Consistently, once my battery charge gets at or below 70% the inverter will no longer power the microwave when not connected to shore power, and the ‘low battery’ light on the Multi-Control turns on. The batteries continue to supply DC to run other items well below a 40% charge. The RV has 2 100Ah lithium batteries and a 3k watt MultiPlus inverter.
I have had it to 2 service centers and I’m not sure if their expertise is sufficient to solve this problem. One told me the ‘current limit’ on the Digital Multi-Control needed to be increased to 20 amps or more to run the microwave on batteries alone - but it seems this limit is applicable to AC in from shore power, not AC out from the inverter. I have had that limit set to 15 amps.
Could that be the issue? Other thoughts on what is happening when my battery charge goes from 80+% to 65% such that the microwave no longer gets enough juice?
From a distance, this simply sounds like switching off the AC Out2 relay … to protect the battery from heavy loads.
Do you have the technical ability to look into your Multi Plus configuration or full VRM Access?
The question would be whether the other loads continue to be supplied and whether at least the display/lighting of the microwave works or not at all.
Your battery bank is half the specified minimum battery bank capacity to supply a 3kVA inverter-charger, so it’s most likely that the voltage is just sagging under the heavy load of the microwave.
@Slit69 firstly your comment about the current limit setting input current from mains is correct. I assume the batteries are 12 volt. My microwave draws about 150amps so it is a fairly heavy current drain from your two batteries so perhaps the batteries are struggling to handle that load.
You haven’t mentioned if it fails after a little while or simply doesn’t allow the microwave to start at all. Perhaps you could just check the battery voltage and see if it drops considerably under heavy load. Maybe the cabling to the multiplus could be checked to ensure no high resistance joints.
My technical ability would be at or close to zero when it comes to this.
I don’t think it’s likely I will solve this myself. I did the post because I don’t trust what the service techs are telling me. It’s an Airstream center, and I’m wondering if the Airstream service techs in my area have the expertise to diagnose and solve the problem.
Two days ago the tech told me the reason the microwave would not run of the batteries when not connected to shore power was that the current limit on the Multi Plus needed to be turned up to 30. That doesn’t seem correct based on what I have read, and I couldn’t get him to demonstrate it because the battery charge was 98% at the time… and the microwave has always worked fine when the batteries are charged at that level.
Regarding your question about the other loads continuing to be supplied, I know the other DC loads off the battery (lights, refrigerator) worked fine, but I didn’t check any other AC loads. I know the microwave was getting enough power for me to try it multiple times, but each time it would only run for 3 - 4 seconds and then shut down.
Why don’t you unplug the microwave and try a much smaller load … Lamp etc … then we’ll see if the whole thing is switched off completely … i.e. the AC Out 2 relay …
Interesting. I wasn’t aware that the ability of the 2 100Ah batteries to supply power would drop so much from 100% capacity to 70% with this inverter.
The RV came with a 1kW inverter and the 2 batteries, and we had the inverter switched to 3kW so that we could run the microwave. But your saying the 3kW inverter only supplies 1300W of AC power when it is connected to 4 100Ah batteries?
Ah no; a 3kVA inverter will supply 2400 watts to a common mixed-load with a power factor of .8, though I’d note that microwaves often have much greater issues due to their duty cycles; that depends on the microwave though, as some are remarkably efficient.
What I was saying was that any inverter will have a minimum specified battery bank capacity to ensure the inverter’s ability to perform to spec; the system you originally had sounds appropriate, with 200Ah of battery capacity supporting a 1kW inverter. When upgrading to a 3kVA (or 2.4kW) inverter, it’s necessary to also provide the supporting upgrades: increase the battery capacity, upgrade the wiring, fuses, possibly switches, etc. A system not installed to spec is unlikely to be able to perform to spec, to put it simply; with that small battery bank, the microwave is pulling nearly 1C from it and the bank -and very likely the wiring and etc if that wasn’t upgraded to spec when the inverter was upgraded- simply can’t support that load for very long, especially when the bank has already been substantially discharged.
While the service centers are indeed wildly off-base with what they’re telling you, and I certainly wouldn’t advise taking the rig back to them if that’s the sort of troubleshooting they’re doing, a good independent installer should be able to sort the issue fairly readily: install an additional 200 to 1000Ah of battery bank as the MultiPlus manual specifies (minimum 400Ah, recommended maximum 1200Ah as I recall), upgrade the DC cabling if needed (should be 4/0AWG or 2x2/0AWG), check the fuses and switches to make sure they’re appropriate, and you should be good to go!