Triumph Thunderbird 900 Starting Issue

Hi, I’ve been doing some reading on the different starter issues but can’t seem to find what matches. I bought a none running thunderbird and starting to get it running. I was trying to check compression and ran into a weird occurrence. I put a large charger on the battery while turning over to make sure I had good power to start. When turning over starter would not engage until several rotations and battery was drawn down. The. Would turn over a couple times and then voltage would be to low to turn over. I am very confused. Based on what I had read if it was the spray clutch it would not engage at all, and just the fact that it would not engage starter until voltage Lowered doesn’t make sense. Please help

 

I first put a tender on for the past 2 weeks to try and top off the battery but I used a battery 40 amp charger/ booster that we use to just trucks and tractors all the time. I can try a new battery. According to our booster, the battery tested good though. So the starter Spinning but not engaging until the voltage had Dropped is normal with a bad battery?