Sounds like a defective battery to me. You need to do some testing, and for starters "pun intended" it's not always easy. You need a carbon pile voltmeter, a multimeter, and an extension wire any size to reach the front to rear of the car
Like this:
What voltmeter tests will NOT tell you without a carbon pile:
What the starter is drawing, IE if the starter is defective, or if the engine is trying to seize.
However you can make an educated guess with just a meter.
If you have another battery ANY decent battery, the quickest way is just to exchange batteries.
You can get a GOOD idea of how good or bad your overall connections, disconnect, added relay/ solenoids, and cables are with a couple of voltage checks.
"String out" your extension wire. Clip the meter to the engine block, and clip the other probe to your wire. Run the wire clear to the back, and have a partner "stab" the remaining connection directly into the top of the battery post.
Crank the engine, read the meter while cranking. Notice the quite low reading. The lower, the better. Post that here
To check the hot side, same idea. Clip one meter probe to the starter hot post, the big post. Clip the other meter probe to your extension, and have your partner stab the probe into the top of the POS post. Crank and read. Should again be a very low reading, the lower the better.
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Have your parter stab the meter probes right into the tops of the two battery posts. Crank and read. Here, the higher the better. Below 10.5 is pretty bad for a battery that "hot." (Maybe the battery is just DEAD, IE discharged)
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