Power problems MacBook Pro Mid 2012?

Hello. I have a Macbook Pro 13" (mid 2012) 2.9 GHz i7, 8Gb RAM, 128 SSD.

The problem is that when it's running on full battery it shuts down after 5 minutes or so... If I plug the charger, it shuts down too.

When the charger is plugged in I noticed: a dim green light (with a slight tick at the DC-in) or a bright blinking green light with the same tick, or a amber light (charging); blinking between amber and green light. With all these different lights the computer acts the same. Sometimes it starts up and boots but I can't use it for more than 5 minutes, sometimes 10, sometimes doesn't start at all.

Sometimes the battery indicator lights show: all 6 green lights from the button to the edge of the computer for 4-5 seconds then fade, when charging displays 5 lights and 1 blinking , sometimes (when battery full) displays 1 or 2 leds.

The first time when this happened was when my Mac wasn't plugged in for 1 week. When I plugged in, showed no lights at all for 15 minutes, then amber and then green and was ok. After that (about 2-3 weeks) it was plugged in (green light) battery showed 97% and out of the blue it shuts down and the green light started blinking. I tried the SMC and nothing. I removed the connection of the battery on the motherboard for 1-2 seconds and then when I plugged in it showed green light and started automaticaly without me presing the power button... then shuts down and same problem.

I didn't try PRAM reset because I can't use the Mac for more then 1 minute now. (10 was at the beggining and 5 was after, tops) . Also, I haven't try to run the AHD because of that.

Please don't tell me to go to Apple because the Mac was bought secondhand (I can't afford a new one) I don't have the bill or the Apple care document (expired anyway), and because of that I guess they will not repair it even if i pay all the costs.

The OS is El Capitan 10.11.4 Thank you very very much and sorry for my english, it's not the first language (I'm from Romania)!