How to 5V 2.1A to 8.4V 400mA?

>>923216
yes.
>8.4vx0.4A = 3.4W
>5vx2.1A =10.5W
plenty of power. that thing is fucking huge though. just search ebay for "buck stepup" to get something lighter(50g or so net mass), smaller and that costs 1$

>>923234
Buck converters reduce voltage. He needs a boost converter. That thing doesn't really look that big either imho

>>923216
http://www.ebay.com/itm/Module-XL6009-Replace-LM2577-DC-DC-Adjustable-Step-up-boost-Power-Converter-BG-/351195147071

This should be good enough, $1.30 shipped.

>>923216
The first thing you should do is dismantle your portable battery and identify whether there is already a switching regulator inside it (highly likely), and if it's not a fixed-voltage type, identify the resistor divider that feeds the feedback input, and determine if you can adjust it to output 8.4V.

>>923355
You'd probably kill it in the process

>>923548
this is /diy/
we dont mind killing a few cheap gadgets
especially if we can learn from eachothers mistakes
and even more so if we succeed

409shop sells a USB charging cable that does just this
5v in, 10 volts out (to plug in to the charger base)

It's just $6,5 and I ordered it yesterday. I'll see how much current it draws.

you could buy the 3.7v uv3r instead and charge it from your current battery without any modding.