Heavy Voltage Drop...normal?

[Ruck_and_Roll]

I recently installed a new DC compressor fridge so I find myself using A LOT more DC power than I ever had before.

One thing I have noticed is that the voltage on BMV-712 is showing precipitous drops. (I have a 160 watt solar). I have a 125 ah Vmax AGM. Float/bulk set to battery mfg recommendations.

Today, I did a little bit of testing as I've noticed this when I've been camping the last few times. Will switch on the power and the battery monitor will read ~13.4. As I power on different items on my camper, the voltage drops, precipitously. Let's say I turn on all the lights, fan, and fridge (which in total is about a 10 amp/hour draw-calculated) the house voltage will within 5 minutes drop to 12.4-12.5. If I turn things off, the voltage climbs back up. I understand I am adding resistance to the circuit but Is this normal?

Going by SOC for AGM batteries, I'm already in the ~50% zone. This doesn't seem plausible and an accurate reading of my battery capacity.

Is this a battery monitor issue? Wiring? Battery itself? Or is this completely normal? Should I be monitoring my battery in another way? Or I am worrying about nothing?

Appreciate any insights.

Best,

Ruck

 

[CoreyTrevor]

Using battery voltage to determine SOC can't be easily done with a load on the battery, as the voltage will drop as you add more load. Then it will creep back up once the load is gone.

You need to check voltage with the battery unloaded and after it has been sitting long enough for the voltage to stabilize.

Have you checked with a voltmeter directly on the battery terminals to see if the BMV is reading correctly with the 10A load? Could there be a resistance/poor connection before the BMV giving a low V reading?

I haven't had a big AGM like that but that sounds like a lot of drop with only 10 amps load for only 5 minutes with such a big battery.

Someone who has had a big AGM will know better than I on that.

 

[rando]

That sounds completely normal.

The 13.4V you initially see is the tail end of your charge voltage. Over time or with a small load this 'surface charge' will go away and the battery will stabilize at the fully charged resting voltage which is something in the range 12.6 - 12.8V. If you apply a decent load the voltage will temporarily decrease due the internal resistance of the battery. An internal resistance around 10 - 20 mOhms is pretty typical, so if you load the battery with 10A you should expect to see 0.1 - 0.2V drop, which is in line with what you are seeing.

As Corey mentions, to use battery voltage to determine SOC, the battery needs to have been at rest for some time, which makes this approach pretty limited.