i drove to my grandmothers and discovered i either have a dropped cell in my battery, a bad alternator or a bad ground. im guessing its a bad ground. every time i hit the brakes, all the lights dim for a second and every time i hit lock or unloak with the car off all the lights dim. also, if you hold the window up button, the car dips to 11V even if its running. then about 10 seconds later its right back up to 14.

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I'm guessing more towards alternator. Seems when under load and low idle the alternator is not able to output enough amps. Battery should not be much of a factor with car running. Bad ground should cause additional issues at times other than when under load, like when hitting a bump, etc.

corroded grounds can seem just like bad battery sometimes. thats why i always start cheap and clean them, if no change then move to the alt. alot of times they last about the same amount of time though i have had to replace both within a few months on a ton of cars, i think the alt may under charge the battery and they dont like that.

I think Michael is right, at low speeds the alternator is not supplying as much of a charge back to the battery, I am pretty sure when you are idling at or below 800 RPM'* the car starts to pull more from the battery, which sounds like what you are seeing.
I have always had my lights dim a little bit when braking, and when rolling a window up or down.

The only possible way I can think of to try to solve this is to upgrade the big 3 wires, and then maybe upgrade the alternator, clean the grounds while at it like Jusin suggested.
Here is a thread where Art did his wiring, and got the alternator upgraded.
https://www.gmforum.com/buick-172/al...please-305375/

Getting an AGM battery may help a bit also, forgot about that.
ODYSSEY Batteries