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Electric Stunt => Gettin all AMP'ed up! => Topic started by: John KruziK on March 30, 2013, 09:10:55 PM
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How do you determine what rate to charge batteries at? Thanks
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Some batteries will specify. In the absence of that, no more than 1/10th of the rated discharge is a good rule of thumb. I push this to 1/5 on some of my RC packs when I'm feeling impatient, for what good it does.
Keep in mind that the proper way to charge a lithium polymer cell is never, ever, let the cell voltage get above a critical value (4.2V springs to mind, but I've seen 4.3 -- I'd have to look it up to know for sure). Chargers are designed with this in mind, and they won't let the cell voltage exceed the limit. The way that you limit the voltage in a charging battery is by reducing the current until the voltage is OK -- so once the cell voltage has reached the limit with maximum current, the charger will start ramping the current down.
What this all means is that if you increase the charging current from 1C to 2C, you won't come close to reducing the charge time by a factor of two -- you'll only cut the current-limited phase by a factor of two, after which the voltage-limited operation will take the same amount of time either way.
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Heating is criterium. You can usually read it from datasheet as Tim wrote. If it is not known, some chargers can measure it in automatic mode (cellpro) and decide what is safe current. If you do not have such charger, then 2C is usually safe, but if you can recognize any warming then you are over. Some batteries will stand only 2C and some batteries even 8C. My experience tells me that those with higher charging C also last longer.