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Old 08-22-2019, 12:35 PM   #1
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How Do It Know? Solar Power

I've been playing with some solar power components I have acquired, but, for the life of me, I can't figure out how some of this works.

For instance, I have 6 wires going to/coming from the solar controller, and 4 wires going to/coming from the batteries. The solar controller is pretty obviously marked and is working fine, the cable to/from the batteries are less obvious, but is also working as designed.

I have power input to my battery bank from my 1) my tiny (50W) solar set up, 2) my 7500W generator, 3) engine alternator, and 4) pedestal when plugged in, and 5) random lightning strikes when holding the wet kite string in the thunderstorm (lol). Power output from the batteries goes to 12V "house" system (2 X 6V GC batteries) and "12V "chassis" system (1 X 12V). And, of course, all the associated transfer switches, and so forth. Pretty basic, right?

What I have trouble with is, how does the system know where to send power when it comes in? Just to the batteries only? Batteries and the ceiling lights if being used?

If the power input is from, say, the solar charge controller to the batteries, and the "house" system calls for power at the exact same time, what/who gets priority? How does all the various systems charge the batteries and light the lights at the same time from the same or different power sources?

What if I were to introduce an inverter into the mix? What if I were using my 12V lights and the microwave (110V AC) at the same time?

Please us small words.

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Old 08-22-2019, 12:43 PM   #2
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Old 08-22-2019, 01:32 PM   #3
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Push over, Winterbagoal.
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Old 08-22-2019, 01:57 PM   #4
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Everything that makes/harnesses 12VDC (11.5 to 14.5 in RV world) sends it to house/chassis batteries if you're lucky.
Anything that uses 12VDC (included inverter) uses the batteries/converter, etc.

Now go read HandyBob's blog.
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Old 08-22-2019, 02:23 PM   #5
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The answer is... it depends. You haven't provided enough info for us to tell you exactly.

IF you have a converter then when plugged into shore power or on the genset your lights and 12v appliances are powered by the converter, it also charges your batteries if they need charge.

If you have an inverter/charger then your batteries power all lights and 12v appliances directly even when on shore power or when the gen is running. The charger section of your inverter/charger charges those batteries when they are in need of a charge. The inverter side of the device inverts 110v power from your 12v batteries, but that's a separate operation.

In either of these two scenarios when on shore power, generator (or inverter) all 110v appliances are powered directly from those power sources.

Do you know which setup you have? Do you have a Converter or an Inverter/Charger?

Solar panels would feed a separate solar charge controller and it's connected directly to your batteries and all that device does is charge your batteries as needed.

Your engine alternator is a separate charging device and is connected directly to your chassis battery and probably, but not always, to your house batteries as well.

So you may have 3 separate chargers attached to your batteries:
1. Converter or Inverter/Charger
2. Solar Charge Controller
3. Engine Alternator

Presumably all of these chargers are "smart" and know how much charge your batteries need and when to apply a charge and how much.
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Old 08-22-2019, 03:27 PM   #6
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OK! Now we're getting somewhere.

Here is some additional info:
* Pretty much box stock '92 Fleetwood Pace Arrow.
* 12V converter
* 7500W Onan
* The usual suspects as far as transfer switches, etc. Haven't molested the existing systems in the least. To be honest, I'm afraid to, since the original electrical schematics are no longer available, and I'm too lazy (and stoopid) to trace all the cabling.

Here's what I added (and, HEY, don't laugh...it's something to play with):
* 50W solar panel, approximately 2' X 2' propped up against the front bumper * Panel MP4 connected to the $17 PWM controller from Amazon. Makes about 1A in full sun.
* Controller output is via the same gauge wiring to the 2 X 6V GC batteries and connected to the battery bank via alligator clips, so I can connect/disconnect it quickly to move it.
* The only connection left on the charge controller is for "Load" and is unused. I suppose I could connect it to a third-party inverter, but I figured "why bother" since I have a bunch of other 110V sources without the battery bank/inverter route.

So.....hope the additional information is helpful. I MAY spend a couple of $$$ to improve and enlarge my system (for something to do) including acquiring bigger panel(s), permanently affixing the panels to the roof, acquiring a MPPT controller and maybe even a single circuit inverter for occasional use.


I STILL want to know how it is possible that I have power coming in to the batteries via the solar charge controller, AND power coming into the batteries via the converter, yet I am able to use said electricity (either 12V or through "Load" AT THE SAME TIME? How do I have electricity coming in and going out at the same instant using the same wires to the same battery terminals?

Is it not really the same instant?

