Li-Ion single cell charger questions

Thread Starter

jjmalove

Joined Jul 6, 2016
51
I'm working on making some Li-Ion charger designs for relatively small devices. In the past I have used COTS solutions for prototyping, but I would like to start integrating the battery charging aspect onto our own PCB with the rest of our stuff instead of having a daughter board charger, to make it smaller most importantly.

My first design is utilizing what appears to be a pretty well received single cell charger, the MCP73831/2. Here is a datasheet for those interested:
http://ww1.microchip.com/downloads/en/DeviceDoc/20001984g.pdf

I'm pretty confident on the charging aspect of it, but tell me if this makes sense. The way I have it laid out limits it to a 500mA charge rate with a 2k resistor on the PROG pin to GND, which I can reduce via increasing that resistor. I'm planning on first prototype to use a standard 18650 3000mAh battery, which accepts up to 0.5C so 1.5 A charge rate so shouldn't be anywhere near over charging it. A standard 5V micro usb supplies the VDD input from a wall adapter or computer. I've got a yellow LED with current limiting resistor from the micro usb to STAT pin, and a green LED with current limiting resistor from the STAT pin to GND. So what should happen if I didn't misinterpret the datasheet is yellow comes on when usb plugs in and battery is not fully charged because STAT is low. Once STAT goes high current flows through the green LED instead and both LEDs are on. This sound correct?

Main questions however are these:

First, at what level does temperature monitoring start to come into play. This chip has no mention of temperature sensing. From my initial research and reverse engineering I commonly see a temperature sensor (I believe a thermistor?) taped or glued directly to the battery pack and wired back to the charger. Is the way this works the chip they use has a pin which when it sees a resistance higher than a certain amount turns the charger off, and then they pick a thermistor which will meet that resistance number before the temperature gets to a damaging number? At what level of cells/amps/voltage/ect. should I begin integrating this? Is the chip I have selected not appropriate since it doesn't include this?

Second, a brief explanation on cell balancing would be great. My initial goals don't involve more than a single cell 3.7/4.2v battery, but any good resources on how to properly cell balance and integrate a 2 cell 7.4/8.2v pack and beyond? Do you have to cell balance if the batteries are simply in parallel for added capacity. For instance could I parallel two identical 18650s to the MCP73831/2 and assume it will take twice as long to charge but also last twice as long or is there more to it even for parallel batteries?

Lastly, is there anything to consider with designing the output of the battery into a regulator to then power the rest of the device, if I plan to leave everything connected at all times except the usb cable? Meaning I would like to have the rest of the circuit board on while it charges, and the battery will be powering the rest of the board while being charged by the usb cable. This isn't mandatory for my current design but just trying to learn if I am missing anything for that.

Thanks in advance!
 
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