jonsleepy wrote:a) how does the circuit make this determination? Is there some kind of clock that starts ticking from 0 every time I plug it in? Or does it monitor the battery fill level in real-time and adjust accordingly?
Both, actually. The control chip has built-in logic that monitors the battery voltage and adjusts the current to do what's best for the battery at each level. It also has a timer that will stop the charging process if the battery goes too long without reaching the proper voltage.
The chip also has STATUS pins that control the LEDs so the user can find out what the chip thinks of its present situation.
jonsleepy wrote:b) Is it bad to plug in the battery when it still has some juice on it? (If I can only charge it for a short while, will the charger cycle through all 3 stages every time it gets plugged in)?
That's called a 'partial charge cycle' and opinions vary about how good those are for batteries. The balance of what I've heard says they're probably okay.
The chip may go through all three stages every time (low-current preconditioning, high-current charging, and constant-voltage final charging), but it won't follow the same pattern blindly every time. It constantly measures the battery and adjusts the charging cycle to the immediate conditions.
You'd solder a through-hole resistor between the pads marked 'PROG' on the board. According to the datasheet for the control chip ( http://ww1.microchip.com/downloads/en/D ... 22005b.pdf
), the charge current is 1000V/Rprog (equation 4-1, page 14), so a total of 1k resistance between the pads will give you 1000mA.
Now, the photo on the product page shows a 1k surface-mount resistor already between those pads, but the text says the default current is 500mA, which would require a 2k resistor. It could be an older photo, or I could be missing something. I'll leave a final call on that to people who know the product better than me.
jonsleepy wrote:d) Is 1000 mA sufficiently fast that I need to add the thermistor for safety? My battery is 6,600 mAh.
You actually want to check the amount of current the battery can supply at a given time. That parameter is called 'C', and it's a good idea to use a charging current between .5C and 1C unless your battery is rated for higher charging currents.
FWIW, raising the charge current from 500mA to 1000mA probably won't make the charging time much faster. The battery will charge to 70% faster, but the topping-off stage from 70% to full charge will go slower.
Here's a page with good information about charging LiPos: http://batteryuniversity.com/learn/arti ... _batteries
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