LiPo Battery Charger for UPS system

Using Arduino Programming Questions
#1

Hi to everyone. For a university Lab course i have to realize a UPS unit with a internal LiPo battery that i have to charge with a buck converter. I have tried to realize the software that control the charging operation and it works properly.

Few days after i verify that the code works properly i found a problem with the voltage that i measured with arduino, they floats and so the battery charger can't work properly since it can't keep the voltage constant during the constant voltage phase of the battery charging process.

I found that it happens when plug arduino into the USB port of the computer after i have started the PC, while it doesn't happen when i plug the USB cable of arduino and then start the PC. It seems to be a problem with the ground of arduino.

I'm using one arduino, on which i have upload the code of the UPS battery charger, and a second arduino to verify the output of the UPS unit (output of the internal inverter). I verify that the problem is not connected to the Inverter or other component of the UPS since i try the battery charger alone and i can still have the voltage floating of the analog pin from which i read the voltage of the battery and the voltage of the network (output of a AC/DC converter that we simulate with a 4AA battery pack).

Why this problem happens??

Problema con caricabatterie UPS
#2

Probably something to do with your circuit, or wiring. But as you have not provided any usefull information, I guess at a missing GND connection.

#3

I verify all the connection and all the GND are correctly connected. The circuit work well few days before and i used the same cable. I'm on breadboard, could it be the problem?

#4

Going by the information supplied, my new guess is 'banana!'

#5

The total circuit (network, buck converter, Lipo battery and inverter) is the following one

IMG_20210515_211042
IMG_20210515_2110424640×2610 2.57 MB

if necessary I can also upload the overall code and a photo of the real circuit, but due to the high number of cables it is not possible to see everything clearly ...

#6

Schermata 2021-05-15 alle 21.47.41
Schermata 2021-05-15 alle 21.47.411077×679 118 KB

I make the circuit on tinkercad, with the same topology that i have realized on the breadboard.
The value of the resistor, inductance and capacito are the previous one that i have indicated on the draw... the 9V battery represents the LiPo battery of 3.7V (on tinkercad there's no a rechargeable battery model).

There's also a 10Ohm resistor that we placed here only to measure the current that is going to flow on the capacitor and on the LiPo battery. I read the network voltage (pin A0 is connected to the series of the two 4.7kOhm resistors, i read the voltage at the central node of the voltage divider and then i multiply the value for a factor 2, so i can measure the real voltage of the battery pack that will be around 6V), the resistor voltage and the battery voltage. I calculate the charging current as: (resistor_voltage - battery_voltage)/10.00

I also read the value on one side of the output resistor with the same arduino on which i have uploaded the total code of the UPS to apply a reaction and give the correct value of the output (3V peak-peak) while the battery goes down.

If necessary i will upload all the code, but i'm sure that it is correct...
The two arduinos are connected trought USB cable on the same laptop... i can't understand where is the problem...