Hi,
I'm working on a PCB project. It's an HMI with display. This circuit need to be supplied by 18V power supply or 18V battery pack, When the power supply is unplugged, the circuit is supplied by a battery pack (5x3,7V batteries), while When the power supply is plugged the supply came from 18V power Supply.
Below, a simple scheme of what I need to realize.
I would like to understand what kind of circuit I should make, and more or less how to make it.

 

A BMS circuit you can get "off the shelf" will often have load control built in, so I wouldn't bother re-inventing the wheel if you don't have to. I suspect you'll need more like 21V or more from the power supply if you want to charge those cells in series. It'd be better to charge them individually, and some BMSs will accommodate that.

 

Thanks. I know I need more than 21V. Sorry, I newbue on this arguments.

 

The OP didn't actually say he wanted to charge the batteries when the mains supply was connected - though that would make good sense! The better way to do this (rather than as shown in his diagram) would surely be to connect the mains power supply to some form of BMS which, in turn, connects to the battery pack which powers the board. Thus if the mains power is there, the battery pack is charged and runs the board but if it is removed, the board continues to run from the batteries alone - no break! 5S BMS systems are available off the shelf, as vdavi81 has said.

 

Hi,
I'm working on a PCB project. It's an HMI with display. This circuit need to be supplied by 18V power supply or 18V battery pack, When the power supply is unplugged, the circuit is supplied by a battery pack (5x3,7V batteries), while When the power supply is plugged the supply came from 18V power Supply.
Below, a simple scheme of what I need to realize.
I would like to understand what kind of circuit I should make, and more or less how to make it.
View attachment 300433

The simplest way I know of to do this uses two diodes. One diode anode connects to battery positive, the other diode anode to power supply positive. The two cathodes are connected to the load positive, and of course the supply & battery negatives are both tied to load negative. HOWEVER, this only works if the battery provides a lower voltage than the power supply, but it can often be tweaked slightly higher to achieve that criterion. If the voltage drop of silicon diodes is unacceptable, use 40 volt schottky diodes, or search for "ideal diode circuit". You didn't tell us how much current your load will draw at the nominal 18 volt input, so I can't suggest specific diodes. Another commenter here suggested using a power supply to also charge your battery pack, but that is a separate discussion...