Monitoring power use in trailer


I have 2x big-ass batteries in my trailer. The objective is to be able to run my trailer interior lights, air compressor, notebook, be able to pump up my air mattress, make coffee, and run a 12V electric banky if it’s chilly. All for a 3 day weekend. The power invertor is a cheap Harbor Freight 2.5KW. I also have a $88 Harbor Freight generator which will certainly die any day.

There’s also a small fridge and an AC unit, but they need the generator.

I’m always curious about the state of charge of the batteries. Whenever the trailer gets any power, be it 120AC or the cable to the truck, the batteries charge. But sometimes I forget to turn off main power when the trailer is in storage so the batts go flat.

At the last couple events I got to thinking that it would be interesting to know the state of charge of the trailer batts, how much current was going in/out of the batts in realtime, and how much I’d pulled from the batts over time. Something like a voltmeter, ammeter, and KWHr meter, all in one.

So I poked around a bit and found some examples available in China for beer money. I accepted risk, not much given the palty price and ordered one. I installed it tonight. Dang, it really does seem to work. The beauty of the modern designs is that you don’t have to run your huge 150A wire thru them to the batteries. Instead it uses a remote magnetic loop sensor to detect the amount of current flow.

There’s a few functions that I have to puzzle out, it not coming with any instructions at all, but figuring shit out is just one more project.

Here’s what I got.


Here’s the controller running in the trailer. It shows 9.8amp of discharging. The cumulative charge/discharge, amp-hours, shows zero because I’d just turned it on. The pen at it’s base shows scale.