LI charger chips are switching step-down converters with a constant-current mode.
Say it's a single switch buck-type charger. It's input range is 5V to 10V, and is capable of charging cells at 2A.

Will that chip require a 2A input regardless of Vin, whether 5V or 10V?
Or, can the chip accept a 1A input @ 10V, and output 4V charge-voltage at 2A?

thx

 

A buck converter draws the same current from the input as it puts out when the switch is on, and it draws no current (well, very little) when the switch is off. When the input is 10V, and output is 4V, the switch is off 60% of the time.

So, in your example, it will draw an average of 0.8A, but a peak oh 2A.

Bob

 

If you consider a charger optimized for solar panel input, it will vary the current as provided by the source. Constant current is not required for charging. Buck converters transform power, within design constraints. 10 watts of input will provide close to 10 watts of output. Best to breadboard and test.

 

draws the same current from the input as it puts out

Does that mean, to output 2A (at whatever voltage), the buck needs input 2A (at whatever voltage)?
Eg, an input of 10V must be 2A, to get output of 4V @ 2A?

 

If it’s just a switching element, yes. If it has inductance in the output, you’ll realize some voltage to current transformation.