Range question

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Thalass
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Range question

Post by Thalass » Thu, 13 Sep 2007, 21:16

Hey folks, I've been looking at some battery manufacturer sites today, and I've got a bit of a noob question for you all.

With respect to range, which is more important: Ah rating, or Wh? I know increased voltage tends to give increased acceleration, so it's safe to assume that the amp-hour rating is proportional to the range (for the same vehicle). But I'm not sure - perhaps the watt-hour rating gives a more accurate estimate?

I ask because I'm perusing the LionEV site, and I'm comparing their Silver pack to the single cell Flex pack.

Two silver packs in series will give me 70V, 209Ah (so 14.63kWh - which ought to be more than enough for a bike, judging by other bikes on the album), and will cost me AU$6,502 (not including shipping)

However, 46 flex-cells arranged into two banks of 23 in series will give me 73.6V, 40Ah, and 2.944kWh (which is comparable to other bikes). But at 40Ah, I doubt it'll go far. And it'll cost me AU$3,929!

The Silver pack gets me more wH per dollar (Unless I'm screwing up the maths, which is entirely possible at the moment) than the Flex. But would the Flex have a decent range? It'd cost me sixteen grand to get a similar Ah rating to the Silver!

(This is using a DC motor, either the K91-4003 or the PMG-132, though of course this may change as I learn more)

Anyway, sadly I have painting to do, so no more number crunching for me. (good thing, too. My brain hurts hehe)
Last edited by Thalass on Mon, 13 Sep 2077, 21:26, edited 1 time in total.

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acmotor
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Range question

Post by acmotor » Sat, 15 Sep 2007, 06:20

Thalass,
Amps are half the equation, volts are the other. Watts = volts x amps. and simply Wh = V x Ah. Image
Remebering that Wh is the energy available, so if it takes twice the energy to go twice as far then thats twice the Wh required.
You can have 100V at 100Ah in a battery pack = 10kWh and thats the same as having 200V at 50Ah in a battery pack. Suitable choice of motor and controller matches these.

If you want acceleration you need high W (kW).
A 40Ah battery may be happy to discharge at 20C = 800A
Another battery of 200Ah may only give 3C = 600A
If the batteries weighed the same then the 40Ah unit would give the best acceleration (assuming they were both the same voltage).

There are many other factors involved as well of course.....
cost
peukert effect and W/kg (mostly effects the acceleration and peak loads)
weight Wh/kg
depth of cycle
number of cycles
temperature
volume
life expectency even if not used
... and then there are copper losses in wiring / contactors etc.
... and motor efficiency at 100%, 200%, 500% of rated current.
Image
iMiEV MY12     102,980km in pure Electric and loving it !

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Thalass
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Range question

Post by Thalass » Sat, 15 Sep 2007, 19:12

Ah, the power equation I know, but this peukert thing I've never heard of before. How is it derived? What is it?



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acmotor
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Range question

Post by acmotor » Mon, 17 Sep 2007, 04:49

Technically see http://en.wikipedia.org/wiki/Peukert's_law

But simply... you don't get all the power back when you discharge rapidly. Image Image Image
This varies with different battery types within the lead acid range and also with other battery chemistries.
Lithium batteries are better LiPos even better.

In an EV using lead acid batteries the Peukert effect can account for as much as 50% of capacity. So reseach and choose carefully.... not just on Ah.
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iMiEV MY12     102,980km in pure Electric and loving it !

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Thalass
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Range question

Post by Thalass » Mon, 17 Sep 2007, 05:03

Ah thankyew! *reads*

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