MCP73831 Lipo charger PROG resistor replacement with thermistor

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vinodstanur

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Hi,
I am using MCP73831 to charge a 3.7v 115mAh lipo battery. I am using PROG resistor 20K. I designed it to charge it at 50mA (a little less than 1/2 C). ie 20K = 1000/50.

I am plugging the battery on the charger. Now I noticed that the charger is becoming hot which I am parallelly charging a mobile phone because the mobile phone is taking around 1A current to charge it. As the temperature increases, I should reduce the charging current right?

If yes, I am planning to use a thermistor (positive) instead of 20K fixed resistor. Any suggestions?
 

You should note that power loss in charger is Vdrop*I and you only need 0.3V min. drop. If you can maintain efficient input voltage and thermal resistance, temperature rise should never be a problem at max current of 0.5A for chip.
 

SunnySkyguy said:
You should note that power loss in charger is Vdrop*I and you only need 0.3V min. drop. If you can maintain efficient input voltage and thermal resistance, temperature rise should never be a problem at max current of 0.5A for chip.
Click to expand...

Hi Sunny,
I am not worried about the temperature rise on the chip, but I am worried about the LiPo battery.
I know for a fact that the temperature of lithium batteries should be monitored while charging and the charging current should be adjusted according to the temperature. But here for the the battery chaging IC itself have a programmable resistor 'PROG' pin where I can connect a resistor to control the charging current. So I am thinking to use a thermistor instead of that resistor so that I can reduce the charging current on increasing temperature. But I didn't know exactly the graph or any data which shows maximum charging current (in terms of C) vs temperature for lipo batteries. Also I didn't seen any one using a thermistor on MCP73831 before. So I am little bit confused if I am taking a correct decision or not...
 

vinodstanur said:
I am not worried about the temperature rise on the chip, but I am worried about the LiPo battery.
Click to expand...

I have no experience with LiPo type...

But when charging nicad/nimh, there will be a point where the battery no longer converts incoming energy to electrochemical storage, but converts it to heat. This is when the battery is fully charged. The rise in temperature can be monitored as an indicator of this.

Heat also is generated if you are sending high current through the battery in view of (a) its physical size, and (b) its internal impedance. This is different from the rise in temperature which signifies end of charge.

Making the battery become hot is how we shorten its useful life. We should avoid this, as you know.

So the idea is to charge at a slow enough rate that the battery does not heat up too much. Then when it does heat up, it tells us to stop charging.

Again, I don't know specifics about LiPo.
 


Hi,
My battery will not heat up because I am charging at 50mA and the battery capacity is 115mAh. Also I am using MCP73831 charging IC which will shutdown when it reaches constant voltage and current less than a threshold.
Now my worry is, I am keeping the battery on a charger where I am charging many other things at 1A. So the battery charger is getting heated up and so little heat will be transfered to the battery also. So I am thinking should I again decrease the charging current when temperature increases or not.

Basically I am searching for a graph which relates battery temperature vs charging current for 1C discharge rate lipo battery.
 

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