On a Liberty 24V coach of my vintage there is a gauge on the dash that shows house voltage on the 24V side. For discussion purposes the normal displayed voltage with the bus connected to shore power, both inverters on and no serious loads, and fully charged batteries the voltage sits steady at 27Volts.
Yesterday I noticed the voltage needle was swinging from 27 up to being pegged at the top of the scale. For information purposes the inverters will shut down at 31 and come back on when the voltage drops.
To try to figure out what was happening I started by shutting the inverter charging circuits off one at a time. My initial thoughts were to isolate what I originally thought was a bad inverter. I had all the ceiling lights on while this was happening and they show a load of about 9 amps on the AC side so the charging circuits of the inverters were supplying charging current to satisfy the load.
Shutting each inverter off in turn did not stop the fluctuation of voltage. I think that rules out an inverter charger problem since both inverters alone were exhibiting the same voltage variations. I then shut off the lights using the master switch and after a while, although not immediately the voltage settled down to the normal range of 27V, but only with the #2 inverter on and #1 off. If I put #1 on, either in conjuction with #2 or alone both charging current in amps and voltage fluctuated up and down the scale.
With #2 on only, and no lighting loads the charging current sits at float and the voltage stays where it needs to be. But it only gets to a steady state after wild voltage swings for a while, which can vary from a few seconds to several minutes. I believe whatever the problem is relates more to the #1 inverter circuit, but since the two chargers are charging the same set of batteries (4 hooked in series and parallel like the chassis batteries) the problem is not with the inverter.
Since I can make either inverter fluctuate wildly I surmised the problem lies with what is common to the two inverters and not the inverters themselves. So I pulled the master DC panel out for inspection of all the master switches and the main curcuit breakers. I could not see any chafing, loose terminals, bad connections or anything to suggest a problem. The batteries are brand new and their connections are clean and tight.
I added insulation tape where there might be a possiblity of a short, I added some wire ties to the cable bundle (it did not do anything but make the cable bundle a tighter loom) and put things back together with the result being no change.
An inverter can exhibit the characteristics I described if the inverter supply AC power is somehow connected to the inverter 120V AC output which "bootstraps" the charger circuit with its own output power causing such voltage fluctuations on the 120VAC side. I will check that but that is almost impossible because of the fact each inverter supply is on a separate bus bar from the output and the outputs are on two separate bus bars.
The inverters are Freedom 2500 and have remote panels. The panels provide limited diagnostics, but nothing in terms of diagnostics that I cannot get with a multi-tester. I am getting voltage swings and I am getting charging currents that swing from high to low. What I don't know is where the inverters get their feedback that causes them to respond as they do. I know the battery cables from the inverters are the likely sources of the data the inverter chargers process, but what makes the feedback to the inverter drop so the charging voltage and current goes up is still a mystery. The swings are irregular and take place in a second or two. AC input power was 122-123 volts last night and is 121-122 this morning, so input voltage is steady and not an issue.
So what is the problem and where do I look?