raggie33
*the raggedier*
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and thats saying a lot. ok my question if i pay 21 cents per killwatt hour. how much does it cost for me to charge a 5000 mah 21700 cell?
my dumbest question yet
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PhotonWrangler
Flashaholic
Not a dumb question. To determine the answer you need to know how much current your charger draws while it's charging the cell and how long it takes to charge it. My finger-in-the-wind guess would be a penny or so.
raggie33
*the raggedier*
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cool ill check that in the am. i have this crazy cool usb device that looks like a thumb drive it monitors so many things such as volts amps energy mah etc etc. then ya can monitor it on ya phone with blurtooth. .im trying to figute if cheap aa cells are cheaper to use.. but if its a few cents to charge me cell. the 21700 is the way to go
raggie33
*the raggedier*
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thank you both i have so many 21700 like 25 cells now
Just over a penny if your charger is 50% efficient in putting energy from your outlet into the cell. Not sure if 50% is right, it's just a guess for a best case, it may well be less efficient than that. there will also be some losses between the power coming into your meter and the outlet you plug the charger into.
Another word, once charging is complete and terminated, unplug the charger.
Dave_H
Flashlight Enthusiast
It works out to less that 1c per charge.
Assuming the cell is completely discharged you need to deliver 3.7*5 = 18.5 watt-hours. Cell voltage
varies from 3.4v nearly dead to 4.2v fully-charged, average around 3.7v .
Charger efficiency can vary. Powered from ac, it has two stages, stepping down ac to low-voltage dc, and
the charge circuit itself. Suppose it steps down to 12vdc with wall-plug adapter, maybe 90% efficient.
Charger itself might be linear (non-switching) at lower levels, which would make that part about 75% efficient
for overall efficiency 0.75*0.9 = 0.675 (67.5%). This would add about 9W, total 27.5W, kind of worst-case.
Typically you might get 80% overall efficiency, 23.5W total.
Charge efficiency of the cell itself is necessarily high (reported 95-99%). Li-ion cell should never get hot,
or even more than slightly warm during charge. This is unlike NiMH which can get warm, even hot during fast
charge. Ignore it for now.
Cell will lose capacity over life, not sure how that affects charge energy.
At 21c per kWh, that works out to about 0.6c per charge.
Dave
Assuming the cell is completely discharged you need to deliver 3.7*5 = 18.5 watt-hours. Cell voltage
varies from 3.4v nearly dead to 4.2v fully-charged, average around 3.7v .
Charger efficiency can vary. Powered from ac, it has two stages, stepping down ac to low-voltage dc, and
the charge circuit itself. Suppose it steps down to 12vdc with wall-plug adapter, maybe 90% efficient.
Charger itself might be linear (non-switching) at lower levels, which would make that part about 75% efficient
for overall efficiency 0.75*0.9 = 0.675 (67.5%). This would add about 9W, total 27.5W, kind of worst-case.
Typically you might get 80% overall efficiency, 23.5W total.
Charge efficiency of the cell itself is necessarily high (reported 95-99%). Li-ion cell should never get hot,
or even more than slightly warm during charge. This is unlike NiMH which can get warm, even hot during fast
charge. Ignore it for now.
Cell will lose capacity over life, not sure how that affects charge energy.
At 21c per kWh, that works out to about 0.6c per charge.
Dave
