LITHIUM BATTERY UPGRADE, PART 2: AC SYSTEM

I will write these next three posts using the plainest language that I can muster.  My purpose here is to introduce readers to the components of our lithium system, and to how they work together, in a manner that non-technical people can understand.  Most people who want lithium battery systems in their RV or van are not engineers.  They don't necessarily need to learn enough to become system designers, but they should learn the principles behind the systems.  After all, lives are depending on proper system design.

^^ What HE said.
This quote is attributed to him.  The actual owner is uncertain.

If you really want to understand any van or RV electrical system, the best way to go about it is to break it down into the smallest possible pieces, and learn each one.

Dr. Feynman sorta took the original Einstein-attributed construct and, well, he Feynman-ized it.  

It's important to note, however, that there are no freebies with this process.  If you don't have a background in electrical design but you really want to learn a few things about this stuff anyway, be prepared to study hard.  It's time-consuming because it's not enough to simply gaze at this information passively.  You have to manipulate it in your head and visualize yourself applying it, or at least watching it in action.  You'll have to walk yourself through our design diagrams (I stop short of calling them schematics) repeatedly in order to solidify your understanding of this kind of system. 

As is genius, so is learning generally.  Especially when it comes to challenging content like this. 

The first thing we need to do is review the meanings of some words that are used to describe electrical systems.  This is not necessarily the best place to ask "Why?"  These are definitions that just need to be accepted as-is.  I'll try to minimize the extent to which these are needed, but they may come in handy before this post series is through. 

The plumbing analogy can be helpful because electricity cannot generally be "seen" but at the same time, it needs to be visualized.  People are generally familiar with how water moves through pipes or hoses, so this serves as a comparison of sorts.  

Three of those units in that table above relate to each other in a very specific defined way, as this diagram shows:  

The common instruction is to cover the third of the triangle that you are intending to derive, and then either multiply or divide based on the spatial relationship of the other two thirds.  

When describing the flow of electricity through a wire or certain devices, this handy diagram is often invoked:

Now that the internet has barfed up this wealth of handy electrical references, here's our basic simplified AC diagram below.

Tap or click to expand for clarity.
Blogger downsamples embedded images.

I'll generally describe each of the diagram's numerically-superscripted components in terms of:

(a) what the component is (with links, or, where specific models have been discontinued, I linked to the closest-available substitute), 

(b) how it gets the electricity that it receives, 

(c) what it does with that electricity, and 

(d) why. 

I've also set up this narrative to follow the general electrical flow from points of origin to the points where it is expended by doing work in a specific way.  Remember - if this material is unfamiliar to you, then it might help to return repeatedly to the analogy of water flowing through an interconnected series of devices and pipes. 

1.  Shore Power – Our van was wired by Airstream to receive 30 amp AC electrical service (larger RVs with more extensive electrical loads are usually designed around 50 amp service).  This is the electrical supply that is commonly referred to in the vernacular as “shore power”, a phrase borrowed from the boating industry.  It is obtained by connecting a heavy power cord from an electrical outlet (often mounted on a low pedestal in campgrounds) to the plug-in connector on the outside of the rig.

2.  Surge Suppressor - Some shore power sources supply voltage that is not held steady at 120V, which is normal household supply.  Voltage spikes and brown-outs can occur due to electrical storms, overloaded campgrounds, and problems with the grid where the electricity is originating. Allowing electrical fluctuations into a van or RV can cause serious damage to the internal components and appliances.  We installed an integrated surge suppressor (Surge Guard Model 34520) to protect against these events.  This device is mounted on an interior wall under the street-side rear couch.

3.  Generator - Our Airstream Interstate camper van was delivered with an Onan Microlite 2800 series propane generator installed.  Much like shore power, this device supplies 120V AC when it is running.  We were not the first owners of this vehicle, and we didn’t choose to have this generator.  We would not have ordered it given the limited ways in which it can be used - it is too noisy, plus our van's propane tank is not large enough to run it for long periods.  But given that it was functioning well and had low run-time hours on it, we decided to keep it as an active component of the electrical system for now.

4. Automatic Transfer Switch (ATS) – This device (a Parallax ATS301 switch) contains a toggle style of mechanical component called a relay that makes the choice to accept AC from either the generator or shore power, with priority given to the generator.  If an ATS were not present, then accidentally connecting shore power and turning on the generator at the same time would result in the combining  of two out-of-phase AC voltages. Each independent source would contribute 120V and, depending on how they alternated, they could double up to create a 240V difference between the shore source and the generator, and damage to the system would occur from the resulting rush of current.

Components 5 and 6 are first discussed separately and then in terms of how they share the incoming AC from the switch-selected source (either the generator or shore power).

5.  Lithium Charger – This Progressive Dynamics Inteli-Power PD9100L Series unit accepts 120V AC and converts it to 14.6V DC with a maximum output current of 60A (14.6V DC is the voltage at which the lithium battery charges most efficiently).  This process will be discussed in the future Charging / Inverter System blog post.

