My new backup battery

Here in central Florida there is about one power failure a week. Most last a minute or two and are no problem since I have an inexpensive backup battery/power surge protector but it is pretty wimpy. We did have the power go out for a few days once when someone struck a transformer pole. We also get the occasional hurricane passing by like tropical storm Nestor is doing as I type this. Lots of rain but no wind. I decided I needed to upgrade my backup battery/power surge protector and I found a nice one with automatic voltage regulation and a nice LCD screen.

https://www.amazon.com/gp/product/B000QZ3UG0/ref=ppx_yo_dt_b_asin_title_o03_s01?ie=UTF8&psc=1

I installed it yesterday and it took all day. The problem was I have a very complex audio video system and I had to completely disassemble everything and then reassemble it correctly. It has been years in the making and there were many things about my installation the I did not like. Such as the old battery backup and my modem were both located in a cabinet and I had put a bookcase in front of the cabinet making it a hassle to get into. I relocated the new unit and my modem directly under the audio receiver and in plain site from anywhere in the room. This allow me to monitor the power and the Internet at a glance. The LCD on the backup shows all kinds of information. Input voltage, output voltage, number of minutes left on the battery, etc. I am not worried about powering the whole house, just the modem and my laptop chargers. I realize a generator might be a better option in some instances however I have no garage to store it in.

But this post is not to brag about my new toy or to recommend this particular unit. I have a question. I have the LCD on the backup set to show input voltage and I noticed something I cannot explain and I am hoping someone (such as Lou) has a good explanation. My input voltage runs pretty constantly around 118  but once I turn on the A/C the incoming voltage shoots up by about 2 volts. When I turn off the A/C it drops back down almost instantly by two volts. This is the opposite of what I would have expected. Anyone have any idea?

It's called neutral swing.  On a 50 amp 120/240 circuit the neutral divides the incoming 240 volts into two 120 volt circuits.

When the A/C kicks in it unbalances the load between the two sides of the 120/240 volts, sending current through the neutral wire.  Since the neutral is tied to 0 volts at the main source panel, the voltage loss through the wire raises the neutral voltage at the RV end.

This subtracts from the voltage on the more heavily loaded leg and adds to the voltage on the opposite leg, in this case the one with the audio equipment.  The neutral "swings" towards the more heavily loaded leg.

A couple volts swing is normal.  You only need to worry if it gets excessive.

He probably would have thought of this corollary, but he can't hear when the AC comes on  ;D

Thanks for that Lou, I knew you would know. Your knowledge of electricity is mine blowing. I do have a 50 amp connection and I am embarrassed to admit I had to go look at my main breaker to be sure. I haven't plugged this in in three years.

Thanks, Tom.  I'm good on theory, just don't quiz me on everyday practical stuff like wire gauge, conduit capacity, etc.  I have to go and look that stuff up, it's not part of my working knowledge.

You are welcome Lou. I haven't worked as an electrician in 30 years and I would have to look up just about anything. I have been trying hard to forget everything electrical. I even hate changing light bulbs.

Well I finally got around to finishing it yesterday. I had to move the cords around to the right outlets. 5 outlets are on the battery backup side and five are not on the battery backup. All I wanted on the battery backup side is the TV, receiver and the modem. I tested it and I get about a half an hour with the TV turned on and about 600 minutes with only the modem turned on.

On the other hand I just microwaved a tv dinner and the voltage dropped by 4 volts until the microwave turned off. Would it be correct to assume the microwave and the battery backup are on the same leg?