The other day I pulled over to spend the night at a rest stop, and by habit I pushed my Battery Display Switch.
Naturally, there was a lot of "surface charge" and the display indicated 13.1V, which is impossible, because no 12V battery or 4x6V battery bank can have more than 12.8V-12.9V at rest.
Then I pulled some leftovers out of the refrigerator to heat in the micro wave when I got an idea. What if I run the microwave for 30 seconds? ...And then can I check my battery voltage?
Sure enough it worked! ...And the voltage never climbed.
So no you know you don't have to wait until morning if you want to check your SOC!
It's easier to read the 12V voltage of a battery than it is to take an hydrometer and measure the electrolyte inside each cell. So that's what most of us do.
Each battery cell is ~2.15v. So a 12V battery will have 6 cells connected in series just like you do when you load a flashlight up with batteries (in series); and then you add the voltage of the batteries together to get the total voltage.
In this case 2.15 x 6 cells = 12.9V, and anything above that is called "surface charge."
My point is that if you could measure each battery cell then you would get a graph like the one below that emphasizes MadMaxWinnie's points: Batteries will charge rapidly at first and then take a long time to charge the last 10%.
==> And old batteries, that are sulfated, will have so much internal resistance you will never be able to recharge them back to 100%.
So that 420AH bank is now only good for 280AH, and as it ages it will drop to around 250AH. This has to do with the number of charge cycles and for me other factors, like:
* Do you leave your batteries in freezing cold temperatures WITHOUT a battery maintainer?
* Are you always changing your batteries from your alternator?
* How good is your Solar Charge Controller, which is also a smart battery charger?
* Are you equalizing your batteries often enough, and not too often, and are you completing the equalization phase?
In general, most owners, especially me, do NOT treat their batteries very well. This is why I bought a 17A Victron Supplemental Charger/Battery Maintainer; and why I will be looking into a smart DC-DC converter in place of my KeyLine VSR, because when I take my RV out for 3-4 months/year I am always driving, and that means I am not reconditioning my batteries like I should be doing.
MICROWAVE TEST - USE IT TO CONVIRM IT IS TIME TO REPLACE YOUR HOUSE BATTERIES.
Last summer my 420AH died. These batteries were only 1.5 years old and over the last 2 months of using them, my SOC over-night would drop rapidly, and my inverter would shut down before I woke-up in the morning.
My batteries load tested good at the Interstate Battery Store; and when I measured the electrolyte all was normal. So, what then?
I refused to think my batteries were bad, but they were!
...Then I started troubleshooting my inverter thinking that old Dimensions is going to be recommissioned as a lead anchor and I'm finally going to get me a newer PSW inverter.
... Turns out that was just foolish talk, and after I researched the new High Frequency Inverters vs. the cost for a good Low Frequency Inverter, I decided to replace all 4 of my 6V golf cart batteries for $400 at Costco.
(Not to mention there are a lot of junky PSW inverters you should not buy for your RV, but that's a different subject.)
...So, as I said, I bought 4 new 6V batteries and all was back to normal.
These points listed above are best summed up in the charts below.
* Specific gravity of SOC voltage checks cannot be used to determine a battery's capacity."
* The best battery test to know if your house batteries are SULFATED can be preformed by doing the Microwave Test listed above.
* Always replenish your distilled water in the battery bank. (I use a DEF funnel for all my fluids. So then next time you are at a service station you should check the trash. I have 3. One for my oil, one for me diesel additives, and one for my distilled water jug, which I also use for my windshield washer reservoir.
* Be sure your inverter temperature probe is connected to the NEG battery post and not the POS post.
* Clean your 300A fuse to your inverter after 10 years of service. It's normally mounted next to the batteries, on the side wall.
* Make a conscience decision to properly maintain your batteries in storage; or "BEAT THE SH*T OUT OF THEM AND DEAL WITH YOUR BATTERY WARRANTY EVERY YEAR."
Note: One problem with this warranty approach is that when you encounter a short: A) You might be camping and now your inverter will not work; and B) You only short one battery. ...It's hard to say?
==> And you can't throw a new battery in with old batteries and expect good results for very long! So I try my best to take care of mine since I have 4 house batteries to deal with.
==> And I don't know about you, but dealing with 240 lbs of batteries is a real PITA.
==> Best solution, go with lighter LiFeO4 batteries... when the price comes down some more! (Maybe this year!)
==> However, when I had a gasser Class A with only 1 or 2 12V-house batteries, then yes... I would "BEAT THE SH*T OUT OF THEM" and take my chances. Plus the 12V batteries are not as durable as the 6V golf cart batteries, so they say.
==> Batteries also have impedance characteristics and if you like getting nerdy you can check out this website to find everything you really don't need to know about battery physics: https://batteryuniversity.com/index....surface_charge