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70 Posts
Discussion Starter #1
Here is a theoretical question that I am sure has been answered way too man=
y times, but here goes again.

Small car - 2500 - 2800 lbs converted
runs on 12 golf cart batts

Two setups (same small car):

#1 - 12 ea 6 V golf cart batts, 72 V system, 9" ADC motor, Alltrax 7245, 5 =
speed manual transmission
#2 - 12 ea 8 V golf cart batts, 96 V system,7" D-DMotor,Curtis 1231C,=
4 speedmanual transmission

Here is my take:
Range - Car #1 wins. 6 V GC batts will get you farther than 8 V GC batts=
. Smaller controller / lower voltage is easier on the batts. This see=
ms to be everyone's experience that goes from 6 V to 8 V. You lose range=
and batt life.

Performance - Car #2 wins. Hands down, #2 will accelerate better. May=
be 20-30% better

Top speed - Car #2 will be slightly higher, but that 7" motor isn't helping=
. The 5 speed trans and9" motor on #1 helps compensate for the lower =
72 V car.

Batt life - #1 wins. 6V last longer than 8 V. They are being treated =
nicer by the smaller controller and more efficient motor. #1 wins by a l=
ot in this category. Also, since range will be less on the 8 V car, give=
n the same driving conditions, #1 has the added advantage of less DOD.

Car #1 is my car, and I've run it all the way from 132 V on 12 V AGMs down =
to 72 V on 6V GC. I've run it on almost everything in between 84 V, 9=
6 V, 120 V both 6 and 12 V batts. I never used any 8 V batts because my =
experience with 12 Vwas so bad I went direct to 6 V. 16 batts is too =
much weight for it. 14 is iffy. 12 is just about right for weight.

The question is: Did I make the right choice with the 6 V GC batts, or s=
hould I have gone with 8 V GC bats? I put the car up for sale a while ba=
ck. Now pulled it off the EV Tradin post, at least for a while. I alw=
ays get the same questions. Why are you running only 72 V? Is this ca=
r really 55 MPH capable? Well, I've driven it that fast, but it isn't as=
fast as on 96 V. Ithink I made the right component choice for a smal=
l commutter car that rarely goes 55 MPH, typically only 40 MPH. It seems=
the rest of theworld disagrees. So, should I have gone with the 8 V =
batteries? The car is no longer advertized for sale, and I may just keep=
it. If I replace the pack with 8 V and the controller with a 1231C, how=
much of a range hit and battery life hit will I take? All for the sake =
of higher top speed (which I never use) and faster acceleration (which I li=
ke, but is hard on the batts).


=

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Discussion Starter #2
Steve Powers wrote:
> #1 - 12 ea 6 V golf cart batts, 72 V system, 9" ADC motor, Alltrax 7245, 5 speed manual transmission
> #2 - 12 ea 8 V golf cart batts, 96 V system,7" D-D Motor, Curtis 1231C, 4 speed manual transmission
>
> Here is my take:
> Range - Car #1 wins. 6 V GC batts will get you farther than 8 V GC batts. Smaller controller / lower voltage is easier on the batts. This seems to be everyone's experience that goes from 6 V to 8 V. You lose range and batt life.
>
> Performance - Car #2 wins. Hands down, #2 will accelerate better. Maybe 20-30% better
>
"Performance" is what is causing the reduction in range and battery
life. Drive it the same way, and it will get the same battery life and
range.

Cory

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Discussion Starter #3
All other things being equal, if the 8v and 6v batteries weigh the same, the
range will be nearly the same.

And if you drive the car the same way, the service life should be about the
same. Think about it. A 6v battery's reserve capacity is usually specified
at 75 amps, while an 8v battery's RC is usually given at 56.25 amps. The RC
current is usually a pretty good indication of what the manfacturer thinks
the battery can supply in routine service. Do the math : 6v * 75a = 450W,
and - no surprise - 8v * 56.25a = 450W.

So suppose you have a controller with a 400a battery current limit. That
will limit your POWER demands from the 96v pack of 6v batteries to 96 * 400
= 38.4kW.

If you now put in a 120v pack of 8v batteries, but you limit your
acceleration power to that same 38.4kW (say by turning down the controller's
current limit to 320 amps), you should get nearly the same service life from
your new pack of 8v as you got from the old one of 6v.

But most people don't do that. Usually the reason they put in the 8v
batteries is to get more power so they can have more acceleration and higher
top speed. And with everything else the same, they get it - 120v * 400a =
48kW.

And they put their right feet down and use that extra power. So now they
are asking the 8v batteries to produce the same amount of current that the
6v batteries produced. In that case, they WILL NOT last as long as the 6v
batteries did.

David Roden - Akron, Ohio, USA
EVDL Administrator

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