The Tri Metric and other use meters which are similar work like this:
When you set them up, you put the Amp hour capacity of your batteries into the set up process. In my case with two 6 volt golf cart batteries in series I used 220 amp hours as my original battery capacity. The batteries are fully charged, and then you are set to go.
As you use battery power, the meter counts down in either % of original capacity or Amp hours (your choice, I use %). So, if you are not plugged in, or your system is not charging it counts down the amount of power you have used. So, say you have the 220 amp hours of capacity I have. With it set in % display, it will slowly decrease the reading on the meter, 99%, 98%, 97 ... 55, 54, 53%. So that tells you the % of power remaining in your batteries. If you have solar, as I do, it adds
the amount of charge input, so if you are using less than your solar is putting out, the meter will show an increase in % capacity. If you tow, and a charge is put into your batteries, that will also show as a % increase. If you stop and plug in, the converter/charger will put power into the system, and it will go (over the time needed to charge) to 100%. If you remain plugged into line power, the meter will stay at 100% as the converter/charger is supplying all the power, not the batteries.
Once it gets to 100%, it will reset itself (saying the batteries are fully charged, determined by their reaching float voltage) and only decrease if you start taking power from the batteries.
As an example, I am out camping right now (wildfires have driven me from my home) and have been in a Forest Service campground for 8 days. My Tri Metric shows that my batteries are at 87% of capacity. This is because I have solar recharge. It is a poor slow recharge, because of all the smoke, but still I have used only 13% of my battery capacity which has not been recharged by the solar panels. This morning it was 82% of capacity, so I have recharged 5% today, so far. No sun, only smoke, or it would be up to 100% I am sure. BTW, the meter adds a "fudge factor" for partial recharge, of about 10 to 15% as you must put more power into a battery to recharge it than you can take out. That is compensated for.
Now, if your batteries are shot, the original capacity number you put in when you program it is no longer accurate, and the Tri Metric cannot determine that. So, it would assume all is well and NOT produce an accurate number. So, once your batteries lose capacity due to end of life aging, the meter will no longer be accurate. However it does have a voltage function on it, and an amp readout, so if you find the voltage is rapidly decreasing with a moderate current drain, you could safely assume your batteries have reached the end of their life and replace them. I like to start with a fresh set of batteries when setting up a Tri Metric, but if your batteries are in reasonably good condition, it is not necessary to replace them.
You can think of the Tri Metric as similar to your checking account. You put money into it, you take money out of it, the meter watches the in and out and tells you the % of what you have at any specific time point. The difference between the meter and your checking account is that you program a ceiling into the meter, with a checking account there is no ceiling so no % of money can be determined.
But the meter also is great to tell you when your batteries are fully charged after driving, or when they are only partly charged when you come back home and thus need to be recharged so they don't sit discharged which will cause damage over time.
You can watch the meter and see if you are running less than 50% charge and how often it happens, which will give you a clue as to your degree of battery "abuse". You can look at the meter on a cold evening and see if you may have a problem getting through the night if you run your furnace (with some experience that is). If it says you only are running 30% capacity, and you know it is going to be 35 F and the furnace will run a lot, you may wake up to a cold trailer.
Battery use meters such as the Tri Metric are one of the only ways to really know what is going on in your 12 volt
electrical/battery/solar/tow vehicle charge system. I recommend them highly.