6.  Inverter – The model we chose is the Xantrex Freedom Xi 2000W unit.  An inverter is an device that changes direct current (DC) to alternating current (AC) – in other words, it does the opposite of what an electrical converter or charger does.  That being the case, you would be justified in your curiosity if you wondered why a source that was supplying AC to start with (shore or generator) would need to be routed through an inverter before it could ultimately be sent on to do its job in powering certain downstream AC appliances.  The answer is that the inverter is essentially serving as a “pass-through” device in this scenario.  It contains its own embedded ATS that chooses between issuing AC from one of those two upstream external supplies, versus issuing AC from its own self, via inversion from battery supply.  Priority is obviously given to the external supply, because it would make no sense to invert power from the battery if shore or generator current is available. If the Inverter didn’t have an internal ATS built in, an external transfer switch would be needed to safely connect it to the AC supply system.

If you look at Diagram 2 of 4 above, you can see a bifurcation issuing from component 4, the ATS.  So how does the incoming current “know” to split itself between the Lithium Charger and the Inverter? As they are wired in parallel, they both get equal priority, similar to multiple electrical outlets on the same circuit in your home.  In our van scenario, there is a maximum of 30A available on the circuit to be shared by the two appliances.  In a scenario in which the Lithium Charger draws 9A AC (the maximum it can accept) to convert to 60A DC for battery charging purposes, a remainder of 21A is then available to feed through the Inverter to the appliance(s) that are activated.  In some scenarios, charging may need to be disabled in order for higher-consuming appliances to be run.  More detail on this process will be given in the post that discusses the Charging / Inverter System.

7.  Circuit Breakers – The 30A and 20A circuit breakers act as safety mechanisms that prevent excess current from accidentally routing to any of the appliances that are either hard-wired or plugged into the AC outlets.  Most appliances have their own internal circuit protection, so the circuit breakers are primarily designed to protect the wiring from shorts and overheating.  As described above, the Inverter passes through shore power which feeds into the 30A circuit breaker first.  That limits maximum downstream loads on all circuits to a total of 30A (if more is added, the breaker will trip).  From the 30A breaker, the current proceeds to three 20A circuit breakers across which the appliance loads are distributed.

A ground fault circuit interrupt (GFCI) is a device that shuts off an electric power circuit when it detects that current is flowing along an unintended path, such as through water or a person to ground.  Some RV configurations, including our Airstream Interstate, had electrical systems designed to include GFCI-enabled circuit breakers, if those breakers delivered electricity to AC outlets in potentially wet areas.  In certain circumstances, an electrical inverter could interfere with the operation of GFCI breakers because of issues involving the configuration of ground and neutral wires (some technical detail is omitted here for brevity; this Xantrex Technical Note (PDF) provides discussion).  In lieu of maintaining the original GFCI breakers, we opted for individual GFCI outlets as described below.

8. AC Outlets – Our rig contains five AC outlets that were wired by Airstream (galley kitchen, bottom of the street-side overhead cabinet, refrigerator alcove, microwave cabinet, and curbside exterior van body).  When AC is being supplied, these work like regular household electrical outlets, with the caveat being that they are collectively limited to a maximum of 30A, as that limitation is imposed by the 30A circuit breaker.  If GFCI circuit breakers are not being used for the reason described above, GFCI outlets are advised.

Like this one, which we added above our galley kitchen. 

9.  Priority Switch – An Intellitec automatic energy select switch is installed because the available current is insufficient to run both the roof A/C and the microwave oven simultaneously.  The roof A/C (described below) requires approximately 13A to run, and the microwave needs 8.3A.  Both appliances also require start-up surges of greater current, so their total instantaneous demand can be far in excess of the 21.3A that results from simply adding their continuous current values.  Out of necessity, it is an either-or scenario when it comes to running them.  If we need to run our microwave, we switch off our roof A/C momentarily (the microwave and the A/C share a single 20A circuit breaker). 

10. Microwave – Our van came with a Dometic CDMW07 (PDF) series microwave oven which nominally requires 1,000W of power to operate (8.3A).  Some people question why anyone would need a microwave in an off-grid van, and our answer is because it reduces propane consumption by the hot water heater, saves fresh water, and conserves gray water tank space, all because of the extra dishes that we don’t have to wash (throw some tamales on a paper plate and pop them in that microwave and voila – no clean-up).

11. Roof A/C – A Dometic DuoTherm 11,000 BTU coach A/C unit (no longer manufactured) was installed by Airstream in our van’s roof.  The manufacturer represents that this A/C requires 13A of current to operate (3.5A for the fan and 9.5A for the compressor).

12.  Soft Starter – We installed an electronic device called a MicroAir EasyStart 364 into the body of the Dometic roof A/C for the purposes of modulating the amperage demands that are imposed on our electrical system.  Without a device of this type, starting the high-demand A/C would overwhelm the Inverter, exceeding what it is able to supply while it is drawing from the lithium battery, and shutting down the operation of the A/C as a result.  The Inverter is capable of providing power up to 2,000W (16.7A) continuously and up to 3,000W (25A) briefly, but the A/C start-up demands a surge that is greater and for a longer duration than the Inverter can accommodate.  The soft starter contains electronic components that essentially start the A/C compressor motor more slowly to prevent this kind of shut-down.  With the soft starter integrated into the system, our lithium battery can power our roof A/C for about 2 hours before becoming overly depleted. 

If you have questions on this section, please contact me via interstate.blog at gmail.

I had to have at least one dumb meme in here...