Battery comparisson

[tomv]

I was collecting all info I could find about batteries into a spreadsheet for easy comparison. Attached is my condensed list. I didn’t include anything that’s more than $1/wh, which leaves out most pre-built EV packs.

The most interesting battery pack I’ve found is replacement battery for camcorders. Those are built from 18600 batteries and also include BMS. Price per cell is better than the best price you can get on lose 18600 cells! Price per Wh is better than Nimh and almost as good as SLA! The problem is to figure out the pin-outs. Also it’s not clear if BMS would like being wired into high-voltage pack. Any ideas?

 

[xyster]

Another critical criterion when selecting a battery is the "C" rating -- a measure of the battery's current-output capability. The C rating is derived by dividing the battery's maximum continuous current output by the battery's capacity.

If, for instance, you have a 35 amp controller, you'll need batteries that can provide that 35 amps without blowing up, shutting off, heating up too much, or the voltage sagging too far. If the battery had a 1C rating, you'd need 35ah of that battery to reliably provide 35 amps of current. If the battery had a 5C rating, you'd only need 7ah of that battery. Many batteries have a C rating too low to work for EVs without wiring an unwieldy number in parallel.

For instance, the 2.2ah cells I use for my X5 are 1.5C -- meaning each should be able to supply 3.3 amps.
http://www.all-battery.com/index.asp?PageAction=VIEWPROD&ProdID=1643
Since they're lithium cobalt, to play it safe and to minimize voltage sag, I constructed my pack 20s15p -- 15 cells in parallel. The pack thus has 33ah of capacity (2.2ah X 15 cells), and is asked to provide 1.06C max (35 amps/33ah).

 

[Lowell]

Are you sure about the capacity on the BP-945? I have a Canon camera, and some of the batteries are labled 5700mAH.

 

[tomv]

I see your point about C ratting. For smaller battery packs it's crucial. But for my case I'm thinking of >1 kwh pack and 1C rate would be fine. Cost, weight and complexity of building are the most important variables for me.

Replacement BP-945 packs come with rated capacities from 6000 mah all the way to 7200 mah at 7.2-7.4 volts. So I guess they are built from 2S3P 3.7V 18600 cells that are rated 2000 - 2600 mah. Most packs are chinese built but some say they use Japanese cells. I think that's key because as far as I know only Sanyo, Panasonic, Sony LG, Samsung and Moli build cylindrical lithium cells in high volume/quality.

 

[tomv]

Here the link to 8000 7.2V mah battery:

http://cgi.ebay.com/BP-941-BP-945-Battery-for-CANON-C2-FV1-Optura-Camcorder_W0QQitemZ140139513811QQihZ004QQcategoryZ43434QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Seems a bit unbelievable. Cells would have to be more than 2650 mah,
I've never seen such cells for sale anywhere.

But anyway, it's even a better deal than I have in my sheet above.

$16.95+$1 shipping (assuming multiple items) = $18 for 57Wh so $.31 per Wh, better cost than SLA and 10 times lighter.

I'm tempted to try those... I wonder if I could just hook them up to the power supply and rely on internal BMS to terminate the charge.

 

[xyster]

Here the link to 8000 7.2V mah battery:

http://cgi.ebay.com/BP-941-BP-945-Battery-for-CANON-C2-FV1-Optura-Camcorder_W0QQitemZ140139513811QQihZ004QQcategoryZ43434QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Seems a bit unbelievable. Cells would have to be more than 2650 mah,
I've never seen such cells for sale anywhere.

Wouldn't it probably be a 2s4p pack of 2000mah cells?

~$2.25 per cell, including shipping, is about 1/2 the going rate for the cheapest individual 2000mah 18650s:
http://www.batteryspace.com/index.asp?PageAction=VIEWCATS&Category=739

Smells suspicious. Could they be recalled cells? Old cells? I'd be tempted to buy one pack, pull it apart apart and test each cell for capacity under a load of at least 1C.

 

[safe]

One wonders if they simply are overstocked with these?

They might have bought a lot of them to get the volume dealer pricing and then once they got them they didn't sell as fast as they expected. After a while they realized they needed to get rid of them before the batteries aged too much.

It could be a "distressed" sale situation... like a foreclosure for batteries... still a good deal...

 

[fechter]

Maybe they're 5 years old.