Q&A for How to Build Your Own Uninterruptible Power Supply

To test your generator's output, connect a multimeter to the generator output and measure for voltage. Check the instructions that come with your multimeter for more details.

Sure, you can always increase/decrease the amount of energy a premade UPS holds as long as the battery/-ies are the same voltage and the cable you use to connect them to where the original battery was is suitable for the current.

There's likely a short somewhere. Double-check everything and isolate parts of the system if necessary to troubleshoot.

you would need to convert DC 12v or DC 24v to (assuming an Android phone) USB (DC 5v). There have been a few YouTubers who have shown various degrees of success using a wall charger to do the necessary conversion. They don't really care too much what you put into them, the just regulate the output. The basic result showed if you had a quality charger, it works, but don't trust the gas station $5 units to hold up (even in normal use, really). I would get a USB power meter and make sure before you use it.

Each piece of the setup described is at best 80% efficient, meaning you would only extract 80% of the energy you put in. What you want to do would be called a power leech, and it's something that would drain your batteries slowly instead of charge them.

Not exactly, you should ground it to the neutral of your distribution panel, which in turn is grounded to the ground bar and the grounding rod, but the other terminal (the hot) should not under any circumstance back feed the distribution panel. You should also ground the chassis of the inverter to a ground source, such as the ground bar in the distribution panel (it may be the same bar or may not, depending on if it's an auxiliary panel or the primary panel). You should also install a throw-over relay to prevent back feed, which could be a dangerous situation to any utility worker. If any of this doesn't make sense, consult a licensed electrician.

The inverter and charger both connect directly to the same battery - the same place. So the + leads of both will connect to the positive terminal on the battery and the - leads of both will connect to the negative terminal on the battery. Multiple batteries of the same type can be connected in parallel (positive to positive and negative to negative) to increase the outage survival interval if you like.

Lead acid batteries release hydrogen when charging. Hydrogen is explosive when at 4% or higher concentration in air. If charging batteries indoors, have due regard for ventilating the area.

Yes, there are specific devices that are designed, tested and labeled for such purpose. You would need to confirm with people at your local power company which devices they allow. For example, your inverter may be required to have a fail-safe disconnect when "normal electric" goes out, so that you are not "backfeeding" power to the grid, which could kill unsuspecting people miles away when they are dealing with "wires down" that they believer are "disconnected" from all power.