Nasty Inverter / Electrical Problem

[Jon Wehrenberg]

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?

[MangoMike]

I have no solution.

But when Jon is looking for help we have a major "O CRAP MOMENT".

mike

[dreamchasers]

Jon,

Since I just finished up with my inverter upgrade, perhaps I can offer some views. First, when the panel gauge is swinging, does the battery bus follow the swings (check with a volt meter at the DC bus source). If the battery voltage is stable, you could have a gauge issue. Secondly, if you battery bus is swinging as you described, a loose or dirty connection would be suspect. After replacing the battery lugs and cables on my Country Coach, I easily see that connection integrity is important on the heavy DC bus used by our coaches.

I would doubt that your battery buss is actually swinging (Serious!), but if it is, a bad connection or bad battery could be an issue. Also, if you suspect a battery, point a IR heat detector at each battery to look for temperature differences. If a bad battery is loading your DC bus that much, heat will be the by product.

Let us know what the outcome is.


Hector

[Jon Wehrenberg]

All connections are clean and tight. I made sure of that when I replaced the batteries last fall, and verified it this morning.

Yes the voltages are swinging and no it is not a gauge issue. I have verified this with the multi-tester.

I spoke to Troy at Liberty (the Stuart service guru) and he suspects a triac failure in one or both inverters (I am not a believer in simultaneous failures) but until I can access my inverters (located beneath the bed and requiring substantial disassembly for access) I will not be able to verify this. Once I get to the inverters whether they need triacs or not they will be replaced and all cable connections at and in the vicinity of the inverters will be checked and tightened if necessary.

Forgot to mention I am letting the batteries discharge now with the inverter / chargers off. I will look for unusual discharge rates as well as elevated temps after re-charging.

[sawdust_128]

Jon:

Do you see the same result while on shore power, generator and alternator? I have seen something similar to this and in checking my notes it turned out to be bad power comming into the coach from the shore source. But, you and I have explore the differences of the wiring system of my coach.

Have you verified the swing on the guage with actual voltages at the batteries? Is it possible that there is a loose connection/short in the instrumentation and not actually a voltage fluctuation?

Have you taken the inverters through a reset (if your inverters allow it)?

******************On edited *****************
After I posted this up, I saw the other comments.

[Jon Wehrenberg]

The shore power is clean and steady at the voltages listed above. I reset the inverters by removing all power and then restarting them. The voltages at the battery terminals fluctuate as does the dash house voltage gauge and as does BOTH inverter voltage displays.

The alternator does not run through the inverters, but merely charges batteries, its voltage regualted by Mr. Delco Remy.

The alternator cable at some point (after going through the isolator) joins with the battery charger cables from the inverters. I have no diagram so I don't know which cable is which because once they disappear into the hidden recesses of the bus I lose track of them. There must be 15 or more, all are black, some vary from the common size but there are enough of the same size to make identifying one from another without a wire diagram unlikely. There are so many connections and cables I will not start disassembling stuff until I can have several days in a row to work on this issue, something I do not have now because we leave on a trip in one day.

[truk4u]

Jon,

When your getting the fluctuations in house charging voltage, check the D/C amps at both inverters and see if there is a big hit of D/C being consumed.

On my dash I have the 24 volt gauge also, but since mine is a naked lady, my gauge is chassis only. But, When I have the chassis charger on even in the float mode, the 24 volt dash gauge get pegged beyond 30 volts! I checked the actual current at the batteries and they are getting the 13.2 float as they should. I only mention this because the gauge is wacky on the charger, but normal when running.

Wow, what a rare event, the Master asking for help!

[mike kerley]

Jon, It sure sounds like a connection issue. Did you actually remove and clean the battery contacts or just check to see if they were tight? How about a bad crimp on a cable?

Has the bus been colder than normal as of late? Are the inverters always on and online, keeping warm? Perhaps an electrical connection internal to the inverters, with one ghosting the other. Are the Triac's heat sunk to the chassis? Cold causing contraction and poor contact.

[Jon Wehrenberg]

Connection issues will be my focus. I verified all the connections at the batteries and the main panel located in the battery compartment. Two areas I have not yet accessed and will not until we have time and that is where the cables meet at a terminal block under the bed and at the chassis ground near the engine. Tomorrow I will do some of this checking.

When the batteries were replaced this fall all connections were wire wheel cleaned, all cables were unfastened from their terminals and wire wheel cleaned and new bolts were used if there had been any sign of corrosion.

Throughout today I have been looking for warm connections and any other indicators of loose or faulty connections, but except for the ones mentioned above I have seen no corroded or loose connections.

[MICHAEL BELL]

Hey Jon, sounds like pending an investigative inspection of the inverters and connections there, you are on the right track. The thing about the inverters is that there are output filter circuits that can be damaged by severe ripple, which could have been caused by a failure in one inverter. By this I mean that both could have a problem, just not the [I]same problem[I] Please let me know what you find, or if you need further assistance. Wish I was near you to aid in your investigation. Sincerely, Ben 813-830-8619.