I am designing a circuit because purchasing pre-made boards would make the cost too high. I need to charge 18650 cells from usb, have low voltage cutoff (most charger ic's cover the rest of required lithium precautions, but not lvc), and a way to power 4 led's in series (12.6v). The problem is, I need the output to the leds to be constant voltage and variable to be able to turn down brightness. The circuit I have so far will take weeks of development and I was wondering if there was an easier way of approaching this. The configuration of the batteries can be 2 to 4 cells in 4s, 2s, or 1s.

Here is my current drawing in block form
usb--battery--charger--low voltage detection--boost converter--led's
2s^
some ic's I have looked at so far that fit my needs
- https://www.monolithicpower.com/en/products/battery-management/mp2639a.html
- http://www.ti.com/product/lm2698

the leds im using draw around 400ma at 12.6v so I do not need very high current components.
But, this circuit would require a lvd using a comparator or 555 timer (for simplicity) to disable the boost converter when voltage is low and this would require a 5v voltage regulator further complicating the circuit and increasing parasitic draws. I am trying to keep the cost of each board around 10$ and do not need many features, just battery protection and variable output.

So what would be a good way to approach this. It seems to me that the circuit I have so far is overly complex. Or am I embarking on a journey full of datasheets, schematics, and hair pulling.

 

For that power level, forget about charging from USB.
Ebay has plenty of charge controllers that are a lot cheaper than you can make one. And if yo are not sure hoe to design it, buy it. Making an 18650 charger is a good way to have a fire is you get it wrong. There are plenty of example of even "professional" companies having products catch fire as they "got it wrong".
Look into chargers and batteries for the radio control market maybe?

 

A solution, perhaps!?

https://www.amazon.com/s/ref=nb_sb_...sprefix=18650+charg,aps,254&crid=E034RPV5NR2B