2010)

Hi Major,

Now I come to think of it, with identical cells the loss within the cells of the pack would be identical. Good catch. 
(Putting 9 in parallel would be quite annoying. I was quite fed up with putting 3 in parallel already)

Moving on to the crimps, connectors, contactors, power cables, motor... I²R loss still applies. Higher losses with low voltage systems are inevitable etc 


Thanks 
//Steven


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## major (Apr 4, 2008)

steven4601 said:


> Moving on to the crimps, connectors, contactors, power cables, motor... I²R loss still applies. Higher losses with low voltage systems are inevitable etc


Not inevitable, but perhaps uneconomical to use the conductor size required to mitigate resistive losses at high current  And the same applies to motors. Low voltage motors can be designed to match the efficiency and power of the higher voltage ones down to the point where you need less than one turn per coil, and even then you could go to homopolar machines. 

I am not advocating the use of super low voltage systems. Just saying that efficiency argument really should be replaced by economics


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## LithiumaniacsEVRacing (Oct 9, 2010)

steven4601 said:


> Power is power, how sad it appears some keep on pondering about torque and more torque. For a drag racer, start-torque translates to take-off acceleration. Very nice, that makes wheelies pop in the air.
> 
> Once the vehicle is moving you need power! And power is what generally wins the race. Ron is confusing that he is actually building a high power vehicle, the torque follows automagically because of the power input OR if you like to see in the other perspective, Ron is building a very high torque & high rpm vehicle that produces a lot of power.
> 
> ...


Yes, power is power, but you do not need high voltage to gain speed. If your pack and your controller continues the same amount of amperage (not raising voltage) through your entire race, you can use down shifting to gain speed just from torque. 

Example:

My 2011 Drag Car started the season with two 2000 amp Zilla controllers and no overdrive. The pack voltage 200v. The rear diff ratio 3.25. I was stuck at the mid 10's until I added the overdrive unit. I was not able to up the motor voltage because I was at max 190v per motor already, so we looked for other options. Knowing we had enormous amounts of torque, we utilized it by down shifting with the OD unit. During the race I click the OD button and you could feel the car speed back up until the end of the race, this speed was produced by gearing and torque, no extra voltage was added. The rpms would drop and allow the torque to start pulling all over again, this process can continue with a steady stream of amperage and additional gears.


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## steven4601 (Nov 11, 2010)

Hi Ron,

Are you sure about down shift? Down as in gear-ratio sounds logical, not in the physical number of the 'gear' though..

What you might where experiencing is the power fall off that could be compensated by going into a higher gear where the motor through the wheels delivered more torque*1 even with the lower gear-ratio. 

*1 and power


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## fixallthings (Sep 4, 2009)

Thank's for all the information. It's becoming a liitle clearer how things fit together. Hard concept for an ICE guy to grasp that you have to factor in the size of the gas tank (amps and volts) in the equation. I'm still wondering how you figure out power requirements. Bowser, how did you arrive at 284wh/mi for power consumption? Would that be a normal power draw for a 4000 pound car at 60mph? Why is excess amperage overkill? Doesn't the controller limit the amount that you can actually use? Is 1000A common in street driving or is it just something for racing? Sorry for all the questions, but this is just so different from what I'm used to. I can fabricate just about anything I have to, but I tend to overbuild. More, heavier, stronger is better, but it seems that is not the case here. The real goal for me is to make something that if it had an engine soundtrack playing could not be differentiated from the gas version of the same car. I want to build something that anyone could accept, knowing that they wouldn't be giving up anything to go gas-free. I'm still going through all the forums, watching EVTV, and all the YouTube I can for information. There's just so much. Thank you again for the help.


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## Bowser330 (Jun 15, 2008)

fixallthings said:


> Thank's for all the information. It's becoming a liitle clearer how things fit together. Hard concept for an ICE guy to grasp that you have to factor in the size of the gas tank (amps and volts) in the equation. I'm still wondering how you figure out power requirements. Bowser, how did you arrive at 284wh/mi for power consumption? Would that be a normal power draw for a 4000 pound car at 60mph? Why is excess amperage overkill? Doesn't the controller limit the amount that you can actually use? Is 1000A common in street driving or is it just something for racing? Sorry for all the questions, but this is just so different from what I'm used to. I can fabricate just about anything I have to, but I tend to overbuild. More, heavier, stronger is better, but it seems that is not the case here. The real goal for me is to make something that if it had an engine soundtrack playing could not be differentiated from the gas version of the same car. I want to build something that anyone could accept, knowing that they wouldn't be giving up anything to go gas-free. I'm still going through all the forums, watching EVTV, and all the YouTube I can for information. There's just so much. Thank you again for the help.


284wh/mile was a guess and was used for easy math...

All the EV calculations can be closely compared to ICE hp/torque numbers, you just have to learn the connections.

Look at range this way....a typical normal ICE car uses ~20hp to cruise @ freeway speeds (~60mph)....So since 20hp = 15kw....then you can say the car needed 15kw to cruise @ 60mph....If you want watt hours per mile (wh/m) all you have to do is change the units around....15,000watts = 60miles/hour = 250wh/mile. Now since you are adding weight due to heavy batteries, etc. I would assume anywhere from 15-20kw would needed to cruise @ 60mph, this would be anywhere from 250-333wh/mile...

you can surely have excess amperage, yes the controller will only use the amount of amperage you program it too, but what I meant by that comment was that your batteries are better served in Series where they will add voltage and allow your motor to perform at its most optimal.

Just like in an ICE car the peak power out of battery in an EV is temporary.
Lets use the 20AH A123 cells for example again.... 
The specs on the cells are
3.2V
20AH
20C safe-burst ("C" = multiply AH by the #C to get maximum safe current output) = 400A (20C * 20AH = 400A)

Now if you connected one cell to a controller and programmed the controller to draw 20A from the battery it could do it for 1 hour (20AH) till the battery was dead empty. But if you wanted to draw the peak amperage (400A) from the cell, very roughly speaking, it would be empty in 3 minutes... 
20A = 60min
200A = 6min
400A = 3min

So now multiply that up to your whole pack...If you have a 60AH pack (3 cells in parallel) they can put out 1200A peak for 3 min!


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