Electrical system background

Electrical issues, both 12 volt and 120 volt
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skater
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Electrical system background

Post by skater »

Many people new to campers are confused by the multiple electrical systems. This tries to clarify it...

The electrical systems in our B190s is confusing and somewhat complicated. In this article I attempt to break it down.

First, understand there are three electrical systems. They are:

1. The 120 volt AC system, like your home has
2. The engine's 12 volt DC system, like your car has
3. The coach's 12 volt DC system

The 120 volt AC System

This system contains the rooftop air conditioner, the fridge (in 120 volt mode), the power outlets, the microwave, and the convertor for the coach's 12 volt system. That's it.

It is somewhat complicated by the fact that there are two sources of electricity in most of our B190s, with a third one that might've been installed by a former owner:

1. "Shore power" - the 120 volt power cord that you plug in when at a campsite or at home.
2. Generator - if your camper is equipped with one
3. Inverter - takes 12 volt power, usually from the coach battery (see the coach's 12 volt system, below), and inverts it to 120 volt

If you have a generator, there is an automatic transfer switch installed that switches to generator power when the generator is running (even if you are still plugged in to shore power). After you start the generator and it stabilizes and starts producing electricity, you'll probably hear the transfer switch produce a loud "click" as it switches over.

While 120 volt is available, as mentioned above, the converter is powered, so you'll be able to use all of the items connected to the coach's 12 volt DC system. Note the starter for the generator is connected to the coach's 12 volt DC system, too.

To detect if you have an active 120 volt AC power source, mine has a small light in the GFCI power outlet. It should be lit when 120 volt power is available. (See picture - the light is just above the red GFCI breaker button.) If yours doesn't have that, the best solution might be just to plug in a lamp or other known working device.

The engine's 12 volt DC system

This is essentially the same as the 12 volt DC system in a car. This provides power for the engine's starter, lights, radio, etc. This system will usually have a regular car battery under the hood. (On mine, it's the passenger-side battery.) This system also supplies the DC mode of the fridge when the engine is running.

The coach's 12 volt DC system

Here's where things get interesting. All of the interior lighting and fans run on this 12 volt DC system. Sources of power for the coach 12 volt system are:

1. The 120 volt AC system, via the power converter under the couch.
2. The alternator on the vehicle's engine, if the engine is running.
3. The coach battery
4. Solar panels, if equipped

It's important to note that when the engine is running, the two 12 volt DC systems are connected together via a selenoid mounted underhood (which you may hear click when you turn the ignition to "RUN"). Thus, even if your deep cycle battery is run down, running the engine will supply 12 volt power to the interior lights, fans, and the generator's starter.

This is especially useful when starting the generator if the coach battery is weak - just start the camper's engine, then start the generator.

Charging the coach battery

The coach battery is charged whenever 120 volt AC power is available, the engine is running, or the solar panels are in sunlight. However, the engine's battery is only charged by the alternator; the converter does not charge the engine battery.

You can check the condition of the coach battery's charge using the monitor. However, note that while the battery is being charged (that is, the engine is running, or you have a 120 volt power source available), the indicator should show "charging", regardless of the battery's actual charge. To accurately gauge the condition of the coach battery, shut off all 120 volt sources and/or the engine, then check the monitor. Note batteries can have a 'surface charge' that means you might get a lower reading 20 minutes after you shut off the charging sources.

What are the batteries?

The engine battery is a standard car battery. The coach battery should be a deep cycle type. Deep cycle batteries are more resistant to the damaging effects of deep discharges, whereas a couple deep discharges of a regular car battery can kill it. The cheaper versions are wet cells that require monitoring and refilling with distilled water on occasion. Better batteries are the gel-type or AGM, which are sealed, but are also more sensitive to being overcharged.

(Added 3/19/21) Lithium batteries are also widely available now. As of this writing they're still fairly expensive, and you definitely must have a converter that's compatible with them (most are not), but the power capacity and lifetime they offer mean we'll probably all be using them within a few years. Also note if you have a solar system, you'll want the controller to be lithium-compatible as well. If you're replacing your converter anyway, there are some that are compatible with both traditional (wet cell and AGM) and lithium batteries, so consider that option to future-proof your B190.

Rough Wiring Diagram

Image
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jojo
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Re: Electrical system background

Post by jojo »

Thanks Skater for this very helpful breakdown. My 98 power seat runs off the engine battery however.
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skater
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Re: Electrical system background

Post by skater »

jojo wrote:Thanks Skater for this very helpful breakdown. My 98 power seat runs off the engine battery however.
Yeah, that's a good point that probably should be emphasized - this was how mine was set up, but there's no guarantee they were all done the same way.
1991 Airstream B190 - bought, 2005; sold, 2011; bought 2017
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VanDownByTheRiver
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Re: Electrical system background

Post by VanDownByTheRiver »

Anyone else have 2 batteries under the hood, for a total of 3?

