Battery not holding charge

Hi everyone, I'm Dan. I'm a new rider and I recently bought a 1979 Kawasaki kz650. About a week ago I accidentally left the key on in the bike for 15 minutes and the battery died on me, I hooked the battery up to my car battery and for a moment the light comes on again and I was able to start the bike. Then I started removing the jump cable accordingly, black head on my bike first, right after that my bike died instantly. So I went and get a fresh new battery and it works fine again...for 2 days. I notice that the battery is not charging? I have to charge it every day basically to get it running. Every fresh charge I can start the bike with the electric starter, but then if i let it sit overnight the next morning the electric starter doesn't work anymore, and I have to use the kickstarter. The kickstarter works and then it also starts failing as the neutral/oil light etc get dimmer. Until where I can get it started, but the moment I shift gear and start running the bike for even just 10 feet most the bike simply started coughing and died, then neither electric starter or kickstarter will work and I have to rip the battery out again to charge and the cycle repeats. Please help, I don't know if it's the rectifier or stator or whatever it is that broke, because it sure isn't the battery

Have you done any voltage tests as described in the manual. That would go a long way to solving this problem..

Do you have the stock points system? If you are running a Dyna S, it takes more power and will discharge the battery.

Hi, so I checked the rectifier, but I'm don't really understand what the numbers mean. So I set the multimeter on the diode? tested the rectifier, and the middle yellow lead reads around 950, while the left and ride side yellow lead reads around 550 and 650, Does that mean my rectifier is fried or okay?

Are you measuring to ground or to the positive output? Those numbers should be more like0.950 or 0.550. Is that what the meter is showing? Also what polarity is giving you those numbers? An actual diode test on a 3-phase rectifier would have 12 results.... 2 for each diode... one in forward and the other in reverse.

Which manual are you using and what meter are you using to conduct the test? Does it have a diode checker or are you using the ohms reading to do a diode check?

Are you running a Dyna S or stock points?

Have you check the battery acid Level? One dry cell of six will do a huge voltage drop on the first start.
Check for hidden electric consumers, set the multimeter on Ampere and connect the multimeter wires between the battery plus terminal and the battery plus wire. IGNITION SWITCH IN OFF POSITION!!! or you will kill your multimeter.
The Amperes should be near Zero.