Any good / easy way to find power loss on 12V house side?

If the temperature is lower for the 2nd measurement that will account for part of the voltage loss. You probably don't have a short circuit. What you're suspecting is often called a "phantom load". IMO the easiest way to start looking is to have everything you know about shut off, then one-by-one pull the 12 volt fuses and bridge the fuse holder contacts with an ammeter. A multimeter with a 10 amp range is good for this. This tells you which circuits are pulling power, after which narrowing it down further can get more complicated. Measuring battery SOC with a voltage measurement is approximate at best, and requires interpretation based on the the recent charging, load, and temperature situation.

This thread is probably the best "User Ad" for the primary need of an amp hour meter :)

The voltage drop down to 12.7v after charging is normal. This extra voltage is due to surface charge. The drop down to the high 11s seems high for a 4 GC2 battery bank. The battery bank has a capacity of about 450 AH and you apparently are using about 75% of the capacity. For best battery life you shouldn't discharge them lower than 50%.

Somehow, it seems that a lot of current is being drawn from the batteries. Without having current measurements all we can do is guess what the problem is. To use 300 AH in 12 hours means your average current drain is 25 amps.

Thanks for the responses.

It will go down from 13V to 12.7 or 12.6 very quickly (maybe 30 minutes - enough that I thought the batteries were bad but it doesn't do it if the disconnect is 'disconnected') and I'll often go from low-mid 12s to high 11s overnight.

Roadpilot wrote:
What is the start voltage and the end voltage? 13.2 down to 12.7 is one thing, 12.6 to 12.1 is another.

I agree. I have a sticker next to my house battery voltmeter to remind me -

12.8V or more = 100%
12.5V = 75%
12.2V = 50%
11.9V = 25%

Also gotta remember that is open circuit (no load) voltage. If anything is turned on it will pull the voltage down and you won't get a true reading.

---

To help me with troubleshooting, I have a clamp-on DC-ammeter. Makes it easy to check individual circuits. If you go this route make sure you get a DC model - some ammeters are AC-only which is useful around the home but not so much the vehicle.

Good point Roadpilot. Could be just the surface charge leaking off, didn't think of that.

Refrigerator? Maybe the heat strip around the door seal.

If it was me I would see how much current is being drawn from the batteries. With only the fridge and the various detectors on I would expect less than 1 amp drain. If more, I would start removing fuses and see where the current is going.

Without an ammeter you could remove all the fuses and see if the problem persists. If not, replace fuses one at a time and see when the problem returns.

What is the start voltage and the end voltage? 13.2 down to 12.7 is one thing, 12.6 to 12.1 is another.