What brand of sorcery is this?
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Old 08-22-2019, 03:49 PM   #7
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Hydraulically speaking, your batteries are like a water tank. There is a spigot at the bottom of the tank. There is pressure in the tank, AKA the system voltage.

All of your charging sources (engine alternator, converter, inverter/charger, solar panels, etc) are like sources of water flowing into the tank. All of your loads are taking water out of the tank.

All of the sources have a water pressure, or voltage. The max battery voltage is the pressure cutoff. If the tank pressure is low, and the sun comes up, the solar system, as the sole charger, when its pressure/voltage rises above the battery pressure/voltage, current flows from the solar panels into the battery.

If you turn on a big load (bigger than the output of the panels, current flows from the solar system and the battery to the load. Shut off the load, the solar system voltage rises above the battery voltage, and charging begins again.

Now you start the engine. The alternator is capable of supplying more voltage/current/power than the solar panels. So, the solar charge controller can't beat the alternator and shuts down until either its voltage rises as the sun rises, or until a really big load needs more power than the alternator can supply.

Basically, in electrical terms, everything (sources, batteries, and loads) are connected together in parallel, and the sources "communicate" with each other via system voltage. At times, in theory, depending on the circumstance, all the sources can be charging at once.
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Old 08-22-2019, 04:40 PM   #8
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To carry the analogy on a bit farther...

The engine alternator and some inverter/chargers are like fire hoses.

Converters are closer to big garden hoses.

Trickle chargers are squirt guns.

Solar can be anything from a squirt gun (solar "battery maintainers), to garden hoses or maybe small firehoses.
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Old 08-22-2019, 04:50 PM   #9
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Old 08-22-2019, 04:55 PM   #10
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It's FM! (FM=Freaking Magic). A much better thing to ponder in my mind is where can I catch some fish tomorrow, but I'm not highly intellectual. I do, however, like fresh fish dinners and so does the DW.
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Old 08-22-2019, 04:57 PM   #11
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The solar panel is super minor. It's not adding much to the mix. Really. Take care to connect it to the 12v battery - not to one of your 6v batteries - your two 6v batteries combined make one 12v battery.

Your various chargers and your converter have circuitry in them that controls what's being put into your batteries. It's not a problem that there are multiple sources. The only time it would be a problem to have multiple chargers all charging the same battery bank is if one of them was not capable of adjusting the charge current and it resulted in overcharging and "cooking" your batteries. That's not likely even with an older RV.

It can happen - just not likely if everything is working normally.

By the way - your genset doesn't matter in any of this. It should be viewed as a 110v power source exactly like your shore power cable plugged in at a campground. Shore power and generator power are the same thing. You're either plugged into one or running the other.
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Old 08-22-2019, 04:57 PM   #12
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They play well together. Your small panel is to maintain battery(s) as long as it feeds the battery.
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Old 08-22-2019, 06:22 PM   #13
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Originally Posted by SLOweather View Post
To carry the analogy on a bit farther...

The engine alternator and some inverter/chargers are like fire hoses.
Converters are closer to big garden hoses.
Trickle chargers are squirt guns.
Solar can be anything from a squirt gun (solar "battery maintainers), to garden hoses or maybe small firehoses.
This is a very good way to conceptualize these concepts.

I apologize if this sounds overly lecture-y... In a previous career I was a college professor and technical director. I have a strong background in this, I will try to explain things further.

You don't have a modern inverter/charger in your coach, so I will cover that first and fastest.

A modern RV inverter/charger (Magnasine is a good example) is three things: It is a 120v (High Voltage AC) transfer switch for the 120v loads connected behind it, and those loads can be powered EITHER by incoming 120v power or from the batteries by inverting. It is a charger for the batteries, and it is an inverter drawing power from the batteries to maintain the HV loads no matter which power source is available.

The incoming HV power to the inverter/charger is either generator or shore power - switched with another transfer switch elsewhere that you DO have.

The second function in the inverter/charger is the charger. This works to detect the current voltage of the batteries and when there is incoming HV, it "splits" that power - using the internal transfer switch to pass the HV power directly through to the HV loads, and at the same time the charger attaches and becomes one of those HV loads to refill the batteries.

Now, about your coach specifically. Every RV has two primary power systems: high voltage and low voltage. The HV system is what you are most familiar with, wall outlets and 120VAC that powers your toaster and TV and everything else that you have in your house.

The LV system is 12VDC and is the same as what powers your car, cigar lighter cellphone charger, radar detector and any other car-accessory type things. An RV uses MOST THINGS on this LV system. Interior lights, water pump, fridge circuits, small fans. All are on the LV system pulling from the batteries.

