CaptainKlapton
10 µW
Hi all,
I have been working on a simulation program for electric motorcycles for the last year. Recently I have been focusing on the battery functions and realized that comparing cells in terms of lifetime energy output is not so easy to do. So here is what I came up with:
Most manufacturers give an approximate cycle life of the battery cells. But do we really care? I mean if the battery was only ever cycled to 50% DOD, the cycle life would approximately double from the spec sheet value for a given set of charge/discharge conditions. Or, consider the strangely realistic case where the cell is not cycled exactly the same way every time causing the cycle life count to get complicated or become completely useless. Counting cycles is not the best way, in my opinion, to get an idea of how long the battery will last because it is too application specific. Energy on the other hand is a constant measurement for all battery driven applications. Know how much you need? Then you can size a battery for it. So that gave me the idea for something I call a “throughput factor.” This number is a simply multiple of the beginning of life (BOL) nominal capacity of the battery. However, the number is calculated from the cycle life such that it reflects the number complete, nominal cycles to 80% of BOL capacity. That means all you have to do is multiply your cell count by the throughput factor and it will tell you how much total energy you can get out of the pack before end of life (EOL). This allows you can keep all your calculations working with values and units you already have with no reason to introduce a new term involving cycle count. It also allows you to more easily compare cells of different sizes and chemistries. For instance, say you are comparing LEAF cells and 20Ah A123's. The LEAF cell is about 1.67 times the Amp-hour capacity and it operates at a higher voltage. But the A123 maintains its voltage better and is rated for more recharge cycles. So which one will give you the highest energy output, aka throughput, for its useful life? Counting cycles won't help you. But if you use the throughput factor you can directly compare the two in terms of energy, which is what we really care about anyway. The one with the highest overall energy output, throughput factor times the number of cells, is the best option in terms of energy. Of course it is still dependent on temperature, mechanical pressure, current draw and all the other things that effect battery cycle life, but at a minimum the comparison between cells is now easier and more meaningful.
So here are the calculations:
Throughput factor = integral(capacity retention by cycle count)d_cycle (also known as the area under the cycle life curve)
Total throughput (Lifetime Energy Throughput) = throughput factor * BOL nominal capacity
Then I discovered this after I did all that math: See “Lifetime Energy Throughput” at http://www.mpoweruk.com/performance.htm
Make sense? The throughput factor is basically a fraction of the Lifetime Energy Throughput. Then the total throughput is expressed as a multiple of the BOL nominal capacity. So I reinvented the wheel a little bit but got to work through some things I never had before, plus I got some conformation that it is actually useful.
Here are the ones I have done so far:
A123 20Ah: 2918.7
EIG C020: 963.6
GBS 60Ah: 1859.2
And here are the graphs from the manufacturers I used to calculate those:
View attachment 1
Questions? Comments? Criticism? Have at it.
I have been working on a simulation program for electric motorcycles for the last year. Recently I have been focusing on the battery functions and realized that comparing cells in terms of lifetime energy output is not so easy to do. So here is what I came up with:
Most manufacturers give an approximate cycle life of the battery cells. But do we really care? I mean if the battery was only ever cycled to 50% DOD, the cycle life would approximately double from the spec sheet value for a given set of charge/discharge conditions. Or, consider the strangely realistic case where the cell is not cycled exactly the same way every time causing the cycle life count to get complicated or become completely useless. Counting cycles is not the best way, in my opinion, to get an idea of how long the battery will last because it is too application specific. Energy on the other hand is a constant measurement for all battery driven applications. Know how much you need? Then you can size a battery for it. So that gave me the idea for something I call a “throughput factor.” This number is a simply multiple of the beginning of life (BOL) nominal capacity of the battery. However, the number is calculated from the cycle life such that it reflects the number complete, nominal cycles to 80% of BOL capacity. That means all you have to do is multiply your cell count by the throughput factor and it will tell you how much total energy you can get out of the pack before end of life (EOL). This allows you can keep all your calculations working with values and units you already have with no reason to introduce a new term involving cycle count. It also allows you to more easily compare cells of different sizes and chemistries. For instance, say you are comparing LEAF cells and 20Ah A123's. The LEAF cell is about 1.67 times the Amp-hour capacity and it operates at a higher voltage. But the A123 maintains its voltage better and is rated for more recharge cycles. So which one will give you the highest energy output, aka throughput, for its useful life? Counting cycles won't help you. But if you use the throughput factor you can directly compare the two in terms of energy, which is what we really care about anyway. The one with the highest overall energy output, throughput factor times the number of cells, is the best option in terms of energy. Of course it is still dependent on temperature, mechanical pressure, current draw and all the other things that effect battery cycle life, but at a minimum the comparison between cells is now easier and more meaningful.
So here are the calculations:
Throughput factor = integral(capacity retention by cycle count)d_cycle (also known as the area under the cycle life curve)
Total throughput (Lifetime Energy Throughput) = throughput factor * BOL nominal capacity
Then I discovered this after I did all that math: See “Lifetime Energy Throughput” at http://www.mpoweruk.com/performance.htm
Make sense? The throughput factor is basically a fraction of the Lifetime Energy Throughput. Then the total throughput is expressed as a multiple of the BOL nominal capacity. So I reinvented the wheel a little bit but got to work through some things I never had before, plus I got some conformation that it is actually useful.
Here are the ones I have done so far:
A123 20Ah: 2918.7
EIG C020: 963.6
GBS 60Ah: 1859.2
And here are the graphs from the manufacturers I used to calculate those:
View attachment 1
Questions? Comments? Criticism? Have at it.