Share some inverter ops experience for comparison

[nwclassic]

I'm gingerly testing our new system. It's composed of four (<2yr old) 6V/220AH batteries, a 2000W inverter and 4/0 cabling.
Arriving at camp, batteries showed 12.99V. Engaged inverter to power the microwave for 30sec (draws 13A), which operated properly.
The resulting voltage was 12.22, which climbed back to 12.7 fifteen minutes later. Should I consider this expected performance (must I calculate draw from both microwave and inverter)? If not, where should l look for problems or diagnostic tests I can perform? It is intended as a boondocking convenience, but only if we can learn to trust it. Our two 120W solar panels can replace at least some of the usage and a 2000W generator can save us if need be. Thanks in advance for some thinking on this.

[OTRA15]

As a lay person following issues like this, in hopes of upgrading our systems someday with lithium and solar capacity . . .

12.7 volts would appear to be fairly normal, after that usage [plus misc concurrent 12-volt DC usage] IMO. Keep experimenting and logging data often -- both 12-volt DC, and 120-volt AC inverter usage.

When possible, like after this trip, establish a baseline carefully. Charge the batteries fully, remove all charging sources [tow vehicle, shore power, or solar if any], wait an hour, with no battery/inverter draw ["Store" if possible], then take a voltage reading for each battery by meter at the batteries, then on your trailer's monitor. 12.8 volts+/- would be the norm, from what I have read IMO.

The 12.99 volts you reported was high, because you had just arrived, and the batteries had not rested long enough after being charged by the tow vehicle as you drove IMO. Gotta wait an hour or so to be accurate IMO.

Keep using and logging data. There are fancier measures of battery performance which are above my pay grade. Suggest research and getting more data.

Sorry if the above is sophomoric . . . just a layman chiming in in the middle of the night! Hopefully some real pro's will show up.

Good luck,

Peter

[OutdoorIdaho]

Battery voltage will be lower while under load. Resting voltage is what should be used as an indication of remaining capacity.

What you describe sounds within the bounds of normal performance. Of course, I am not a pro, either.

To answer another comment you made, the inverter will have internal power consumption with a higher surge at startup, on the order of a few amps.

Like you said, you'll learn to trust your system and get used to what it can support and how much your 240W on the roof can actually produce with different weather conditions.

[fran&frank]

Add a battery monitor like the Victron BMV 712 and you will be able to more accurately monitor and assess your system.--Frank

[uncle_bob]

Hi

Battery voltages vary with temperature. What you carefully read and log with a battery at 50 degrees will not apply to a battery at 80 degrees or at 20 degrees. Just how the voltages change depend on the alloy used in the plates (if it's a lead acid). That alloy is modified by manufacturers to make the battery "fit" better in a given region. ( one alloy for places it gets really cold, another one for places it gets really hot). Since batteries get shipped here and there, there is quick way to tell what you have.

As noted above, battery voltages under load are going to be a bit low. Battery voltages while charging are going to be a bit high. Any time you fiddle with a battery, it takes a while to get back to equilibrium. All of this makes doing useful voltage readings problematic.

If you are serious about running off grid, battery usage *is* part of the equation. Knowing what you are pulling from the batteries ( or putting into them) right now is important. Being able to capture the history of these things is very useful. Something like the BMV-712 is a great tool to do all this. On top of the "what's going on now" stuff, it will also track exactly what's in your batteries. A shunt based monitor (of which a 712 is one example) is the only practical way to do all this.

Indeed, there is a bit of setup involved in getting the absolute best accuracy from a shunt setup. It needs to understand what type of batteries you have. It also needs to know a bit about your charge profiles. None of this is crazy hard to come up with. It just takes a bit of digging and observing what's going on.

Fun !!!

Bob

[GetawA-S]

Amps at different voltage s are apples and oranges. I prevent you use P= IV to determine watts. This makes comparisons easier.
Running your microwave at 13 A , 120V (1560W) for half a minute (1/120 hr) should have used 13 WHr. The inverter base load will make this higher.
If you like working with amps, the battery voltage at 1/20 house current voltage would have you drawing 260 A (again, allow for inverter inefficiency).
Against your battery capacity of 5.28 kwHr (half of which is usable), you have drawn a very small amount. Estimating capacity from voltage is tough to accomplish reliably.
IMO, meter inaccuracies or battery equilbration lag make your initial test too imprecise to be useful. You need to use more of your capacity, and not take readings too soon.

[TouringDan]

Originally Posted by nwclassic

I'm gingerly testing our new system. It's composed of four (<2yr old) 6V/220AH batteries, a 2000W inverter and 4/0 cabling.
Arriving at camp, batteries showed 12.99V. Engaged inverter to power the microwave for 30sec (draws 13A), which operated properly.
The resulting voltage was 12.22, which climbed back to 12.7 fifteen minutes later. Should I consider this expected performance (must I calculate draw from both microwave and inverter)? If not, where should l look for problems or diagnostic tests I can perform? It is intended as a boondocking convenience, but only if we can learn to trust it. Our two 120W solar panels can replace at least some of the usage and a 2000W generator can save us if need be. Thanks in advance for some thinking on this.

Your testing results sound about right. We have a 2,000 watt inverter and 3 BB lithium batteries so the voltage barely drops at all when we run the microwave. This, of course, is one of the advantages of lithium, but as long as your voltage stays high enough while you are operating the microwave that is all that really matters. I would just operate the microwave normally until you have a problem.

Dan

[nwclassic]

Responses are thought provoking, so thanks. To fill in a bit, the inverter and cables are the new part of our electrical system (batteries, panels and Solar Boost + Link10 gauge were a previous upgrade). We can observe +,- current flow and will do so when we next test. Seems like a good activity while we're at home... with the ultimate goal to know what we'll use BEFORE we do it .

[SuperTrouper]

"Arriving at camp, batteries showed 12.99V. Engaged inverter to power the microwave for 30sec (draws 13A), which operated properly." Remember, 13 amps at 120 Volts is 144 Amps @ 12 volts (assuming an inverter efficiency of 89%). That's a big instantaneous amp drain and the battery reaction you recorded seems normal to me. - brad

[OTRA15]

Originally Posted by SuperTrouper

"Arriving at camp, batteries showed 12.99V.
. . .

Chances are good that the 12.99 reading was artificially high per Post #2 IMO:

Originally Posted by OTRA15

. . .
The 12.99 volts you reported was high, because you had just arrived, and the batteries had not rested long enough after being charged by the tow vehicle as you drove IMO. Gotta wait an hour or so to be accurate IMO.
. . .