confirmed, developer
1,668
edits
Difference between revisions of "Battery capacity"
Jump to navigation
Jump to search
no edit summary
(Created page with "To understand battery capacity, you have to understand that power (P, measured in watts) is equal to electromotive force (V, measured in volts) times the current (I, measured in ...") |
|||
| (2 intermediate revisions by the same user not shown) | |||
| Line 1: | Line 1: | ||
Battery capacity is typically measured in milliamp-hours, or how many milliamps you can draw from a battery for how many hours? If you can draw 200 milliamps for 8 hours, then the capcity is 1600 mAh. The only way to actually measure the capacity is to do just that, drawing a known current and measuring the time until the battery is exhausted (or reaches some voltage that you set as the minimum). Some battery testers will try to measure the remaining capacity without discharging the battery, but this is not accurate. | |||
When comparing batteries of different voltages (for instance 1.2V NiMH vs. 3.6V li-ion), you have to determine the total energy of the cell. Power (P, measured in watts) is equal to electromotive force (V, measured in volts) times the current (I, measured in amps). So, for instance, a 60-watt lightbulb requires 60 watts of power to light up. But the power company bills you for watt-hours (converted to kilowatt-hours), because the total energy (E) they sold you is based on how many watts you were using for how long (t, time). For burning a 60-watt bulb for 24 hours, you would have to pay for 1440 watt-hours (or 1.440 kilowatt-hours). | |||
P = I * V | P = I * V | ||
| Line 13: | Line 15: | ||
There is more to a good battery than its measured capacity. An LED might draw 1 amp of current at 3 volts. So that is 3 watts or 3000 milliwatts. If you have a 2400 milliwatt-hour NiMH battery (whose 1.2 volts of power are being boosted to 3 volts to run the LED) then you can figure that you could run the LED for 2400mW*hr/3000mW=0.8 hours, or 48 minutes. But you're having to provide a lot of current (I=P/E) of 3/1.2=2.5 amps. That's a lot for any battery to deliver and it will at least get very hot and may only be able to drive the LED at something less than 3 watts. So the ability of a battery to deliver a high current is also important. "High drain" devices require more current. | There is more to a good battery than its measured capacity. An LED might draw 1 amp of current at 3 volts. So that is 3 watts or 3000 milliwatts. If you have a 2400 milliwatt-hour NiMH battery (whose 1.2 volts of power are being boosted to 3 volts to run the LED) then you can figure that you could run the LED for 2400mW*hr/3000mW=0.8 hours, or 48 minutes. But you're having to provide a lot of current (I=P/E) of 3/1.2=2.5 amps. That's a lot for any battery to deliver and it will at least get very hot and may only be able to drive the LED at something less than 3 watts. So the ability of a battery to deliver a high current is also important. "High drain" devices require more current. | ||
Cells that have high internal resistance will not be able to deliver the same current as ones that have less internal resistance. Alkaline batteries have higher internal resistance than NiMH batteries | Cells that have high internal resistance will not be able to deliver the same current as ones that have less internal resistance. Alkaline batteries have higher internal resistance than NiMH batteries. | ||