1984 Mercedes 500 no start

Mercedes 500 SEL 1984 200k miles. Ok, the car will start up really good most of the time, and I notice no problems driving it. If Im driving and say I turn on the heater, the radio sometimes goes off and
the lights dim and my rpm guage shoots up for like a second, then everything is back to normal. I will shut the car off and try to start it again and I get nothing at all. No lights work AT ALL. Nothing works on the car, its like there isnt a battery connected. No starter clicks or nothing. Any Ideas? Its on a charger right now
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I also just changed the battery out with a older one and the car came to life, but it needs to be charged. I dont get it, because I put a battery tester on the old battery, and it was reading good with around 12 volts, hit the load test and it went down into the yellow zone, but not by much at all, and nothing in the car would work at all. The battery is charged too. What is going on here.
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Suggest you check the ground connections between battery and chassis and chassis and motor. Replace any battery > 4 years old.
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Found it. The mechanic that installed my starter didnt connect some wire to the starter. It was just resting against the bolt and was making contact only sometimes. Drove back and told him to look at it and he said "he has no idea how it could have "came loose". He connected it and there was no more problems.
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