# Yet another Q from a Newbie



## Guest (Oct 30, 2008)

I assume you mean two strings into a 72 volt Alltrax controller? If so then you still have only 72 volts but more range due to the fact you have double the batteries. If you put those two strings in series then you will be pumping in 144 volts and MAGIC SMOKE HAPPENS. Can't make a 144 volt controller out of a 72 volt controller. You will only have 72 volts available to use. More batteries equals more weight and that means you go slower. Spend the money on the proper controller. It is an expensive part of the EV. No way around that. Don't skimp unless you are fine with 72 volts. 72 volts on a small car is fine but many want a bit more performance. So I say go for that good 120 to 144 volt controller and a good 9" motor. Make sure your controller is up to the task of the motor. Big Motor/Big Controller. 




c_rickey said:


> I have been reading here for a while and finally decided to actually post something. I am looking to attempt my first conversion in the near future and I am looking to do it on the cheaper end. My question is, would it be possible to run 2 strings of 72v into a 72v controller like the alltrax instead of paying almost $1000 more on a decent 144v controller? I am assuming the performance wouldn't be as good. I figure since I haven't seen any conversions like this its not that great, would the amperage be the problem or is it just the efficiency.


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## kugmo (Oct 31, 2008)

good luck on this conversion thought, 72v from 144v? hmm, I'm no expert though, keep this post updated so we know the progress


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## TX_Dj (Jul 25, 2008)

kugmo,

This pic from my scooter is an example of what c_rickey is talking about.










That photo has 72v worth of 12v batteries, but is split in half to run at 36v. Notice that at the most positive and most negative ends of the packs, there are two connections to each end. That is the parallel connection.

Those batteries are 6 Ah each, but because they are in parallel, they become 12 Ah. 

36V (nominal) * 12 Ah / 1000 = 0.432 kWh

At the same time:

72V (nominal) * 6 Ah / 1000 = 0.432 kWh

When running parallel packs, the amount of amps that need to be supplied by each pack is reduced, thereby minimizing the effects of peukert's law. 

However, when you run a higher voltage, you need less amps from the battery to provide the same watts, thereby minimizing the effects of peukert's law.

1 = 1.

But... and there's always a but, isn't there? With a lower voltage, your motor RPM is reduced, meaning that you can't accelerate or go as fast as you could if you ran 72v.

Moral of the story (imho)... if you can only carry 72v of lead, use 72v of lead. But if you can carry 144v of lead, use it as 144v of lead.

I ran parallel strings in the scooter because I am entering a competition in the 36v class, and really don't have enough room to fit 72v motor/controller, nor do I think I want to go that fast on this scooter.


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## vgslimo (Oct 21, 2008)

TX_Dj said:


> I ran parallel strings in the scooter because I am entering a competition in the 36v class, and really don't have enough room to fit 72v motor/controller, nor do I think I want to go that fast on this scooter..


lol thats funny,i can picture you doing 80mph and wheelies, good luck in competition and nice scoot.


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## c_rickey (Oct 30, 2008)

TX_Dj said:


> kugmo,
> 
> This pic from my scooter is an example of what c_rickey is talking about.
> 
> ...





Thank you for the info. I knew there were issues i just wasn't completely sure of the specifics. 

Anyway I am looking to convert a neon and hoping to get 40 miles on a charge with 60mph speeds. I am looking to keep the cost down so lifepo is of coarse out of the question I am thinking 12v lead acid. But I cant seem to nail down which motor and controller to use. I am thinking that the noise of the curtis would annoy me to high heaven but I am nervous about the Kelly because of the horror stories. I really look forward to this project. any help would be greatly appreciated.


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## TX_Dj (Jul 25, 2008)

Personally, I would avoid Kelly like the plague until they can resolve their quality and service issues. They show potential, but from what I've seen most of the potential is in an application as fire starter and/or kindling. 

The curtis whine is not really as bad as some people think. Get in touch with your local EV club and see if you can go for a ride-n-drive with someone there who has a Curtis controller. The whine is only present at low motor RPMs, as the motor looks like a short to the controller at those speeds, and it is far easier to limit the current at a lower frequency, hence the "whine" (because it drops from 15 kHz to 1.5 kHz when doing this). It's great for making a soft subtle noise to alert unwary pedestrians to your otherwise silent EV... then again, so is something like a soft-tone horn or whistle, or even a bicycle bell. 

The key thing to keep in mind with parallel strings is that if you have any mismatch whatsoever between the two strings, the stronger one will always try to discharge into the weaker one until they are "balanced". There is typically nothing to prevent this from happening, or to control the amount of current that will be delivered. This is not usually a problem for fresh batteries all from the same manufacture lot- however, as they get older, they will tend to stray farther from "matched" and it will exaggerate the problem. Additionally, if you have a failure in one cell in one string, it could cause a thermal runaway condition where the stronger string continually tries to dump current into the bad cell, and it could theoretically start a fire.

If you can fit enough batteries to go with 144v, that will do you best, especially if you can find a motor/controller combo that will meet your budget. If not, you can still acheive ranges of 40 miles with a 72v system, however your speeds will be lower, and you will need to be more diligent in keeping the battery amps down during acceleration in order to minimize the effects of Peukert's law.

The real keys to range are weight and drag- so if you can reduce those, you will do well regardless of what voltage you use, but battery quality is also a factor. No two battery types perform the same. All battery manufacturers advertise Ah capacity at the 20-hour rate, which is not applicable to EV use. Some batteries with identical advertised Ah capacities have wholly different Peukert's exponents, thus when you draw them harder (at the 1-hour rate or less) one can supply more capacity while the other will be dead sooner.

Hope it helps.


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## TX_Dj (Jul 25, 2008)

vgslimo said:


> lol thats funny,i can picture you doing 80mph and wheelies, good luck in competition and nice scoot.


80 mph would not be advisable on this scoot.  The steering rake is so close to vertical that it becomes very unstable at 15 mph. I am scared to see how it will do when I get my go-fast sprocket on there, which will give me a top speed of (still undisclosed and classisfied). 

It will wheelie as it sits right now, though. Last night I was trucking up a hill at about 12 mph, and shifted my weight back as I crossed a transition in the concrete. Between the full throttle, the jolt to the front tire from the transition, and my weight shifted rearward, the front tire rose to about 1 ft clearance, and gracefully dropped as I shifted my weight forward.

Can't complain!


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