House Battery Charge Question

very good meter

State of charge can also be deduced from measurements of the density of the electrolyte. Inconvenient.

The only really easy method is the battery monitor which counts the amp-hours going in and out.
I am very satisfied with this low cost one. eBay.com

Oh one more thing. You said you have two 12 volt in Parallel

Are they MARINE/deep cycle? If so the recommended minimum State of charge is much higher. like 75-80 percent SOC

DEEP CYCLE (normally in an RV those are two GC-2 class six volt batteries BUT NOT ALWAYS, can go to 50%, but not MARINE/deep cycle unless you want to replace frequently. z

One of the reasons I like six volt in series pairs.

NOTE: you can get 12 volt DEEP CYCLE. just a bit harder to find and more expensive.

Lots of potential variables when looking at a Voltage value, and calculating SOC level.

-For accurate readings, the batteries should 'rest' for ~12 hours with no drain/charge going into them, before taking a voltage reading. A Multi Meter reading at the battery itself, is the most accurate way to measure.
-Current battery temperatures should be factored too. The measure voltage can be adjusted up, if the batteries are at low temperatures, or down if the batteries are at higher temperature. (Look for spec's for your specific battery manufactures, on temperature compensation values. And keep in mind the 'mass' of some battery banks, even two batteries have a pretty good mass to them. So even if the outside temp in the AM has warmed up to say 65 degrees, the batteries could still be much cooler (Reverse if it was hot, and now cooler temps:)!). (For example, we have X's 4 L16's Lifelines. Took a reading today, after a 14 hours 'at rest' period. Outside temperature was 68, but the Magnum Temp Sensor reading reflect battery temp was still 54 degrees. A infrared heat reading showed 58 on the side of the batteries closest to the still shaded basement doors. A temp in the middle top (These are paired 2X2 next all together like a four by four square.) reflected 55.6 degrees. The Magnum temp sensor, is sandwiched half way down and between the barrettes, as close to the intersection of the four batteries as possible.) So use some wiggle room, when calculating temperatures.)

Since we're usually using the coach, and thus the battery systems, it's not always easy or practical to get a resting battery measurement. Many of the devices used to display battery health, have pretty sophisticated logic to look at Total Out/In and Temps of the battery, when calculating displayed SOC's.

Long winded way of suggesting you validate the actual SOC level:)!

Best of luck to you,
Smitty

An Inverter/Charger ONLY CHARGES the battery bank when it has 120 volts INPUT to it. You have 2 sources of that 120 volts. Shore Power or a Generator. The Truck engine(alternator) has nothing to do with the Inverter/Charger. Doug

Awesome info thanks guy. I woke up to 12.24 and the solar power charging it up again, but I am surprised that I would reach 50% batt on only one night of boondocking. Is this normal? I am in a 20’ born free van that has basically no amenities lol. The only thing I ran last night was tv for a couple hours, Xmas lights for a couple hours and then my Engle fridge which is always running on dc but supposed to draw very minimal power. I am also having trouble understaning how the inverter/charger comes into play? I have a freedom 10 model inverter/charger. My understanding was to turn it off if I’m getting low on batteries? Am I wrong and this thing somehow actually charges them while giving me power? My understanding was the only way to get charge is from shore power, solar or running the truck.? Sorry if it’s a noob question but I am new to having an rv and learning as I go. Crossing into Baja tomorrow. Happy New Years guys!

Temperature and battery manufacturer all affect the full charge open circuit voltage of a battery. Some commonly available batteries are full charged at 12.65V at 80F while others are 13.1V.

Another key is that this method of estimating state of charge only works when the battery has no load and hasn’t had a heavy load on it or been on a charger for a while in order to allow the electrolyte to mix and surface charge to dissipate.

You need a chart from the battery manufacturer that has temperature compensation information and open circuit voltage vs. state of charge.

DiskDoctr wrote:

Not all voltage charts agree with what you posted I think it is important to factor in temperature into the voltage equation.
Temperature vs Voltage

I always try to stay above 12.2. When recharging you should be fully charged when your converter goes into float and this often takes a surprising amount of time. You dont have to put 100 % back into the battery every day but after a week of two of incomplete charges you will see a lack of performance and possibly not make it through the night or your voltage will start dropping off quicker.

The voltage may go up when charging but your battery capacity becomes smaller and smaller if not fully charged. If you have a wet battery you will see this with a corresponding drop in the specific gravity. After doing incomplete charges for x days and you return to shore power it should charge fully and go into float. If you are boondocking long term there will be days that you need to do very long charges to get the converter or charger to reach float.

We use a lot of power and I shoot for a complete charge every day. If I'm down a lot by my standards I will charge off my converter while we run the genny to make coffee and use microwave and then let the solar bank finish off. The specific gravity is the true indicator of battery charge and sometimes differences in each cell. This is not to say you should need to dip the battery often. When you get the rhythm of how you operate you will know when performance is dropping.