No power 09 Rogue- need help!

I have a 2009 Nissan Rogue and have been having problems with it starting. I’ve had to have it jumped several times the last few months and I’ve assumed it was a dead battery from not using the car in a week or more. A few weeks ago it was totally dead, no dash lights, my Viper wasn’t working all, nothing. But it jumped instantly and started fine. Brought to autozone and the tested the battery and said it’s voltage was fine and suggested getting the connectors changed. Go to garage and tell them (the second time I went there for this problem- said the battery tested fine last time so they didn’t do anything) and they only cleaned the connectors and said everything was fine and nothing needed to be replaced, just cleaned out some corrosion. Couple days ago it wouldn’t start again. Mechanic I know came and looked around under the hood and he said he couldn’t see anything out of the ordinary. Brought over his scanner and jump pack today and jumped it. Scanned it and the voltage in the battery was jumping around from over 16 to under 12 while it was just idling after the jump. Had me turn it off and try turning it back on and everything was immediately dead again. Said it may be the alternator but he couldn’t get a good scan with the voltage jumping around.

Any ideas? He also said it could be electrical with the viper alarm but he said he honestly never seen anything like this. (He did say it was odd when I told him the other day I hit a couple bumps and the stereo muted itself. )

I have a video on my phone of trying to start it before it was fully dead but I don’t know how to post it since it says it’s not a correct file type.

The battery voltage at idle shouldn’t be jumping around much, and shouldn’t be over about 15.5 volts max. So if that’s actually happening, addressing that problem is where to start.

If the above turns out to be a red herring, and a battery load test passes, the next step is to measure the voltage at the two starter terminals during attempted cranking (key in start). Both should measure at least 10.5 volts, probing from terminal to starter case. Do they?

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The battery voltage should be pretty steady. It appears the alternator has a problem. I assume the voltage regulator is inside the alternator. You should also fully charge up the battery using a battery charger and not make the alternator charge it up a discharged battery. That puts excessive strain on the alternator. Have a shop check to see if there is an excessive current draw on the battery while the car is parked. That will discharge a battery also.

Many new vehicles don’t have dedicated regulator but use one of the control modules to regulate the alternator output. The alternator is not designed to recharge a completely depleted battery. It was dead. You jumpstarted it and may have watched the control module enabling charge, seeing it maxed out and then turning it off to prevent damage. It continuously cycling due to the state of the battery. Shut it off and ithe battery is still too depleted to do anything shows it’s deeply discharged. In that case, the battery should be recharged by an external trickle charger until it’s fully recharged before doing anything else. At least you risk chasing your tail on the diagnosis and could potentially damage the alternator if you don’t recharge it properly. Then test the battery. Then reinstall it and test the alternator. All looks ok, start looking for a parasitic load that is draining it…