volumetric value power off lipo?

[dutchlincoln]

Hello,
I'm trying to calculate how much energy i can put in my electric scooter.
Is there a " easy" way?
I would like to know if there is a volumetric value of a lipo cell versus watts it can deliver.
For instance: 1000 watts for each liter of volume. Then i estimate as an example that a cell of 4.2 volts and 250Ah would give me a volume of one liter.
Is this value known?

I would like to use this value as well to be sure that a complete pack offered, for instance Ebay, is actually correct as advertised. If someone offers a pack of say 20Ah, 60V, it would be fairly easy to calculate if this would fit in the size of the pack thats offered..

Thanks!!

 

[pdf]

What you want to know, I think, is the volumetric energy density. Watts is power, not energy. You want to know watt-hrs per volume. While that number can be easily estimated, I don't have a battery with me to measure. Just take a LiPO cell current-hr rating, multiply by an estimate of the average voltage, and divide by the measured volume. I think a value of 3.75V for the average voltage is probably close, but others might know a more accurate value. If you have a cell in front of you, then you can measure it. There is be slight variations based on the "form factor" of the cell, but this will be close enough for gub-ment work.

 

[Drunkskunk]

Roughly 270 WH per Liter, and around 140 WH per KG, based on my Turnegy packs.

 

[Hillhater]

These Boston cells claim to be as much as 490Whr/ltr. ..from their data sheet.....
http://www.boston-power.com/sites/default/files/documents/2012-03%20Swing5300%20DS%20Rev1%200.pdf

and i suspect some of the 18650 cells (Panasonic) will be much higher still ( >600Whr/ltr ?)

...For instance: 1000 watts for each liter of volume. Then i estimate as an example that a cell of 4.2 volts and 250Ah would give me a volume of one liter.

Be very careful when extrapolating data like that.
Energy density is based on the individual cell volume, and some of them are odd shapes ( cylindical etc) so you cannot just assume all the volume of an assembled pack is 100% cell content..there will be air spaces !

 

[dutchlincoln]

hi,
They are A123 pouch cels if i believe.

How do i recalculate Wh? i know watt, as is voltage * amps

Point is, i want to buy a 48V/40Ah pack, but i find the pack a bit small to be true 40Ah...

 

[Hillhater]

you need to know the capacity of the cell ..Amp hours.
Most of the A123 pouches are 20Ahr, but they do/did make 16Ahr too i believe..maybe other sizes also ?
Best to ask for the cell specification/data sheet.
EG..
http://www.mavizen.com/A123.html
Note ..."Wh/Liter: 245.6"

 

[pdf]

hi,
They are A123 pouch cels if i believe.

How do i recalculate Wh? i know watt, as is voltage * amps

Point is, i want to buy a 48V/40Ah pack, but i find the pack a bit small to be true 40Ah...

Forgive me if I am being obvious, but if you are comparing pack of different voltages, you can't directly compare Ahr; a 40 Ahr 48v pack will be twice as large as a 40 Ahr 24v pack. Whr are comparable because they denote energy, independent of voltage. If two packs have the same voltage, then their Ahr ratings can be compared. To estimate Whr, you multiply the Ahr by the average voltage during discharge. Of course, in an application, you have to account for energy you can't extract, such as the last 15% or so of discharge energy.

 

[Hillhater]

A 48v, 40Ahr pack of A123 20Ahr pouch cells would need a 2p, 14s config of 28 cells.
So the MINIMUM size would be 230mm x 161mm x 210mm ... thats 7.7 liter volume..
... and would weigh over 14kg ( most likely 16-18kg )

 

[dogman dan]

For us to comment on the specs for a battery on ebay, we'd need the link, or you could type in the specs.

20 ah A123 pouch cells should be a good pack, you should expect very close to the real 20 ah from each pouch. Lifepo4 is not the most dense chemistry, but in pouch format you at least don't lose any volume to the air space between round cells. Worth it to use lifepo4 for the increase in cycle life.

Real world use differs, the pack will keep balanced much nicer if you only used 80%. So using a wattmeter to let you know when you have used 32 ah would be a good thing. You could continue till 100% discharged without great damage of course, but the wattmeter will help you plan ahead to typically stop by 32 ah. So ride a bit slower if you need to stretch range, and the wattmeter helps you know if you must slow down to get there or if you can continue to ride fast the whole ride. By the time you reach 10-15 ah discharged, you know how you are doing with range.