Newbie question - SeeLevel under load and Battery Charge

elbereth

Hi all!  We just picked up our new 2018 T@B CS-S and went out on our maiden voyage last weekend.  I learned a lot from these forums before we went out, thank you for all the helpful info!

We are tent campers upgrading, and plan to spend the majority of our camping on “last minute” trips to state parks and the like, and so will be doing 2-3 night trips without electrical hookups.  I’d love to be able to get away with using the 2-way fridge in the Clamshell on 12V if I could.  This weekend, I found within a few hours of parking our rig that the voltage on the SeeLevel was showing up as 12.1, so I turned off the fridge.  Immediately, the SeeLevel showed 12.5 or 12.6.  

I know from the forums that the voltage on the SeeLevel will “drop” under load.  My question is, which is the “correct” one for knowing when the battery is discharged down 50%? 

pthomas745

On this page, scroll down to the section on "Battery Voltages" and there will be a discussion of "surface charge."

".....To properly check the voltages, the battery should sit at rest for a few hours, or you should put a small load on it......"

https://www.solar-electric.com/learning-center/batteries-and-charging/deep-cycle-battery-faq.html#Battery Size Codes

The battery State Of Charge will read lower when under load.  The higher the load, the lower the State of Charge will appear to be.  When you turned the fridge off, the battery then gave you a different State of Charge,...but to be completely sure, you would have to give the battery some time before it settles out.  In my experience, that hour wait hasn't really made much difference if the battery is showing 12.5.....but if it is down around 12.2 or so I pay very close attention.

I also make a mental note of 1.  How many amp hours in my battery 2. How many amp hours am I using or have used.  3.  How many do I have left.    I use those calculations along with what the State of Charge is showing to advise me how my battery is doing.

Sharon_is_SAM

Another question, did you pre- chill the frig on shorepower as that will help limit the frig draw down of the battery in route.  How large is your battery capacity?  The less capacity, the less likely you will be able to go a couple of days without recharging via AC, generator or solar.  Also, some people have had a battery monitor installed like the Victron or Morningstar units that provide a more accurate assessment than the SeeLevel volt meter.

jkjenn

Battery monitors, like the Sea Level, will show the current voltage, which is not a very accurate way to determine the state of charge because it shows e everything that is drawing power, including phantom power draws. The same is true if you have a solar panel connected, so you could have a false sense that the battery is 100% charged, when it might not be so. 

Battery monitors that use a properly connected shunt will give you a more accurate state of charge. I use a Victron battery meter for this. There are other good meters, as well, like those made by Trimetric, Bogart Engineering, and Xantrex.

elbereth

Thanks for the info!

We did pre-chill on shore power, which helped, as did the cool night up here in the northlands.

After we got back, I looked and we have the Interstate HD24-DP which I believe only has 42Ah - so I’m also realizing a battery upgrade of some sort is in our future.  You know, somewhere in the midst of buying tent attachments and hitch locks and levelers and...  so much for “hitch up and go” right? 

In the meantime, it’s just helpful to know I’m not totally destroying my battery on an overnight!  Will keep learning.

ScottG

@elbereth, the voltage under "no-load" condition is what you should use to evaluate the state of your battery's charge.

As you have noted, if anything is running the displayed voltage will be reduced. The bigger the draw, the greater the reduction. Once you get used to it you'll even get a sense of which devices really hog the power.

Also--as jkjenn implied--you also don't want to have any incoming voltage from a converter, solar panel, or other type of charger. In those cases, they display will show the voltage from your charging device rather than you battery.

Technophiles and those who really need precise info to monitor their battery may benefit from one of the pricey metering systems, but they are by no means necessary for rank and file campers provided you have a basic understanding of your equipment.

Michigan_Mike

elbereth said:

After we got back, I looked and we have the Interstate HD24-DP which I believe only has 42Ah - so I’m also realizing a battery upgrade of some sort is in our future. 

Keep in mind here too that although your battery is rated for 42 AH, in reality you only have 21 AH (50% of the battery rating) as you never want the battery voltage to fall below 50%.  

Battery monitoring devices such as the Victron or Trimetric will give you real time voltage reads at the battery and also the percentage of power svailable in the battery (among other things), which you never want to fall lower than 12 volts. And as Scott noted, over time it will damage a battery.  

SteveK

As others have stated, under the conditions you stated, the no load voltage is the one to use to estimate the battery condition.

I believe the Norcold in your trailer draws 3.6A when running.  Under normal conditions you could expect it to run about 25% of the time or 0.9 Amps average.  Your battery is rated for 42 Amp Hour.  Rule of thumb is that you want to only use half of the available amp hours, but that is not ironclad, end of the world.  Using more will only have a minor effect on battery life, however going too far will permanently damage the battery.  For this reason a conservative approach is usually recommended.

So back to your Norcold.  Assuming lights and such are small part of the load and allowing the fridge all of the "safe" 21 Amp Hours, you could expect to get about 24 hours of use before you would be concerned.  You stated you want 2 to 3 days so I would suggest a bigger battery or a method of recharging your battery daily.  I know that an 80 Amp Hour 12V AGM battery will fit and that would give you about twice as long.  Beyond that, it will take a little work but a number of forum members have been able to get over 200 Amp Hours to fit with multiple batteries.

The information above has a lot of rules of thumb, estimates, and typicals thrown together so your mileage will likely vary.  Higher temperatures will cause the fridge to run more often, using the radio and television will eat up some of your battery, etc.

jgram2

We boondocked for 48 hours-fridge on propane if you have that option, used a battery powered lamp, solar powered twinkle lights on the canopy, no Aldi use, some radio, charged pda and laptop, very little use of interior lights, no use of door light, used the propane stove and the 35 year old Coleman (which just died on us, sob). Battery got down to 12.1 . We did miss the AC as it got into the 80’s, but we survived as it cooled down at night in the mountains.