Xperia Z3 :: Any Way To Charge From Fastboot Or Flashmode

i have an xperia z3 that has been working fine for five moths now on lollipop , however today i noticed that the battery indicator changed from 17 percent to 50 percent to 7 percent every couple of minutes, i looked up online and somewhere it said to let it discharge completely then recharge in all stupidity i used m phone till it suddenly blacked out, i was like

step 1: check

step 2 however wasnt working as when i plugged the charger it put up the charging battery screen with the red light, then it blacked out again and it just keeps on turning it self on and off from black screen to battery screen

id like to let it survive for a while till i can afford a new battery or something, can i do that?.i will let it charge for the night, however i need the phone in 3 days and cant afford a new battery

i found that i can get to fastboot mode and flash mode , but thats it cant reboot normally.i changed cables and chargers and even switched to magnetic charger,charged other devices using my charger to make sure, forced shut down using power+volume up and using the killswitch but its all in vain and the problem is just the same, many people who had this problem got it fixed by forcing shut down and changing chargers but i let it for hours and hours to charge

i also updated from lollipop 5.0 to 5.1 via flashtools since i got to flash mode, i erased user data and everything and it installed perfectly but still when i disconnected and pressed power button it showed the low battery icon and it still wont charge.is there anyway to chrge it from fastboot or flashmode and boot it from there cuz they work