Your coach is a motorhome, so it has a SEPARATE 12VDC system for the "chassis" systems of starting the drive engine, headlights, dash radios, etc. There will (likely) be a momentary-contact relay switch somewhere to connect the two LV battery systems together as a booster if one or the other has a dead battery. Usually this is to boost the engine batteries with the house system so you can start the drive engine.

The chassis alternator is primarily charging and powering the engine batteries, but MAY have a diode (one-way valve for electricity) set up to send power to the house batteries when the drive engine is running but prevent draining the engine batteries if you run down the house batteries.


Everything on the low voltage side (LV) is determined by the voltage in the batteries and cables. Much of the rest of this will be concerning the low voltage house systems.

Electricity in the LV system is like a container of water - The battery is the container. Charging pours water (power) into the container until it is full of voltage, then the charge controller turns off the supply.
Turning on a light or using something else creates a drain from the container - and power flows from the battery or supply to the drain (load) and work is done - the light is lit.

After a certain amount of time, the battery has supplied everything it has and the container is empty. Don't let it get that low! I will address battery management later.

Solar is a small-refill capacity for the batteries. The 120V charge controller that your coach has is a large refill capacity, but this only supplied when there is a HV source such as your generator or shore power available.

Your solar system at only 50 watts / 1 amp is basically useless for anything other than keeping the battery fresh when your coach is stored. Sorry, but that just is too small for anything other than a couple LED lights. Batteries (even when disconnected) will naturally lose about 10% of their charge per month while stored, so having this panel on the roof of your RV and connected to the batteries while you are stored will actually do some good. In normal use while you are using your coach... It just can't keep up with the demand. There's a reason I put 1020 watts of solar on the roof of my coach! 50 watts... That's a demo system, but a good way to learn about solar and wiring.


So how does the power "know" where to go?

Electricity flows based on "potential," which from an outside perspective looks like "balance" in the system. Potential is "does the power have someplace to go from a higher power source to a lower power destination. Everything flows downhill, the path of least resistance. The charger has a higher voltage and the battery slowly increases in voltage as it stores energy until they are almost equal, and the charger can detect this happening and it stops charging to avoid overfilling the battery.

When the charger or solar controller has power to supply, it has a higher "potential" than the battery, and the power flows there. If you simultaneously turn on a light, then that light ALSO has a "potential" but it is much much lower than the battery - so some of the power from the highest source (the charger) will be diverted to power the light, and the rest will still add power to the battery. If the load is too large for only the charging supply, then the total potential will drop below that of the battery, and power will flow out of the battery to make up the difference.

The easiest way to see all of this in action is with a Battery Monitor that uses a shunt. Magnum Energy sells one that works with their inverters, but there are many others available on Amazon as well for a variety of prices.

A shunt is a special type of rated terminal that sits at the negative terminal of the battery, and ALL power to the batteries must flow through it. It has two small sense terminals that connect to the monitor device, and the flow of electrons through the shunt can be very precisely measured. When the batteries are accepting power from a source, the shunt and monitor will indicate a certain amount of positive amperage. When the batteries are draining / supplying power to the loads, the shunt and monitor will show that as negative amperage. This will usually be able to also tell you the current SoC (State of Charge) of your battery bank.


Battery care is important!

GC2 golf cart batteries are 6V and need to be connected in series pairs and if you have 4 or more, parallel groups of those series pairs. If this is confusing, STOP NOW and don't connect them until you know how! The worst thing to do is short circuit a giant battery like that.

Use of a flooded battery (standard lead acid / car battery / deep cycle marine, GC2, etc) is that you want to keep it above 50% SoC to avoid damage. Even so called "deep cycle" batteries will suffer if you drop them below about 40-30% SoC, and the overall lifespan will decrease substantially. A full 12V battery with no load on it should read about 12.7V, and at 50% SoC is 12.1V.

Note that this is NO LOAD and you should let the batteries "rest" for about 5-10 minutes after disconnecting power to test the voltage. A battery monitor SHOULD take the overall load into account when telling you the voltage and SoC, but this may be problematic as loads pull down the voltage in the wire, affecting the accuracy of the reading. It is better if you can minimize the loads while checking voltage, or just try to avoid operating your batteries below 11.9 volts indicated while loads are connected.
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Old 08-22-2019, 08:04 PM   #14
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Geordi: The nicest, best, most comprehensive explanation I think I have ever received. (love the screen name, too, BTW). You have given me the courage to go on with my new-found knowledge. 10/10.

Would you consider moving down the street from me, to consult on some of the more complicated plans I would like to eventually execute? I can go as high as $1.00/hour and all you can eat.

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