Hi. In the Cessna 152 there is a master switch that is split into two parts. The right side is a battery master and the left side is an alternator switch. If I turn on the battery switch the electrics power up and the amp meter goes to positive. If I instead turn on the alt switch nothing powers up and the amp meter goes negative. If I then turn the battery switch on as well the amp meter remains negative. I would have thought that the amp meter would go positive if the alternator was charging the battery.
What’s going on here?
TIA!
Battery switch controls all power. Alternator just whether the alternator is charging the battery. You can google the 152 flight manual which includes details of the electrical system function and wiring.
Hi, this is a bug within the MSFS C152 aircraft. In real world Cessna 152 you (roughly) perfom this steps. I did my real world PPL training partly in C152, so I’m familiar with it. 
- Open fuel valve
- Turn on BCN light
- Turn on BAT switch (Power from the battery)
- Fully enrich the Mixture
- Set (push) Throttle a little bit (half inch or so, 10% within the MSFS)
- Make some strokes of fuel using the PRIMER
- Start the engine and check oil pressure is growing to the green range
- Set Throttle to 1000 rpm
- Turn ALT switch on and check Ammeter. Needle should show slightly positive value - battery is charging. Red light bellow should not be enlighten.
I’ve already reported this to ZenDesk.
If you want correct behavior of C152, you may install this C152x mod GitHub - DRF30q/C152X: C152 Realism Mod for Flight Simulator 2020 into the Community folder. It fixes and adjusts couple of things, including this one. 
EDIT: Check this for some procedures https://i.redd.it/yfo0nely97g31.jpg
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Thanks SO much that was exactly what I was looking for!
