Battery test

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Jim Hilburn

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Jun 13, 2017
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OK, me again.
I'm trying to figure out if my 2 12v in parallel is going to run my funace intermittently through the night. The furnace fan pulls about 7 amps. I don't want to actually run it and burn up propane and the only way to run the fan alone is to continuously turn it on and off to get the 2 minute cycle.
So instead I have every light in the house on and come up with just over 7a. The batteries are freshly charged from my solar suitcase and began at 12.8 but the load brings it to 12.2v. The question is about at what voltage to terminate the test, knowing the voltage will increase once the load is removed. As low as 11.8 maybe? Don't want to do damage just to test how long it will sustain this load.
 
From the site's library:

http://www.rvforum.net/joomla/index.php/39-batteries-and-dc-12v-stuff/309-battery-charging-basics

Taking them down to 11.8 is way too far to discharge your batteries. 12.1 is the recommended 50% mark.
 
But is it really 11.8 when under load? It's reading 12.1 now but I bet I can go turn off the lights after 1 hour and it will rebound to at least 12.5.
 
So at 1 hour I removed the load and it climbed back to 12.5. I had actually done this last week but only for 2 hours and wanted to do an extended test.
This tells me that because I'm reading 12.1 with the load that doesn't mean the batteries are actually depleted to 12.1. So wouldn't it be true that if you read 11.8 with the load that it should rebound to 12.1?
 
Now to post this I have to make some assumptions
FIrst: You said 12 volt batteries so I am assuming MARINE/deep cycle or MARINE/rv (Same thing)
These batteries like to have a 75-80% state of charge,, Basically if you run 'em down farther than that they start aging very fast

True DEEP CYCLE (it is possibly you have this type) can go to 50% State of Charge before serious Aging kicks in... Continuing

Group 24, about 75 AH
Group 27  About 95
Group 29  About 105
Group 31 About 130
That is each,, Multiply by two than by 20% to get usable enegry.

Remembrer also  This is at the 20 hour rate. you are likely drawing faster than that
And you have OTHER LOADS PRESENE

Now some other tings.

A dual 1141 Lamp fixture is about 1.5 amps... EACH (3 amps total) So two and 1/2 you got your 7 amp test load. 

Replace those lamps with LED's and the current draw goes down below one amp per fixture.
 
Glad to hear from you John.
I've changed out all my lights to led and use them sparingly. And it's a small rig, 23". Just want to be able to keep it cozy when it drops to 45 at night.
By chance I ended up with one new Group 27 and one new Group 29. Carquest and Walmart. On the 3rd walmart for the original $100.
The first time I wrote to this forum it was about installing a switch between these batteries since they were dissimilar but someone here convinced me to go ahead and parallel them because they are so close in size. And it seems to be working.
The question at hand is about the voltage drop when you apply a load like the furnace fan. How far to drop the voltage before calling it quits. Today I ran 7 amps for 1 hour. The voltage immediately dropped from 12.8 (right after charging) to 12.2. Within 1/2 hour it was at 12.1. At one hour I removed the load and it eventually rose back to 12.6. 
This leads me to believe I could run the fan until I got an 11.7 reading but the batteries will actually still have at least 12.1. What am I missing?
 
Jim, I now see what you're trying to do (didn't understand at first) but what you're trying to do is very difficult to do using voltage as a parameter. The best (most accurate) solution is to install a battery monitor, like the Trimetric RV 2030. It not only indicates battery voltage, it also indicates the number of amps going in or out of your battery bank. More importantly, it displays your battery capacity, in percentage, at any given time. At a glance you'll know whether your batteries need to be charged or not, and because it's counting amps in real-time, you'll always knows your battery's SOC.

Kev
 
To followup, I ran a 6 hour test yesterday. Turned on lights till I read 7.1 amps at the battery and let it go. I was going to let it drop to 11.9v but it was getting late so after 6 hours it was reading 12.0 after starting with what I believe was a full charge and a reading of 12.8.  I turned everything off, waited 30 min. and the reading was 12.3.
So these batteries should easily run the furnace enough through the night to make it comfortable. Next problem will be getting them recharged the next day. I camp at some festivals where generators are a no no but in heavy trees making getting full sun difficult.
 
With a group 27 and 29 batteries you have close to 200AH of battery when the batteries are new. 

So assuming your batteries are truly 100% charged.  From your replies you state that you had a 7.1 amp load on your batteries for 6 hours.  That would be 42.6AH used of your 200AH of battery capacity. As a percentage the 42.6AH is 21.3% usage of your battery or 78.7% SOC.  You did let your batteries rest for 30 minutes and they came up to 12.3V.  That should be a reasonably accurate reading.  Using the battery chart below 12.3V is about 70% SOC. 

http://www.marxrv.com/12volt/voltchart1.gif

Given that your batteries are not new, and possibly not at 100% SOC when you start I think your batteries are preforming about right. 

BTW, where are you reading the voltage in the tests?  Right at the battery or farther up the line? 

I strongly agree with Kevin's reply earlier about installing a battery monitor like Trimetric for accurate monitoring of your batteries status.  Using voltage readings are a rather inaccurate way to monitor the batteries. 

However, if you are just going to have intermittent usage such as one night w/o shore power and then back to shore power, what you are doing is fine.  On the other hand if you are going to go multiple nights w/o shore power and trying to charge with a generator or solar, you really need the battery monitor. 
 
AST, These batteries a nearly new and very lightly used, but I guess I'd be suprised to see 200 AH out of them given their pedigree. But they don't list AH on these marines.
 
Kevin Means said:
Jim, I now see what you're trying to do (didn't understand at first) but what you're trying to do is very difficult to do using voltage as a parameter. The best (most accurate) solution is to install a battery monitor, like the Trimetric RV 2030. It not only indicates battery voltage, it also indicates the number of amps going in or out of your battery bank. More importantly, it displays your battery capacity, in percentage, at any given time. At a glance you'll know whether your batteries need to be charged or not, and because it's counting amps in real-time, you'll always knows your battery's SOC.

Kev
Click to expand...
Kev, did you install one of those?  Your Tour didn't come factory with it, did it?
 
Jim Hilburn said:
AST, These batteries a nearly new and very lightly used, but I guess I'd be suprised to see 200 AH out of them given their pedigree. But they don't list AH on these marines.
Click to expand...
True, I would be surprised to see 200AH out of those batteries.  I was pretty much accepting the AH figures given earlier by John from Detroit for deep cycle batteries. 

Bottom line, using voltage readings to guesstimate the SOC, your testing would indicate your AH's to be in the range of 150-200AH and seem to be working pretty well. 
 
Mile High said:
Kev, did you install one of those?  Your Tour didn't come factory with it, did it?
Click to expand...
Hi Brad. No, I installed it when I installed the solar system. Next to our solar setup, the Trimetric has been the most useful boondocking tool I've installed. Can't imagine not having it.

Kev
 

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Kevin Means said:
Hi Brad. No, I installed it when I installed the solar system. Next to our solar setup, the Trimetric has been the most useful boondocking tool I've installed. Can't imagine not having it.

Kev
Click to expand...
Thanks Kev, I may have to look in to one.  I never really know where I stand on batteries other than I see the voltage drop.  Unfortunately, that's how the backup autostart sees it on the generator too.
 
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