How can I limit a device's current draw from a cheapo Boost/Buck?

[monster_of_electronics]

I'd like to use one of these cheap boost/buck converters w/a 3S li-po battery to charge a laptop, but the laptop can draw as much as 4 amps continuous @ 15v, it doesn't seem wise to trust the said cheapo boost converter for more than 70% of it's advertised max continuous ampacity.

A slower charge is fine in this case, how can I restrict how much current the device (laptop) can draw without modifying the device.

[mikerj]

This will be very difficult indeed. You'll need to reverse engineer the laptops battery charging circuit and determine which component sets the maximum charge current.  Obviously you can't really lower the power consumption of the rest of the mainboard (CPU, north/southbridge,hard drive etc..) but this shouldn't be a problem if you never actually power up the laptop.

However the current consumption when only charging the battery (laptop powered down) may be a fair bit less than 4 amps.  Why not measure the current consumption in this condition (with a discharged battery) before modifying anything?

[Ian.M]

Try and see if the laptop will charge from a current limited bench supply set to 15V O/C and 2A S/C.  If it will, there is a fair chance it will also charge from a current limited boost/buck module, though there may be an undesirable interaction between the module's current limit feedback loop and the laptop charge controller that may cause the output to pulse.  If you are going to experiment with this, I would advise a heavy duty crowbar circuit set to 16V in case the module output overshoots.