calculating current per cell for powerwall

wattwatt

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Joined
May 21, 2018
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55
I'm losing my math somewhere and can't figure out where. I'm planning the battery config for my DIY 48 V powerwall using harvested 18650 cells. Let's say my baseline power usage is 2500 watts/2.5 kW, so 2500 W / 48 V isroughly 53 amps of current I need to pull from the battery/inverter (let's ignore DC to AC conversion lost for now to keep things simple). The first phase of the battery installation will be 14s245p.To figure out how much current each 18650 cell needs to provide I divided 53 A by 245 cells to get roughly 0.217 A /217 mA per cell. I thought this was right, but out of curiosity I tried another route and divided the total watts usageby3430 (the total cells in the battery for the first phase) and I got roughly0.73 watts per cell. This equates toroughly 0.195 A / 195 mA per cell (0.73 W / 3.7 V). I know 217 mA is close to 195 mA, but I imagine they would be the same if not very very close. I'm doing something wrong here; how do I properly calculate the amperage/current needed per 18650 cell in this case?
 
Joined
Dec 20, 2018
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Your Error is to calculate with 3.7V per cell, but 48V divided by 14 equals ~3.429V. When calculating 0.73W / 3.429V you get ~213mA.

I always calculate with 50V wich would be 3.65V per cell. Its easy to calculate and close enough to "real" in my opinion.
 

wattwatt

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Joined
May 21, 2018
Messages
55
Okay, I think I see my issue - I'm confusing A and Ah... to run the house for an hour I need 2500 Wh of energy and EACH of the 3.x strings has to have 53 Ah capacity. But, if I need 2500 W of power at any given moment then the WHOLE battery has to be able to put out 53 A and NOT EACH 3.x string...??
 

daromer

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Oct 8, 2016
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Wrong. To run your house pulling 2.5kw for One hour you need some where between 3-3.5kwh useable Stores energy.

You have conversion losses all the way between the battery a d the appliances. More important If the inverter isnt pf1 you need more and IF you have Huge inverter with high Idle load even more ;).

Current per cell later on also depends on type of cells. IF your average is 350mA per cell its more like 250 in One and 450 in another depending on what type of cells you used.
 
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