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While I’m waiting to hear back from the mechanic who is doing the chassis upgrades (it’s been darn near a month since I’ve heard a word from him), I’ve been trying to make sure the Vintage Bus project progresses. Look out… here comes a somewhat technical post that not everyone will appreciate.

Researching Inverters

This week, I’ve been doing research on Inverters. An inverter is what converts 12 volt or 24 volt direct current battery power to household 120V AC power. The inverter also usually serves as a battery charger when plugged into shore power at an RV park. Most inverter setups are pretty simple: 1) when running on battery power, you can run a limited number of electrical devices because the inverter has a limited capacity of delivering power (40 amps of 120V AC for instance compared to the 100 amps you can get from shore power), 2) when charging, the inverter draws a high amount of power from the shore while the rest of the AC loads are supplied from the same source… shore power.

The problem with the above setup is multi-fold: 1) When you’re connected to a standard household 15 or 20 amp circuit (like when I’m parked in your driveway), the battery charger will suck between 8 and 15 amps, which doesn’t leave much power left to use in the coach. That means that you can’t make coffee, use a hair dryer, use the microwave or run the hot water heater because you’ll draw too many amps which would cause the shore power circuit breaker to trip. When that happens, you loose all power! 2) When you’re connected to the most powerful outlet you’ll run into (usually referred to as “50 amp service” which delivers two legs of 50 amp 120V power), you still have to manage your loads because running two air conditioners, a water heater and the battery charger will get one of the legs to go over 50 amps and cause the shore power breaker to trip (the third air and other loads would be on the second leg of the 50 amp service), again, causing you to lose all power!

I’ve been investigating a special type of inverter which allows for a much more flxible lifestyle. The first thing this inverter can do is limit the amount of power drawn off shore power. That means that I can indicate the maximum number of amps I want to draw and the inverter will make sure it never goes above that figure… and that means that I should never trip a shore power circuit breaker! How does it accomplish that? By supplementing the shore power with battery power via the inverter. Not just any inverter can do this… in fact 98% of them can’t. The problem is the AC power is delivered as a wave form (just think of regularly spaced ocean waves) and in order to supplement one source of power with another, the wave form of shore power and the wave form of the power coming out of your inverter have to be in sync. There are four inverters that I know of that can synchronize the wave forms and therefore supplement shore power with power coming from the batteries: 1) the Trace SW 4024 which hasn’t been made in many years, 2) the Victron Quattro, 3) the Mastervolt Mass Combi, 4) Studer Xtender which doesn’t have any distribution in the USA.

With any of these inverters, you can set a maximum amp draw to be pulled from shore power (let’s say 13 amps when plugged into a 15 amp shore power circuit). When the coach demands less than that amount, then any excess is used to charge the coach batteries. If the coach demands more than the max that you’ve specified, then the inverter stops charging the batteries and starts to supplement the shore power via the batteries for up to 40 amps of additional power. That means that air conditioners, hair dryers, water heaters and more can be run when plugged into an outlet that does not have sufficient power for those loads. The idea is that hopefully, the excessive power draw will be short lived and that the inverter can switch back to battery charging mode. This will most likely happen for many hours while the coach owners sleep through the night and don’t have many loads calling for power.

Running air conditioning off battery power will quickly deplete even a large battery bank. Having the capacity to run air conditioning can be useful though for when you return to a coach that has been sealed up and parked in the hot sun. Run the air for 20 minutes, then open the windows and run a few roof vents and you’re back to relative comfort. The batteries will be run down quite a bit, but would hopefully get recharged by morning if you’re plugged into a 15 amp plug in a friend’s driveway. When not plugged in, solar can be used to slowly charge up the batteries, which makes using an all electric fridge and appliances workable.

By using a synchronizing inverter, we should be able to do such simple things as wash our hair, make coffee or use a hairdryer without fear of tripping the shore power breaker, which will allow us to park in many more locations without being frustrated.

Using a synchronizing inverter also allows you to purchase a much smaller generator than what would usually be required. Generators are usually sized by figuring out what your maximum load will be and making sure you buy a unit large enough to supply that power. Air conditioners and other appliances have starting loads and operating loads. Let’s say one air conditioner draws 16 amps when it’s running, but needs upwards of 40 amps to start up. Next, let’s say your water heater and a few other loads might be running at the same time that you start that generator and you can see how you might end up needing a good sized generator to handle that kind of power draw. Use a synchronizing inverter and you only have to size the generator for the average draw because the inverter will supplement the generator power and handle any loads that are above the average! That allows you to buy a much smaller, lighter and less expensive generator.

Another issue I’m researching: Inverter power is specified as either Watts or VA and many people seem to think they are identical concepts, but it sounds like they’re not. The real power in a 5000VA inverter would really be more like 3000 Watts! For more details, see this web page and then do a google search if you want to learn more about the differences. It sound like VA=Watts when discussing DC power, but not when you’re talking about AC power, which is what is causing some of the confusion.

So, while I’m overly frustrated at not hearing from the mechanic who’s supposed to be working on my bus, I have some comfort in knowing that the vintage bus might have a more usable electrical system than my current bus.