I have what appears to be a regular car battery under the driver's side of the hood, and a smaller battery under the passenger's side of the hood.

To be clear, I also have a marine deep cell coach battery under the couch.
'94 B190 w/ 460
Wakeslayer
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Re: Electrical system background

Post by Wakeslayer »

I have two under the hood. Laid out as you describe. Mine are actually jumped together, which is not right, but I don't really mind it. I have never looked under the couch.

Mike
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skater
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Re: Electrical system background

Post by skater »

VanDownByTheRiver wrote:Anyone else have 2 batteries under the hood, for a total of 3?

I have what appears to be a regular car battery under the driver's side of the hood, and a smaller battery under the passenger's side of the hood.

To be clear, I also have a marine deep cell coach battery under the couch.
This sounds like an addition that someone did to have more battery capacity (someone added a third battery to our trailer for the same reason).
1991 Airstream B190 - bought, 2005; sold, 2011; bought 2017
1995 Airstream Excella 30' trailer

WBCCI #13270, Washington, DC Unit
VanDownByTheRiver
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Re: Electrical system background

Post by VanDownByTheRiver »

Yeah I suspect the old deep cell under my couch needs to be topped off or replaced, because my coach voltage is ALWAYS low without generator/alternator running, like immediately after shutting off gen/460.
'94 B190 w/ 460
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skater
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Re: Electrical system background

Post by skater »

VanDownByTheRiver wrote:Yeah I suspect the old deep cell under my couch needs to be topped off or replaced, because my coach voltage is ALWAYS low without generator/alternator running, like immediately after shutting off gen/460.
Probably the latter - the single stage converter that came standard with these can easily overcharge a battery and kill it.

Note, if it's in the cabin I suggest getting an AGM battery that is sealed so it won't vent dangerous fumes. Downside: If you don't replace your charger, you can kill an AGM battery quickly, too, and they're more expensive to replace.
1991 Airstream B190 - bought, 2005; sold, 2011; bought 2017
1995 Airstream Excella 30' trailer

WBCCI #13270, Washington, DC Unit
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Planck
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Re: Electrical system background

Post by Planck »

I think another down side is that with that deep cycle getting ruined by the single stage charger, and the owner not knowing it, an extra load will be put on the alternator trying to charge it, possibly damaging the alternator.
VanDownByTheRiver
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Re: Electrical system background

Post by VanDownByTheRiver »

My alternator does sound pretty overwhelmed.... Thanks, Planck, I hadn't connected those issues!
'94 B190 w/ 460
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skater
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Re: Electrical system background

Post by skater »

VanDownByTheRiver wrote:My alternator does sound pretty overwhelmed.... Thanks, Planck, I hadn't connected those issues!
My van had the 130 amp alternator as part of the prep kit. You shouldn't be able to overwhelm something like that... I'm honestly not even sure what "overwhelmed" means in terms of an alternator. Are your headlights dimming?
1991 Airstream B190 - bought, 2005; sold, 2011; bought 2017
1995 Airstream Excella 30' trailer

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dougb
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Re: Electrical system background

Post by dougb »

Skater,
What powers my system monitor panel, the cigarette lighter and water pump on my 1993 b190?

Thanks,
Doug
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skater
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Re: Electrical system background

Post by skater »

Those are normally powered by the coach system, assuming you mean the lighter near the panel and not the one by the radio.
1991 Airstream B190 - bought, 2005; sold, 2011; bought 2017
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Moosedog49
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Re: Electrical system background

Post by Moosedog49 »

Is there a battery isolator between the engine battery and the RV/Coach Battery to prevent crossfeed. ('96 B190) I seem to be having a short problem on the RV side draining my battery within 2 days. Replaced battery thinking bad cell (closed top battery, no way to use a hydrometer) after a recharge and the dead battery again. So much equipment so little room.
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Planck
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Re: Electrical system background

Post by Planck »

Don't know where your RV battery is, but should be able to disconnect the positive and put an ammeter in there to see how much current is being drawn. Then start pulling fuses until there is no current draw. My guess is your fridge could be on the 12v setting and running the battery down. Before any other testing I would pull the fridge fuse to see if that fixes the problem. If so, need to find out why fridge is not turning off.

Good luck.
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