Is there no diode in series? The ones I've seen appear to have a 1n4148 in series which will drop the 5v supply down to 4.3V. The 200 Ohm resistor will limit the current to <1mA at full charge.
Hello there,
The first rule with diodes and voltage references is that you can never depend on a diode to drop a certain voltage. That's because it's voltage can vary for one or more of several different reasons.
The first is current. The diode drop can be anything from around 0.4v for a very light current up to around 0.8v for a heavier current. Applying this to the 5v circuit, that means the battery could get up to 4.6v and that is much too high for a standard Li-ion battery and even a high capacity type. That's a bit extreme, as we would probably see at least 0.5v or even 0.6v in real life where there is more than a tiny tiny current flowing. So if the leakage of the battery is a little higher we could see 0.6v. But even with that we see the battery voltage go up to 4.4v, which is still far to high. The limit is 4.225 max. And then what happens if the 5v supply is a little higher, like 5.2 volts? Many voltage regulators have a tolerance of at least 5 percent even at room temperature, so with 5.2v we could see 4.6v which is far, far too high.
Then we have the temperature coefficient. If the diode gets warm the voltage drop goes down. If it goes down by only 0.1v we have 0.1v more on the battery. That could make the difference between charging well and destroying the battery.
So you can begin to see why this type of charging circuit is no good. At best it could easily ruin the battery when operated from a regular 5v power supply. Luckily, when run with a 3.3v power source there will be little or no charging so with that power supply it's ok.
The 200 ohm resistor is in series with the diode, and that limits the current yes, but not the voltage. My measured results from the past few days show the battery voltage rising first to 4.29 volts and then today up to 4.37 volts. That's much too high for the Li-ion battery and will shorten it's life or maybe worse.