Agreed, whatever is going on here is gonna require a professional to probe out and sort. There's likely a bypass or some other weird splice going on, but where and what else is going on with the circuit? Only God knows till someone is able to probe out the whole run.

Is there any voltage between those wires, what's the voltage between the red and neutral, and whats the voltage between the black and neutral?

To expand.

Voltage is a relative measurement - think of it as "what is the difference in electrical pressure between these two points".

If you measured two ends of the same wire, no matter what else is happening on that wire, voltage will read close to 0, unless the wire is extremely long (150 ft or longer). Most of the time when we say "oh that's a hot. It's 120v" we're using a reference that is always constant - the ground. Like the literal ground beneath your feet (or the ground wire, which eventually goes into the actual earth outside your home). Because that measurement tells us how likely (and painful) it's gonna be to get a shock from it. More volts = more pressure = more likely and more pain.

When the switch closes (aka in the "on" position), it's the same as having one long chunk of wire rather than two chunk, when it's open (aka in the off position), it's separated agian. So when it's open there is a difference in electrical pressure - one side has 120v (referenced to ground) and the other side has 0v (agian, as referenced to ground) as they are now two separate wires, one with a connection to the panel, the other with a connection to the lights.

Still, it's unlikely to be the culprit. A loose wire would been giving intermittent issues as it made and lost its connection till it finally came completely off.

It could be a dead switch (aka one that failed in the off position) though, I'll give you that. Depending on the age of the switch it might even be the issue.

If no one messed with the wiring, how would it suddenly become incorrect?

The problem isn't "I tried to swap out a switch, and now nothing works right" it's "my lights don't work even though no one messed with them and they worked fine an hour ago"

>Check the GFCI outlets in the bathroom to see if any of them have been tripped. Sometimes those get daisy chained together, even though that's not the proper thing to do.

This in particular is common in homes built between the 60s and the 80s. They used to have a two prong outlet for a plug in electric razor, tied to the bathroom lights but otherwise provided no GFI or even basic grounding protection (they did have a transformer to step down the voltage, so it wasnt a complete death trap). Homeowners and landlords typically just swap the two prong for a GFI receptacle but change nothing else about the circuit, which can take out the lights when the GFI trips. If you have an unusually large cover plate around a bathroom GFI, this has happened in your home

Speaking as an electrician that constantly has to un mud all his boxes... yeah the stuff just cracks right off. Give those tiles a month and half would probably fall off on thier own. Any flat-ish tool should be able to pop the tiles loose.

Do you understand how small 0.008" is? A human hair is 0.003".

You're literally splitting hairs just so you can walk around pretending you're clever.

Found the pain in the ass engineer every tradesman hates.

Seriously, 8 thousandths of an inch is entering the territory of CNC machining, not hanging a hose reel.

You could also use a screwdriver (preferably a square or Roberson) to sit inside the anchor and tap the driver with the hammer. A little easier to knock it in if you're in a weird spot or lack hand eye coordination.

If you absolutely, positively cannot afford off the shelf lithium system, at least consider going lead acid (aka car batteries). They require more maintenance than lithium (you have to top up the water in them) and more ventilation (they produce hydrogen gas while charging) but carry less risk of catastrophic fire if mishandled.

It's still risky, and something that I still don't fully advise doing, but if you're gonna be bullheaded about it, then at least pick the option with less chances of burning your home to the ground.

As a bonus, you can later upgrade to lithium cells, so you can still get your system up and running soon, and upgrade to lithium whenever funds permit.

As an in field electrician, I also echo the advice to buy a pre-built pack. Batteries can be little mini grenades, especially when you overcharge them or overheat them. Go ahead, Google "battery explosion" and have a feast of firey goodness.

Bigger battery, means bigger fire. And at 300+ cells... thats a lot of battery.

This most definitely not in the realm of "an internet search, a soldering iron and some gumption". The whole install sounds closer to "get at least three quotes from a local electrician or solar installer" territory.