I'm currently trying to build a USB charger device for my mobile phone using an old 12V car battery as power source. I purchased different step down modules (see below) to regulate the voltage from approx. 12V down to 5V.
My problem is, that as soon as I connect my mobile phone to the output of the step down regulator, the previously adjusted output voltage (via potentiometer) drops for about 0.7V causing the mobile phone to not charge properly. Example: I configured the step down module to provide 5.2V at the output. Without load, there is no problem. With my mobile phone attached the output of the step down regulator, the voltage drops to approx. 4.5V.
I checked the input voltage of the regulator from the battery. Without Load: 12.85V, with mobile phone connected: 12.74V.
Is the "unstable" input voltage responsible for the massive voltage drop on the step down output? Or is there anything I can do to improve output voltage stability under load? Because I don't want to set the regulator to 5.9V only that it will really deliver 5V when connected to my phone - I think that's not very comftable for the mobile phone?!
I tried the following "ready-to-use" modules: