# Can't figure out correct resistor for charge current

Hello, I am making a lipo battery charger for a 3.7v lithium-ion battery. I found a charging IC that I want to use but don't know what resistor to use. This is the IC I'm using and this is the data sheet for it. In the datasheet there is this formula on how to find the correct resistor.

I am a little confused on what to do to get the correct output current. The output current I need is 400 - 500ma. Thanks for your time.

The formula requires you to FIRST determine the charging voltage, Vprog, What will that be?

@Paul_KD7HB, I will be using a Arduino uno to charge it, so 4.98v is the voltage output from my IO pins.

An arduino is NOT a power supply. Rework your design.

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But the charging IC's input range is 4.25v - 6.5v, an Arduino Uno IO pin can output 5v. Shouldn't that work?

get a single cell 1S control board, and use 4.2v adaptor. many of those 3.7 lipo already have control boards so all you need is 4.2v charger so it won’t overcharge no matter what you do

Not everything is about comparing the voltages, there is also current consideration

So an Arduino Uno doesn't give enough current to charge the battery?

As @Paul_KD7HB already told you, Arduino is not a power supply

Alright, so if I get a 5v power supply that has enough current, what resistor should I use for 400 - 500ma output?

NO

The way I read the specification: Rprog = 1000 * Vprog/ Ibat

for 500 ma (aka 0.5A)

Rprog = 1000 * 1/.5 = 2k Ohms

However if Vprog is on the high side of the specification (aka 1.07) the battery charge current is:

Ibat = Vprog/Rprog * 1000 = 1.07/ 2k *1000 = 535 ma

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