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 Post subject: batteries' energy equivalency
 Post Posted: Sat Oct 03, 2009 3:08 am 
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Joined: Fri Sep 18, 2009 8:53 pm
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Location: University of Alabama
We're curious as to how the energy equivalency works for the battery bank(s). I've seen the equivalency equation and wanted to check what the 0.8 factor was exactly? Is it a correction factor for the batteries' capacity after the DOD has been accounted for? If it isn't, how is this usable capacity accounted for?


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 Post Posted: Wed Oct 07, 2009 3:59 pm 
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Joined: Tue Oct 30, 2007 6:50 pm
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Location: Illinois Tech
I think you are correct. I think that the 0.8 factor is put there because they are assuming that you will not be able to extract that last 20% of the energy in your batteries. That is why the minimum voltage of capacitors is not 0V but 10% of the maximum voltage.


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 Post Posted: Wed Oct 07, 2009 4:07 pm 
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Joined: Fri Sep 18, 2009 8:53 pm
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Location: University of Alabama
I thought batteries shouldn't be discharged past 40% charge otherwise permanent damage can be done to the batteries? Am I mistaken?


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 Post Posted: Wed Oct 07, 2009 4:41 pm 
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Location: Illinois Tech
I think batteries with different chemistry will have different acceptable depth of discharge levels. The point of the rule I believe is to provide a baseline of how much energy will be assumed you have in your accumulators when you start endurance. It is up to you to make the correct choice of accumulator type and size and drivetrain configuration to take full advantage of the allotted energy.


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 Post Posted: Wed Oct 07, 2009 5:11 pm 
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Joined: Mon Jun 05, 2006 4:05 pm
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Location: Rules Committee
Yes, and acceptable depth of discharge once in a race vs. every day for a year are very different things.


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 Post Posted: Wed Jan 20, 2010 12:41 pm 
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Joined: Tue Nov 10, 2009 2:10 pm
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Location: Wentworth Institute of Technology
Perhaps this has been answered, but I did not see it in the rules PDF:

At what discharge/C-rate is the Ah rating used for the calculation of battery energy?


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 Post Posted: Sat Jan 30, 2010 4:01 pm 
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Joined: Tue Oct 30, 2007 6:50 pm
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Location: Illinois Tech
I think the C number comes from the manufacturer.


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 Post Posted: Sun Jan 31, 2010 8:42 am 
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Joined: Wed Sep 12, 2007 5:22 am
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Location: NCTU MECE
The C number have nothing to do with energy calculation for FH.
The manufacture will decide which C rate they'll test it with.
Some company even provides data at different C rates.

However, the C rate you use greatly affect the actrual % of power you can get from the battery.

Li Battery typically have around 5C dis. and 1C (or under) charge.
But these numbers can be tweaked. By increasing the weight, you can get a high C rate cell.
eg. a 10C dis. 20Ah cell will be heavier than a 5C dis. 20Ah cell. (and bigger too)


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 Post Posted: Sun Jan 31, 2010 5:14 pm 
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Joined: Tue Oct 30, 2007 6:50 pm
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Location: Illinois Tech
The Ah in the energy calculation of the batteries is the C rating of the battery.

A 1C discharge rate for a 2Ah battery equals to a 2A discharge for an hour.
I don't see a reason why increasing the weight of a cell would increase its maximum discharge rate. It might increase its capacity and thus the maximum current you can draw for the same period of time but not the maximum discharge rate. So the numbers in your example are not possible. If you have 20Ah cell with a max discharge of 5C doubling its weight/volume will give you a 40Ah cell but the max discharge rate will remain 5C.


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 Post Posted: Tue Feb 02, 2010 12:58 am 
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Location: NCTU MECE
The reason for that involved the process of making Li cell.

The way they make Lithium cell is that they apply Anode material (usually graphite) and Cathode material (LiFePO4 or similar) on to thin aluminum sheet ( backing).
Then they apply the material and compress it to specified thickness. And put a barrier in between.

The amount of Anode and Cathode material dictates the actrual Ah of the battery. But how thin you apply it onto the backing will affect the C rate. (thin application will provide bigger surface area, which will allow more electron exchange.)

So if you want to have a high C rate cell, you'll need to apply the material in a very thin layer, which increse the use of the backing ('cause you'll need longer backing to accomondate the material), thus increased the total weight. (due to more backing presence in the cell)

This is how cell manufacture makes "Power type" and "Energy type" cell with same capacity, same material, but different C rate.

Of course if you keep the thickness constant, doubling the weight would double the capacity with C rate stay the same.


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 Post Posted: Tue Feb 02, 2010 5:15 pm 
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Joined: Tue Oct 30, 2007 6:50 pm
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Location: Illinois Tech
I think that there is a difference in terminology at play here. The constant C is usually taken to mean the capacity of the battery, which depends more on the chemistry used than on the plating thickness of the anode or cathode. The maximum discharge current, which you call the C rate is specified in multiples of the capacity hence 5C or 10C, and it is a function of cathode/anode construction and battery chemistry.

So to sum up:

C = Capacity in Ah or mAh. It is defined by the manufacturer and it is a very basic spec that manufacturers use to advertise their cells. It might change at different discharge rates but that change is not taken into account by FH rules.

xC = Discharge Rate in A. Manufacturer's usually list the maximum amps you can draw from a battery continuously and in bursts in multiples of the capacity. An example would be a 10Ah battery with a max discharge rate of 13C and continuous of 10C. This does not mean that at 13C you will get the same capacity as in 10C or that at 10C you will be able to get all 10Ah out of your battery. (The testing for the capacity of a cell usually happens at or less than 1C.


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 Post Posted: Tue Feb 02, 2010 10:30 pm 
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Joined: Wed Sep 12, 2007 5:22 am
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Location: NCTU MECE
I was talking about the xC part... lol


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