Try to charge your battery to 100% on other phone (I know you have one
),or use this one,but wait untill you will see the green light
connect your testphone to the regular charger (without battery) and stick stylus (like you do soft resets),unplug charger.
Put the battery back,connect charger and try flash from SD
I saved my testphone few times but its dead now coplitly lol.in my case ,I think,it was a build-in battery problem