MacBook Pro (A1990) dead - How to diagnose if battery or mainboard issue?

Hi everyone & thanks for your help in advance! My MacBook Pro (A1990) won‘t start (no reaction at all) after I haven‘t used it for a couple of weeks. Is there any possibility to check via multimeter or other tools if the reason is the battery (repairable) or the motherboard (financially not reasonable to repair)? **Things I have tried to narrow down the cause of error to either battery or motherboard:** I tried all the keyboard combinations for different types of reset. I also tried different chargers. Open it up and disconnected & reconnected the battery (one „block“ looks a little bloated“). Also bought a USB-C diagnosis tool and it doesn’t charge at all (no current, voltage ok), no matter which port is connected (and same thing when I had the battery disconnected). Thanks a again and looking forward to your replies :)