Why the difference in amps display?

[wpatton]

While running a test on an Everchill 12 volt refrigerator I observed the following and wondered why the difference in readings? Plugged into shore power (household 120 volt) my Surge Guard 34830 showed a draw of 2 amps with the refrigerator compressor running during initial cool down. When I disconnected the shore power and ran on battery only, with everything turned off (and radio/antenna fuse pulled), my battery monitor showed 5.0 - 5.5 amps. A response to my Southwire inquiry assured me that the Surge Guard display "is the amount of amperage I am consuming in the trailer". Any ideas as to why the difference in the readings?

[ncitro]

wpatton Welcome to the forum! The surge protector measures 120v amps, whereas your battery monitor reads 12v amps. You can draw some comparisons to the two, but basically when you are plugged in the amp draw you're seeing is charging the battery, when unplugged and looking at the battery monitor you're looking at the battery discharge (from the fridge and whatever else is on).

Hope this answers your question.

[TucsonJim]

Welcome.

Typically, you'll see a 10:1 ratio of amp draw between the battery and shore power. For example. If you are running a television on shore power, it may draw about 3 amps. If you were to run that same TV with an inverter, you'd see 3 amps on the AC side, and 30 amps being drawn from the battery.

What muddies the water is that you're likely running the converter (battery charger) while you're on shore power. The converter takes 120VAC, and converts it to 12VDC. And this is what your refrigerator is using to run while on shore power.

Jim

[howson]

While running a test on an Everchill 12 volt refrigerator I observed the following and wondered why the difference in readings? Plugged into shore power (household 120 volt) my Surge Guard 34830 showed a draw of 2 amps with the refrigerator compressor running during initial cool down. When I disconnected the shore power and ran on battery only, with everything turned off (and radio/antenna fuse pulled), my battery monitor showed 5.0 - 5.5 amps. A response to my Southwire inquiry assured me that the Surge Guard display "is the amount of amperage I am consuming in the trailer". Any ideas as to why the difference in the readings?

I'll pile on with a slightly different angle. A direct comparison can be done if you do a little math and convert the readings to watts. It's not too hard since voltage x amperage = watts.

Let's say the Surge Guard is showing 118VAC and has a 2 amp draw. 118x2= 236 watts.

Assuming the battery is 13.2VDC and the 5.5 amp draw, 13.2*5.5= 72.6 watts.

That's a significant difference! As the others have written, the converter running (and charging the battery) could account for the extra ~160 watts being consumed when on shore power.

Howard

[Jlawles2]

The only way to get an accurate reading on the amperage is to use the same source of power and the same meter. Using AC amperage for one and DC for the other leave a lot in between to figure out.

For example: DC converter may be charging the battery at the same time increasing amp draw, to that we add the inefficiency of say 90% (For 10 amps AC in, 9 amps AC gets converted to approx 90amps DC roughly).

Let's also not forget that the battery may be partially damaged or approaching the end of life thus the charging amperage never really reaches that magic 0-1amps for maintaining.