Edited 2017 - changed recommended long life storage voltage and added comments on fast charging using some recent systems. RM.
What YOU do as regards several of these questions depends largely on what YOU are trying to achieve or test.
Discharge to cutoff is fully discharged (to whatever remaining % that voltage represents). That's the easy one :-)
Percent dropoff of current in tail sets final % of max possible charged reached. There was a superb table given here within last week or so. Can supply later if you don't find it.
Real Men™ plateau at 4.2V and tail down to 10% or even 5% of the constant current rate. This gets the battery full and knocks the stuffing out of it.
Others terminate the current tail at say 25% of cc value.
Optimum lifetime for ongoing usage is at about the end of the constant current phase. That makes it very easy to locate - charge at specified current until desired max voltage is reached, then charge at constant voltage as desired. Here "desired" is to stop immediately. This is the point at which batteries tend to give significantly longer whole of life mAh of storage without grossly reducing mAh capacity per cycle. This is liable to be the point where older "fast chargers" tell you they have finished. Actual % total claimed varies but probably 70% - 80% range.
Newer USB input fast chargers use the term differently. In the case of USB the maximum available charge current at 5V is 5A so that the battery MAY be able to be charged at ~= 6A for the CC part of the cycle using an efficient buck converter to drop voltage and raise current.
[For a buck converter: Vout x Iout = Vin x Iin x efficiency_of_conversion]
Some systems such as QuaqlComms Quick Charge system allow the use of higher charger voltages (9, 12, 20) with specifically designed equipment, so battery charging can be faster for a given voltage provided that the battery specification allows this.
Maximum charge rates for LiIon and LiPo batteries are usually C/1 = 1A per Ah of battery capacity.
At 5V, 5A a USB charger can charge a 6000 mAh 1 cell LiPO battery at max rate - so eg a 10,000 mAh single cell battery used in some larger tablets can not be charged at the allowed 10A ! rate.
For long life storage where actual stored capacity is unimportant, LiIon and LiPo cells should be stored at about 3.7V.
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Using cells without protection adds to the rich tapestry of life. As long as you don't mind the occasional scorch mark on the tapestry that's fine. Note that part of the protection is a one time high capacity fuse under the cap for when things get out of control. Undervoltage discharge destroys. Charging from below a certain voltage at full rate can get fun, I'm told. Charging at reduced rate can bring cell up, I'm told. Below another second level they say don't even think about it. I've had very poor success in trying to get LiIon to misbehave. I have a box of unprotected cells that are very uncooperative about venting with lame etc. Strange. Sony and Apple and even HP seem to be much better at it :-).
When I understand you right, you want to know whether you can connect your project directly to the battery, even when the battery is charged?
The answer is somewhat hidden in the MCP73831 data sheet, on page 13:
during constant-current mode, the programmed charge current is supplied to the battery or the load
So yes, your project can be connected to the battery all the time. When you connect the power supply to the charging circuit, your project will be powered by it.
The current your project needs gets subtracted from the charging current (that means the battery will be charged at a lower rate)
Likewise, when your project needs more current than you have programmed as charging current, the battery will need to supply the additional current.
During charging, your project will be powered by the charging voltage, which will be at most 4.2 volts (OK, maybe 4.5 volts when your battery needs this), so you will be on the safe side with the Arduino Mini.
I need an Arduino based lithium battery tester that uses N channel Mosfets to switch On charging from and external power source and discharging through a 2 Ohm resistor. I have no issues with either charging or discharging circuit schematic, however i can not combine them together. I'm attaching the schematic for discharging the battery into the 2 Ohm resistor, please help with adding the components for making it able to charge the battery from an external, let's say 4.2V power source, all controlled by Arduino.
Best Answer
Surely a high side PFET connecting the battery + terminal to the 4.2V external supply is enough. I hope you aren't doing anything unsafe, I really hope you've read about thermal issues, charge currents and voltage cycles with Lipo batties.
If you have the default logic state as the 4.2V thing connected to the battery, then have the system with a pull down resistor on Pin 12. Then when Pin 12 goes high, it will turn on the NFET low-side switch to provide the 2 Ohm load, and the gate voltage of the P fet will rise and turn it off, thus allowing you to test the battery without the 4.2V thing interfering.
You can swap the logic (not recommended, unless you also detach the battery when not in use) by putting a pull-up resistor on Pin 12 and driving it low to allow the battery to charge again.