use of an off the shelf psu as a controllable battery charger - Page 1
Forum section: electronics / projects

author: jmibk
author_rank: Regular Contributor
timestamp: January 22, 2022, 03:12:53 pm
content: I like to build up a battery charger for (primary) lithium kind batteries with high charging current capabilities.After brainstorming about suitable designs for step up and down converters I spotted some cheap china SMPS on amazon for around 30 bucks. They come with 5 volts and 60 amps of output current.Now I ordered some of these supplies to explore their electronics to hack them into a battery charger.I found a schematic on the internet that matches nearly perfect with the design of my PSU. You can see it down below.It is a PSU built up around the TL494 (not the original part, but its the same) and is secondary switching.Now the tasks to use it as an battery charger:Batteries of this type are charged by a constant current following by an constant voltage regulation of the voltage source. So I have to figure out, how the voltage and the current can be controlled externally of this supply.For voltage control there are two voltage dividers on pins 1 and 2 of the 494. These inputs regards to an error amp. On pin 2 there is a kind of setpoint voltage of 2,5V built up with a voltage divider 50/50 of an reference voltage of 5V generated by the chip itself.On pin 1 is also a voltage divider that is feed from the output voltage. The main tap of is 2,5V when output voltage is around 5V. There is also a trimmer on the board to fineadjust the output voltage in a tiny range.My idea is connecting the output voltage directly to pin 1 of the 494 and on pin 2 a potentiometer that is connected between VREF (5V) and Ground (V-). So you can select any voltage between 0V and 5V.This voltage is also ground referenced, so I can use an ADC or Arduino to drive this voltage from an uC.The supply voltage of the 494 comes from the same transformer than the output voltage (different windings). So a change of the output voltage also affects the supply of the 494, called AUX voltage in the schematic. I decided to supply the AUX voltage from an tiny 3W / 12V supply - so the 494 is always powered on if the supply is powered.The current can also be regulated by the error amp pins 15 and 16 of the 494. Pin 16 is ground potential, 0V. so there is a voltage divider on Pin 15, center point is connected to Pin 15, one end is sensing the voltage on the current shunt and the other end is pinned to the VREF (5V). If the center point goes below 0, overcurrent is tripping and the psu is skipping some switching to reduce its output voltage.If I adjust the voltage divider and make the 5V variable voltage I should be able to adjust the output current, that is also the charge current of the battery/cells.Here you can see the schematic (credits to the man in the internet) and I drawed in my changes.Now to the problems I spotted and I like to invite you for helpful tips and hints for the project:One supply is doing terrible peeping noises (like a mouse which is dying) and the output voltage should be 2,5V but is going up and down terribly.At the second one I tried one switching transistor died at second switch on. It gets hot after a few second. I tested it and it morphed into a resistor of around 1kOhms. No blown fuse here, the primary dc voltage is available.What are the things I'm not thinking about?