My school converted a 1987 Mazda RX7 last school year and competed in the SMARTT Challenge. We did well in the competition, but this year we would like to make some improvements in the vehicle. We powered our vehicle with 12 8volt batteries, as we could not exceed 96volts. This year we are thinking about switching to 12volt batteries. We would have 16 12 volt batteries in a series parallel configuration. We would parallel each two batteries and then serial the 8 parallel connections. I want to know if that would be more efficient. The SMARTT Challenge has both a range competition and a speed completion. Most people set up for one or the other, but we are going for a middle of the road setup.
Battery Configuration
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In general, higher voltage will be more efficient as losses are relative to the current, and power is current times voltage. If you use a the same (or better - a higher) Ah capacity of the batteries you will have less voltage drop, and increase your range. Basically, if you drain lead-acid slower, you will get more energy than if you drain them fast. I guess you have already made sure the car can handle the higher voltage.
Lithium is Expensive, but better!
Lithium is Expensive, but better!
According to the rules of our competition, we cannot exceed 96 volts. I teach electronics, and I know that if you parallel two batteries together, your voltage stays the same, but you double your available current. Also by having 8 series connections as opposed to 12 series connections, I was figuring that I would reduce my total resistance. All of this is good in theory, but I do not have a lot of practical experience when it comes to electric Vehicle design.
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its more that 1 105 ah battery weighing the same as 2 50 ah's, is usually "better" it kind of depends on the particular battery, some are better than others.
you would be better off with 16 6v batteries in series, compared to 16 12v batteries of same dimensions (half the AH, double the volts) in series-parallel setup.
also, in this case, maybe standard car starting batteries make sense, you only use them a few times, (as opposed to as many times as possible) and they can deliver insane current, should have awesome acceleration.
and, if you ask me, if its a competition, its worth the few 1000$ extra to buy lithium, really, it is. easily doubles performance and range, at half the weight.
you would be better off with 16 6v batteries in series, compared to 16 12v batteries of same dimensions (half the AH, double the volts) in series-parallel setup.
also, in this case, maybe standard car starting batteries make sense, you only use them a few times, (as opposed to as many times as possible) and they can deliver insane current, should have awesome acceleration.
and, if you ask me, if its a competition, its worth the few 1000$ extra to buy lithium, really, it is. easily doubles performance and range, at half the weight.
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I see now that I misunderstood the question, sorry! 
Well, you seem to have got your answer though! But it would be much easier if you bough bigger batteries instead of parallelling them. And using 16 6 volt batteries, as was suggested would help you if some cells are bad (3 cells per battery instead of 6) in that you only need to replace 3 cells (one battery) if one cells goes bad!
I can confirm that your theory is correct in real world EVs as well
Do you need acceleration or range? For acceleration starting batteries would be good, as suggested above!
Well, you seem to have got your answer though! But it would be much easier if you bough bigger batteries instead of parallelling them. And using 16 6 volt batteries, as was suggested would help you if some cells are bad (3 cells per battery instead of 6) in that you only need to replace 3 cells (one battery) if one cells goes bad!
I can confirm that your theory is correct in real world EVs as well
Do you need acceleration or range? For acceleration starting batteries would be good, as suggested above!
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well, the reason why parallelling 12v batteries is bad, is that no battery is created equal, and if you have 2 in paralell, one is bound to get more loaded than the other. this can be a mismatch in cable length, or even a slight randomisation in better/worse during the manifacturing, just no battery is equal, and in paralell, they create trouble.
16x 100 AH 6v or 8x 105 AH 12v is pretty much equal, 120+AH 12v starter batts and better are quite commonly availible, probably the best choice in this situation, you can get a new one around the corner if one blows up, 6v are more rare, but would be cheaper to replace in theory. the golden middle road of 8x2 12v is actually not (see above).
16x 100 AH 6v or 8x 105 AH 12v is pretty much equal, 120+AH 12v starter batts and better are quite commonly availible, probably the best choice in this situation, you can get a new one around the corner if one blows up, 6v are more rare, but would be cheaper to replace in theory. the golden middle road of 8x2 12v is actually not (see above).
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Well, cable length should be negligible if we are not talking thousands of amps in too small cables! Don't forget that every battery itself is actually made up of several parallelled lead-plates (although in the same electrolyte, so it is still the same cell) and no cell is equal either. If some cell is bad you have trouble, and with the double amount of cells the probability for that is statistically doubled!
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you're totally right about the number of cells thing!
