No power and no lights car not starting up.

[Antique]

Hey All,
I am new to this forum and have found great resources. But today I ran into a problem here are the facts that I have.

1. drove the car all morning(in the rain)

2. went out to drive it in the afternoon

3. put key in first time radio and dash came on and half a click on the starter when i turned the key.

4. putting key back in no power at all, no lights, dash, radio, buzzer . nothing.

5. hooked up trickle charger, battery was fully charged.

6 figured it might be a bad tumbler so I took it out and tried to start it with a screw driver. No dice

7. I checked and made sure it was in and out of park OK.

8. ground cable on battery is good.

9. fuse box dry as a bone.

10. steering wheel locks and unlocks OK

Only way I can start the car is by jumping the starter
I jumped starter and it fired right up https://streamable.com/l7v87d


is it something as simple as a bad battery? Alternator is 3 years old. there have been no signs of starter trouble as she fires up very quickly. it is a 1984 300d turbo. this my second 300d and love it more than the first. just need to get it back!

thanks in advance......