Dreaming About Amps

One of my favorite movies is Apollo 13, and there's a scene where engineers in a room are thinking about the problem of trying to get the astronauts home.  These engineers are all arguing their positions until one of them comes up with the one, searing insight - "power is everything!  Without power, they can't re-enter the atmosphere and they don't get home, period!"  From that point on they totally re-think the problem and they turn their focus to squeezing every possible Amp out of their spacecraft to successfully get the astronauts back to earth.

To build a battery powered, all-electric RV, you have to re-think everything about how conventional RV appliances and power systems have been done.  For example, I'll show below why conventional roof-top RV A/C systems are hopelessly inefficient for a battery powered application.  Fortunately, the off-grid living movement has made tremendous strides in recent years and can teach us much about how to successfully change from the old propane / 12V / plug-in paradigm to an all-electric battery system.

First lets look at a shorthand way to think about our power consumption and battery needs.  If I replace the RV propane cooktop with an electric cooktop like this DUXTOP 1800-Watt Portable Induction Cooktop, it is rated for 1800 watts max power usage in one hour.  We don't cook on highest heat all the time, and for probably 15 or so minutes at a time.  So lets say we use 900W for 30 minutes of cooking a day.  900 Watts / 120V = 7.5 AMPs for an hour, 3.75 AMPS for 30 mins.

Now we need a shorthand way to convert 12V DC battery power into 120V A/C.  I'll cover inverters and batteries in another post, but for now assume it will take roughly 12 -12V DC amps to equal 1 - 120V AC amp.  Our shorthand is a 12X conversion rate for 12V DC amps from a battery to equal 1 - 120 V AC amp for our appliances.

Our simplified calculation of power needs for this cooktop is 4 - 120V amps X 12 battery amps = 48 amp / hours of battery storage to cook 2 meals a day. That's easily doable with normal deep-cycle batteries and inverters and we can use this shorthand to figure the battery needs for all our appliances.

To see why we have to re-think conventional RV systems though, lets look at A/C - easily the biggest energy draw for the GearBox.  The smaller conventional rooftop RV units use 17 - 120V amps per hour max.  When you're plugged in a 30 amp service at your RV Park, these rooftop units work great.  But we don't want to plug in all the time, and 17 120V Amps X 12 battery amps would mean that running the A/C for one hour would use up 200ah of batteries!   A bunch of heavy, expensive batteries and a big-ass inverter are not what I have in mind for the GearBox, so I've got some ideas how I can reduce that 17 amps to less than 5, which should allow a few hours worth of A/C on one day's battery charge with a reasonable sized battery system and inverter.