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Join Date: May 2007
Posts: 18

# Battery Charging Problem

10/30/2008 5:09 AM

Hi to all, I have a constant current source of 20 milli amps at 2.7 volts, and i have a 4 F capacitor(2.5 volt). Can some one suggest me a good circuit to charge a 2.7 volt Li-ion battery. can the capacitor alone be used as a current source after charging, ommiting the need of the battery??

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#1

### Re: Battery charging problem

10/30/2008 10:53 AM

If your Li-ion battery has a nominal voltage of 2.7 V, it is very likely that, while charging, this voltage go up. The data sheet would tell you exactly by how much. For example, a car battery that is 12 V rated, when fully charged reads some 13 -13.5V. While it is charged, the same battery would read 14-15V (depending on the charging current, the make and age of the battery etc).

Coming back to your battery, as a rule of thumb you would charge it with 1/10 to 1/7 of the capacity. So if it says that the battery is a 200mAh, use a charger that is, practically a current generator of about 20mA and that has an output (measured) of at least 4V. Now, for a small size Li-ion battery, charging with this current (or even larger in fast charging) might rise the temperature of the battery. The Li-ion chargers have a feature that, varies charging current as a function of temperature, voltage on the battery. You can get a Li-ion charger on ebay for a very low price, and disregard all my posting.

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#2

### Re: Battery Charging Problem

08/02/2010 1:23 PM

I think you will destroy the capacitor ( Condenser.) as the rating is too low for the supply.

{can the capacitor alone be used as a current source after charging}

Yes of course it can.

{Can some one suggest me a good circuit to charge a 2.7 volt Li-ion battery.}

If your battery is about 200mAH then the constant current source you have will be fine.

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