Is my battery test valid?

I have questions about testing a battery from one of the major solar suppliers. I've used the battery for a couple of years for occasional summer weekend camping. It is part of a small solar rig with battery, panels, charger and inverter all from the same manufacturer. The battery was stored indoors over the winter. I've always suspected that it doesn't have the advertised capacity so I recently tried to test it with equipment I already have.

My test was to fully charge the battery in the solar system, then disconnect the panels and run a small AC load through the inverter and measure the delivered power with a Kill A Watt meter. I wasn't after a precise measurement. I just wanted to know if the battery is in the ball park of its specified capacity.

The battery is a common 12 volt, 100AH battery and the inverter is a 3000 watt inverter. The AC load was purposely kept low, at less than 600 watts throughout the test so the inverter wouldn't draw too much current from the battery.

With this setup I assumed that ideally the battery can hold 1200 watt hours or 1.2 kWh. The inverter is advertised as >90% efficient so it should be able to deliver about 1000 watts to the load. Assuming I can draw more than 50% of the stored energy from the battery that should put at least 500 watts into the load. I actually measured about .32 kWh or only about 320 watt hours before the inverter shut off, or barely more than 25% of the ideal battery load.

Inquiring about a refund under warranty I was told that the battery could not support a 3000w inverter so essentially my test was not valid. While agree that I could not draw 3000 watts out of one battery, I think that the battery should be able to supply any >90% efficient inverter with something less than 1000 watts.

So am I mistaken or missing something? Thanks for any insight.

BTW, I bought the larger inverter because it was on sale and I assumed that over time I could add more batteries to the system, not because I used it to drive larger loads. I've since been told that you shouldn't mix old and new batteries together so lesson learned.

Buy a battery load tester . And test just the battery. What's $50 compared to getting a new battery?

MakerHawk Electronic Load Tester USB Load Tester 150W 200V 20A Resistor Adjustable Constant Current Battery Capacity Tester Module Intelligent Discharge Resistance Power Tester https://smile.amazon.com/dp/B07F3NH...abc_Q4YV9SVZS969098079A3?_encoding=UTF8&psc=1

I assume the battery is lead acid.
The 100AH is specified when discharged over a time period of 20 hours. That would be a 5 amp discharge current.

In your test you discharge with 55 Amps. There is a peukerts formula for a lead acid battery that will recalculate the capacity for the bigger load. With such a big load your 100AH battery will shrink to a lesser AH battery (maybe 60 or 70 AH)

Thanks for the replies. I neglected to mention that the battery is one of the common 12 volt 100AH Lithium Iron Phosphate batteries. I've already replaced it with other batteries so I'm hesitant to spend any more on it.

But I am curious if my test is valid or if there's some reason I shouldn't expect to draw more than 320 watt hours from a 1200 watt hour battery in that configuration. The manufacturer claims that I can't drive a 600 watt load with a 3000 watt inverter on a single battery but that doesn't make sense to me yet.

For LiFePo4 the 100AH capacity is specified with a discharge of 1C or 100 amps.

And you’re right, it’s no problem for a 3000W inverter to drive a 600W load. The 600W load will draw about 55 amps from your battery. So the battery should be able to supply the 55 amps for 100/55=1 hr 50 mins.

Just make sure that your battery is 100% full when you start the test.

Is your battery DIY or some ready build product?
At what voltage does the inverter switch off?

Second on verifying a full battery.

Measuring it's voltage is not real sufficient, and your inverter may have shut off due to simple voltage drop over insufficient cables or a bad connection.

The battery was purchased new in the summer of 2019. It is an off the shelf, ready to use battery from a domestic (US) supplier.

I charged it to full capacity with the solar panels and charge controller just before starting the test. The inverter was connected during charging and during the test. When the panels were first disconnected the charge controller read: 100%, 13.5v although it soon dropped to 100%, 13.2v. (How can they both be 100%?)

To start the test I turned on the ~560 watt load and the battery monitor showed 12.8v, 95% capacity.
After 42 minutes driving the ~560 watt load the inverter beeped and shut off. Monitor showed: 12.7v, 84%. Kill A Watt meter showed: .32 kWh just before it shut off.

After a minute or two I restarted the inverter and the load and continued the test. I did that a few times because the inverter would shut off after a few minutes. I stopped after the monitor gave me a low battery warning. The shorter tests ran for combined totals of about 18 minutes and another .14 kWh according to the Kill A Watt meter.

So the initial test was 42 minutes, 320 watt hours.
The subsequent tests totaled about 18 minutes, 140 watt hours.
Grand totals of about an hour and 460 watt hours.

After all that the battery monitor showed: 11.1v, 26%.

I suppose a bad connection might explain the inverter cutting off, but the cables were indoors, on tight and sized for the 3000 watt inverter. But if my measurements are to be believed, I got less than 500 watt hours out of the inverter before the battery dropped to 26% capacity. It seems to me that something doesn't add up and I don't think its the oversized inverter.

You need to check the shutoff voltage of the inverter and check the voltage at the inverter terminals under load and the battery terminals under the same load and verify you're not just cutting off on a voltage drop.

You need to charge to 14.6v and discharge to 10.0v to get actual full capacity. 13.5÷4=3.375 volts per cell. So you may have only been 70% charge.

100% full is 14.6v, 0a
13.6 is a fine use charge, but for a capacity test you need 100% full.

For a good capacity test, I would use a string of incandescent light bulbs, or a fixed resistor set to 300W.
Inverter will pull waste current, and be inefficient output.
Unless you have a battery shunt that tracks actual usage...

The inverter is a fine load, but you have to measure the battery watts, not the inverter watts.

I get that my setup isn't suitable for an accurate measurement but it seemed that I wasn't even in the ballpark.

So apparently my charge controller isn't configured correctly. I'll have to dig into that some more. That makes more sense to me than the manufacturer's excuse (the inverter is too big). I did set the battery type but apparently that wasn't enough.

Thanks everyone for the clarification.