I just can't remember where I read this today, about discoloured wiring (that got hot on one side and not so much on the other because of parallel cells and unequal lengths of quite thick wire)
and we're probably talking about thousands of amps.. well, if it was my race, we would
I just can't remember where I read this today, about discoloured wiring (that got hot on one side and not so much on the other because of parallel cells and unequal lengths of quite thick wire)
and we're probably talking about thousands of amps.. well, if it was my race, we would
Ok, I have done a little more research for you guys. The batteries that I currently are using has a rating of 116 minutes at 75amps, and the ones that I was looking to replace them with have a rating of 77 minutes at 75amps, but with the parallel conection that should increase to 154 minutes at 75amps correct? But with the new set up I would have to add 4 batteries at a total of about 280lbs. The 12 volt batteries are deep cycle, not starters. I need more speed, without suffering much in my distance.
http://smarttchallenge.com/ (rules are there)
So, are you Weaver High School "rookie of the year"?
Do you have a web site to point us to?
If not give us as clear a description of the car as it stands now.
Also details of last year's performanxe.
Gerhard
PS are you student or advisor?
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no, more like 77 minutes at 150 amp, or 160-170 minutes at 75 amp. look into the peukert effect. (basically, the faster your discharge compared to the battery capacity, the less total capacity you're gonna get, and the other way around, if you double the battery AH on the same draw, it slightly more than doubles the time) combined with air drag, its best to drive 'slow' (40-50 mph?) at the range test..
We are Weaver Academy. We also have a web page, but it is not complete. It is
http://webstarts.com/WGRT
I am Richard Loftis, and I am one of the advisors.
http://webstarts.com/WGRT
I am Richard Loftis, and I am one of the advisors.
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not complete.. well, it looks quite good already, but theres.. well.. nothing at all about the car on there. it really needs a page with some photo's and details about the conversion ah well..
why choose deep cycle batteries by the way? or did you already have them?
ah well.. why choose deep cycle batteries by the way? or did you already have them?
Weaver Academy Electric Car Wins Awards
An electric car constructed by Weaver Academy students helped the school take home numerous awards in the SMARTT Challenge last weekend. The challenge is a national program designed to educate students about innovative alternative fuel technology. To prepare for the competition, Weaver students converted a gasoline powered Mazda RX7 into an electric car they call the WHIP. At the competition, the students participated in a range event, oral presentations, troubleshooting tests, vehicle design, school initiative, website design, auto cross and safety inspections with the car. Weaver earned first place in the oral presentation, third place in the range competition, third place in the auto cross, second place in design and the Rookie of the Year award for its first time entering the competition. Students and teachers from several different disciplines joined forces and spent countless hours creating the award-winning automobile. For more information, please contact Principal Pete Kashubara at 370-8282.
An electric car constructed by Weaver Academy students helped the school take home numerous awards in the SMARTT Challenge last weekend. The challenge is a national program designed to educate students about innovative alternative fuel technology. To prepare for the competition, Weaver students converted a gasoline powered Mazda RX7 into an electric car they call the WHIP. At the competition, the students participated in a range event, oral presentations, troubleshooting tests, vehicle design, school initiative, website design, auto cross and safety inspections with the car. Weaver earned first place in the oral presentation, third place in the range competition, third place in the auto cross, second place in design and the Rookie of the Year award for its first time entering the competition. Students and teachers from several different disciplines joined forces and spent countless hours creating the award-winning automobile. For more information, please contact Principal Pete Kashubara at 370-8282.
Alexander_B what type of batteries do you suggest. Remember that my comp. rules require lead acid batteries. Our current set up is twelve 8V batteries connected in series. We went with 8V over 6V in order to save weight. I know that the 6V batteries have a high Ah rating, but my caculation told me that the added weight would counter act the longer life. The motor that we have is 6.7" in dia. x 12.28" long. I think this motor is part of our speed problems. We are only getting about 40 mph on a flat surface. Would I be better off changing battery configuation, or spending my money on a bigger motor?
What is your drive train?
What is the brand and model of the present batteries?
Are you running up against the Gross vehicle weight limit?
Do you have space for a larger (or more) battery box(es)?
Gerhard
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You cannot tell the power of the motor?
And how much current can the controller deliver?
Lead acid can often deliver a lot of power at the cost of energy, so lack of speed is probably either because of motor or controller. Make sure you have the right gear ratio to let the motor run at it's highest efficiency rpm as much as possible!
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I cannot see why you should not be able to reach much higher speeds... 550 A @ 96 V gives you about 50 kW, more than the motor peak, and should be more than enough to reach higher speeds.
Have you tried different gears? Does the car accelerate good, or do you have slow performance over-all? Maybe the curtis limits current to protect from overheating? can you monitor the motor voltage and current while driving? Check voltage sag, maybe some of the batteries are poor?
I am running 120 V at 300 A max, and going 56 mph (top speed) is no problem, still have power left (top speed is limited by my motor RPM, which is max 7000 as I have fixed gear). My motor is 25 KW peak, about 34 HP that is, but I almost never use 24 KW... My controller can handle 600 A peak, and overheating is no problem in my case.
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