Battery Readings. 12.8V/13.5V At The Same Time

RZRBUG

OK, I thought I had this figured out, but my mind seems to come and go these days.

I hooked up to shore power to cool down the refrigerator and charge the battery in preparation for a little trip tomorrow.  I plugged my little 12V monitor into the plug inside the trailer and got a reading of 13.5V.  I then plugged it into the 12V plug that I intalled on the LP cover (connected directly to the battery) and got a reading of 12.8V.

Does this mean that the actual charge state of the battery is 12.8V, and the voltage being sent to the battery by the converter is 13.5V?

Verna

Yes, I do believe you have it correct. 

Michigan_Mike

RZRBUG said:

Does this mean that the actual charge state of the battery is 12.8V, and the voltage being sent to the battery by the converter is 13.5V?

Sounds plausible and agree with Verna.  When plugged into shore power the reading will vary and mine runs around 13.7 volts via the electric converter.  

When you suddenly unplug from shore power the voltage will run a tad bit higher (e.g., 13.5 volts DC, etc.) and begin to dissipate some as the battery plates cool and the voltage evens out or is distributed across the battery plates.  You can watch it on the LED meter and see it start to drop off and on a good battery it will fluctuate but should be reading around 12.5-12.8 volts, depending on the age and condition of the battery.  It does the same thing when you connect a solar panel to the battery and fluctuates the same way when it is removed.  

Cool beans!  

ScottG

Mike's explanation makes sense, but your description implied that you were still plugged into shore power when you took your reading at the tub. If I use a multimeter to do essentially the same thing you did, I get 13.66V at the power port and 13.63V at the tub. This makes sense as the converter voltage should be getting passed directly to the battery terminals.

Now, if you were plugged into shore power and your battery was fully charged but somehow disconnected, than, for the same the reasons Mike described, I would expect to see 12.8V at the tub. (And that's exactly what I saw when I when I repeated the test with my battery disconnect off.)

ScottG

Another thought came to me in a dream--it's just a guess, and a weak one at that...

Assuming your battery is connected and you are still hooked up to shore power, you could actually be measuring two different things. The power plug reading represents the voltage being sent from the converter to the 12V house wiring, while the tub reading represents the voltage being sent by the battery charger to the battery.

I see two weaknesses with this idea, however. First, 12.8V seems a pretty low charging voltage, even in "float" mode. (Mine reportedly floats at 13.2V). Second, I don't think converters (mine anyway) split their output that way--they provide a single but variable output based on battery charge and load state.

I'm throwing this out there anyway because I notice you have a 2013 T@B, and your components may differ from my 2015. 

Verna

There were major updates between the 2013 and 2014 and even more updates to the 2014. I do believe the converter stayed the same and the wiring stayed the same. The 2013 is still a Pleasant Valley manufactured T@B.