Project advice ("pass through" charger/UPS)

[virgosystems]

I would like to design a "pass through" charger with optional solar input for 4 18650 cells. The load will be a raspberry Pi with some number of USB connections.

-If external power is connected, battery should continue to charge while powering the load

-If external power is disconnected, it should switch to battery power without current or voltage drop.

What IC/devkits should I look at? Max budget is about $50.

Many thanks in advance

Joe

 

[Bluejets]

-If external power is connected, battery should continue to charge while powering the load

-If external power is disconnected, it should switch to battery power without current or voltage drop.

I can't see any difference.....