Hi! I want to power my Arduino with a battery and be able to charge it via usb. I bought li-ion 3.7V battery, TP4056 li-ion usb charger (amazon link) and SX1308 step up regulator (amazon link).
See the attached diagram of my power circuit.
The voltage between battery's + and - is 4.15V (a bit weird, I expected to see 3.7V).
The voltage between OUT+ and OUT- on charger is 0.8-1V (again, I would expect the same voltage as on battery).
The voltage between OUT+ and OUT- on voltage regulator is 0.8V (that makes sense, it needs at least 2V on input to work properly).
I've set step up regulator to output 5V by connecting it directly to the battery and turning the potentiometer. No problems with it.
Why is the voltage on charger's OUT pins so low? It's the second unit I tested, previous one had the same problem. Did I miss something?
Do you get these results with no power provided at the USB port? If so, then it may be that the protection function in the charger is turning on for some reason. But if the battery works fine driving the boost regulator directly, I don't know what the problem would be. Do you have the OUT- terminal of the charger connected to the GND terminal of the boost regulator? Which Arduino are you using?
I should say that even if you get this to work, there is another function that needs to be added, and that is "load sharing", which allows the USB 5+ line to power both the battery charging and the boost regulator independently so power for the Arduino does not come from the charger. Without that, you would have to turn off the boost regulator and Arduino while you're charging from USB power. If I know which Arduino you're using, I think I can give you a schematic for the load sharing circuit.
Fully charged a Lithium Ion battery is about 4.1V, when there is virtually no energy left in it, below 3.3V.
You're right, thanks for pointing that out.
ShermanP:
Do you get these results with no power provided at the USB port? If so, then it may be that the protection function in the charger is turning on for some reason. But if the battery works fine driving the boost regulator directly, I don't know what the problem would be. Do you have the OUT- terminal of the charger connected to the GND terminal of the boost regulator? Which Arduino are you using?
I should say that even if you get this to work, there is another function that needs to be added, and that is "load sharing", which allows the USB 5+ line to power both the battery charging and the boost regulator independently so power for the Arduino does not come from the charger. Without that, you would have to turn off the boost regulator and Arduino while you're charging from USB power. If I know which Arduino you're using, I think I can give you a schematic for the load sharing circuit.
USB not connected. I have a wire going from OUT- to GND on step-up regulator.
"Without that, you would have to turn off the boost regulator and Arduino while you're charging from USB power."
Why would it be a problem? If I'm not mistaken, OUT voltage would come from USB and the regulator would simply pass that voltage to the board, which is fine in my case. I will be using either Arduino Nano or Digispark ATTiny85.
I also found this post. Apparently this is a known problem with these cheap li-ion chargers. And indeed, after a few reconnects I managed to get expected 5V on OUT pins.
prk3:
"Without that, you would have to turn off the boost regulator and Arduino while you're charging from USB power."
Why would it be a problem? If I'm not mistaken, OUT voltage would come from USB and the regulator would simply pass that voltage to the board, which is fine in my case. I will be using either Arduino Nano or Digispark ATTiny85.
The OUT voltage comes from the output of the charger circuit, not directly from USB. The charger knows to stop the charging process by tracking the charging current, and when that drops to 1/10 of the full charging current, it shuts down the charging process. But if you are also drawing current for the Arduino as well as for the battery, the current will likely never fall far enough to turn off charging, and that creates a potential safety issue.
The attached schematic shows a charger circuit similar to yours, but with load sharing added. That part of the circuit consists of three parts - a P-channel mosfet, a diode and a resistor.
When USB is not connected, the gate of the mosfet is pulled low, which turns it on fully, and the battery supplies current to the boost module. But when USB is plugged in, the USB 5V raises the gate voltage, which turns off the mosfet, and USB powers the boost module through the diode while it is also charging the battery through the charger. When the battery is fully charged, charging will shut down as it should even though the Arduino is still on.
The Nano already has the diode, so you would just need the mosfet and resistor. But the diode on the Nano is a schottky diode, so you would need to change the resistor to about 10K because schottky diodes can have significant reverse leakage, and you might need the lower resistor value to keep the gate voltage low when USB isn't plugged in.
The switch is used to disconnect the battery from the rest of the downstream circuit, specifically the boost converter. The converter will sink some current even if there is no load on it. So the switch just allows you to shut everything down and prevent the battery from being discharged needlessly. But of course it isn't required.
