Which voltage to use to determine RV battery level

[MarkEHansen]

We lost shore power at the park and had to run overnight on just our batteries. I was watching the voltage on the batteries (I don't have a current monitor) to determine how low they were getting.

Most of the time there wasn't much running - just the CO detector, refrigerator control board, a few LED lights, etc. During this time, the battery voltage read as 12.6 (generally).

However, when the furnace fan kicked on, the voltage would drop to 12.1 (give or take) - which is as expected and I understand why this is.

My question is, when determining how low the batteries are, which voltage do I consider? The at-rest voltage (12.6 or so) or the under-load voltage (12.1 or so)?

[JRTJH]

From one of the "reliable information about batteries" sites:

Measuring state-of-charge by voltage is simple, but it can be inaccurate because cell materials and temperature affect the voltage. The most blatant error of the voltage-based SoC occurs when disturbing a battery with a charge or discharge. The resulting agitation distorts the voltage and it no longer represents a correct SoC reference. To get accurate readings, the battery needs to rest in the open circuit state for at least four hours; battery manufacturers recommend 24 hours for lead acid. This makes the voltage-based SoC method impractical for a battery in active duty.

https://batteryuniversity.com/learn/...tate_of_charge

[MarkEHansen]

I responded, but it never showed up, so I'm trying again.

I read that site and it sounds like you need to check with the battery in a no-load condition, but that's not very practical while at the RV park. I guess I'll just assume that the voltage I read while the furnace wasn't running (12.6 from the OP) is the value I should use?

Thanks,

[skids]

That is my take on it Mark. Read when not charging or discharging.

[chuckster57]

Originally Posted by skids

That is my take on it Mark. Read when not charging or discharging.

Only problem is, if it’s hooked up to the trailer it’s discharging while not plugged into shore power and charging when plugged into shore power. We have to physically remove the trailer leads to get a true reading, but we only wait an hour.

[MarkEHansen]

What I'm wanting is to watch the battery voltage while on battery power, waiting to see when it gets to 80%, then 70%, then 60%, etc. I don't really want to have to disconnect the battery from the trailer each time I look at it.

I'm hoping I can just read the low-load voltage and it will be "close enough". Is this just not going to work? Do I need to remove the cable from the battery?

Thanks,

[JRTJH]

My take on "battery charge status" is probably different than someone else's take. That's why I referenced the "experts" (so this thread doesn't become a "My way's better"... "No, my way beats your way".... "No, you're both wrong".....

That said, if you're plugged into shore power, you can't use voltage as an indicator because the converter voltage (not battery voltage) is what you'll be monitoring.

If you're NOT plugged into shore power, your voltage reading will not be a "battery at rest condition" reading, so it's, at best, a "guess"...

What to do? For me, I compare it to the gas gauge and the "miles to empty" estimates <key word: estimate> in any car or truck. The gas gauge may read "1/3 remaining" and the "miles to empty" may reflect an inaccurate quantity... So what??? Does it really matter "that the accuracy is 100% correct" ??? NOPE, none of us are going to use that 103 miles to empty to plan a refueling event at 102.5 miles, we will use either the fuel gauge or the "MTE" quantity as a means to estimate when to stop for fuel...

The voltage measurement of a battery "in use" is also "just an estimate" and close enough, since none (or very few, if any) of us are going to "use the battery until it gets to "11.7692352 volts" anyway...

The entire "need to be accurate" is, for the most part, WAY overstated....

If you're within reason, close to the voltages in this chart, you're probably close enough to "monitoring your battery charge status", whether the battery is "in use" or "at rest"....

Honestly, if it's 30F and the furnace is running, with an inside temperature of 65F, when the battery voltage drops to 11.6, you're going to be faced with one of two options:
1. Keep running the furnace and stay warm as long as you can and deal with any potential battery damage after it happens.
2. Turn off the furnace, cover up and shiver the rest of the night, so you don't risk damaging your battery....

Neither option is "ideal" but realistically, few of us are going to make the trailer "miserably cold" because the battery voltage "is below the recommended charge level"....

ADDED: I was typing this as you were posting your last response. So, to answer your question (with my opinion), Any battery lifespan is based on multiple considerations. Frequency of charging, depth of discharging and number of "charge/discharge cycles". There are other factors, time, temperature, humidity, etc, but they only 'muddy the water' and make understanding the concept more difficult.

So, using the first three, frequency, depth and cycles, if you leave the battery connected and allow it to discharge to 60% then recharge, you'll be "using one of the lifespan charge/discharge cycles" and shorten your battery's life. If you disconnect the battery, you'll not use that cycle, so theoretically the battery will last longer.... It's really up to you whether battery lifespan or convenience is the greater factor. Sort of a "you can't have your cake and eat it too" kind of thing.....

[MarkEHansen]

Well, this is what I was hoping for anyway. The power went out around 1:30am and the furnace was running all night (although it was only upper 40s). By 7 or 8 am, I was checking the voltage and it was 12.6 (when the furnace was not running) - which is pretty near 100% (as an estimate).

Note that I have two of the Costco Interstate 6V deep cycle batteries, so they should last a while before depleting to 50%.

I fully understand that at 12.6 volts, I was no where near depleted. My original question was with regard to "when" I should take that voltage reading, which I now know is when the battery is unloaded (at rest).

Thanks all!