(Battery) Bank Failure

I got a comment the other day on my YouTube channel that reminded me to do a job that I had been long neglecting. I’m glad I did, because it pointed out yet another hole in my preps.

Nice, neat build, but it doesn’t work. Yet.

The comment was about my Battery Bank build series of videos, and asked if the 600W inverter would handle the compressor start-up surge for my fridge. Before I built the battery bank, I bought a Kill-A-Watt meter and did a little monitoring of the fridge, and it seemed to me that the startup surge was about 800W, which I felt was well within the limits of the inverter I was looking at. I was never really satisfied with the measurement, though. The Kill-A-Watt display isn’t really responsive – it only updates once a second or so – and it seems like the surge isn’t accurately displayed. But I went ahead with the Samlex 600W pure sine inverter and built the battery bank anyway.

That was my first mistake. My second was never testing the bank for its primary use case – powering the fridge while the generator is off. No excuses – I just kept putting it off in favor of the next thing. To be fair to myself, I have actually at least used the battery bank. There have been a couple of blackouts where I powered a string or two of LED lights, but that’s not a real test of the system. I also had a hint of failure when I was installing my driveway gate and tried to power a hand drill; when I grabbed the keyless chuck to snug up a bit, the inverter shut down. I chalked that up to the stall current putting excessive load on the system, and figured it would be way more of a load than the start-up surge of a fridge compressor.

Oops.

With a little spare time this morning, I rolled the fridge out from the wall and plugged it into the inverter. I opened the fridge doors, unscrewed the light bulb – that was drawing 40 watts alone – and waited. And waited. And waited some more. Finally, like a half hour later, the fridge kicked on. The inverter kept up with it for a second or two, then dropped out.

So, I screwed up. I need more inverter. And I don’t need pure sine, either, not for a fridge. But again, I don’t have a good idea of the actual surge, since the Kill-A-Watt is somewhat lame for capturing peaks. So I’ll just be guessing again, which didn’t work out well last time. Maybe I’ll just go for a 1000W inverter, like this Cobra CPI1000. Only $80 on Prime; I like that it has a digital voltage display, and the 2000W surge rating should handle the load.

Or will it?

6 Comments

  1. Br0therH@rold

    Looking at that Cobra, I would be skeptical of its surge rating. A good inverter not only lists its surge rating in the specs, but how *long* it’s rated to handle that surge for. Sure it might be able to handle a 2000w peak, but that might only be for 1 millisecond, which might not be long enough for the start-up surge of a fridge compressor. Remember, the surge rating is just marketing, and can’t be relied on.

    Reply

  2. Starting loads on refrigerators can be as high as 12 times running load. Although they are typically between 5 and 10 times running load. Duration is short enough that the “time delay” built into most circuit breakers and fuses is sufficient to cover the start up draw. To accurately measure this you need a Peak measuring AC meter of some kind. They can be rented from some industrial equipment rental outfits. A Kil-a-watt meter is insufficient for proper system design if you are trying to stay on the edge of equipment ratings.

    As Br0therH@rold points out, the rated duration of the peak load of your power supply is significant in your designs, whether that be an inverter OR a generator. Running a generator at peak load, even within design specs for “duration”, shortens the life of the generator. The same usually holds true for an inverter. A good genset or inverter will provide the service factor information necessary to design appropriately based on your intended usage.

    I advise that you avoid designing around the peak load specifications of your power supply if at all possible. All equipment has a design life based on certain criteria. And while peak load specifications do provide for a margin of safety, I believe they are best viewed as a marketing tool used to help differentiate one product from another in the eyes of the consumer. Much better to have equipment designed to handle the maximum you expect to throw at it as a matter of course. Use the rated watts figure and understand what the design life expectancy of the equipment is.

    One additional consideration people often overlook is the load a “frost free” refrigerator adds to the equation. On a timed interval a heating coil is energized for a period of time to defrost the evaporator coils, with the melt off typically running through a tube to a catch pan located somewhere in the airflow of the blower/fan.

    This load must be factored into the amp hour capacity of your system design.

    Reply
  3. APB

    But… but… but Steven Harris said it would work!!

    Yeah, I know – you have to take what a guy who would run a twisted pair of 10 gauge copper wires through and around the frame of a truck to a toolbox in the bed without any sort of looming or protection says with a huge grain of salt. But on the practical side, I know there are people successfully running fridges off of small battery banks like this one – maybe with larger inverters, but nothing “pro-grade.” I know the duration of the surge is an issue, and as I said, this setup nearly kept up before popping. But I’ve never seen any commonly available, affordable inverters with spec sheets listing surge duration or power factors.

    This might be a job for more trial and error…

    Reply
    1. Br0therH@rold

      “But I’ve never seen any commonly available, affordable inverters with spec sheets listing surge duration or power factors. ”
      That should tell you something….

      Reply
      1. APB

        Tells me I might should just buy the cheap 2000/4000W inverter from Harbor Freight and call it a day.

        Reply

Leave a Reply

Your email address will not be published. Required fields are marked *