Powering a 12 volt fridge and battery - Page 3 - Fiberglass RV


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Old 10-27-2007, 07:38 PM   #29
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We've talked before but I don't remember a consensus as to what happens when you present two different batteries at different states of charge to an automotive alternator... In the sailboat marine application the preferred set up seems to be to use a heavy duty continual use alternator with an external regulator with two or more circuits and sophisticated 3 phase charging programs.

Har! We should submit the whole recharge your trailer battery with the tow vehicle thing to the Mythbusters gang!

(Edited to add some links found later.)
Oops. I responded to Byron's message before I read yours and I basically covered the same topic. The marine method of charging with a multi-stage regulator would probably be the perfect method for someone who strictly dry camps for long periods and then drives medium distances between camp sites. It would probably be worth the cost of a high output alternator and special regulator to avoid having a battery bank that was never more than 75 or 80% charged in that situation. My requirements probably aren't so strict. I can see using state parks with electrical hook-ups and the occasional RV park so I could get by with the tried and true but less efficient method for short periods. Maybe if I manage to fry my alternator I will look at the more sophisticated solutions.

I'm not sure how the typical alternator's voltage regulator sees dual batteries but I suspect it is just reading overall system voltage. Generally a car or truck's primary battery is close to fully charged. The alternator charges it up to replace the amperage needed to start the vehicle but the battery doesn't do much when the car is running as far as I know. The alternator handles the vehicle's electrical system: lights, defrosters, stereos. The battery kicks in when the alternator can't handle the load which shouldn't happen often or when we forget to turn our lights off. Otherwise the vehicle battery is drawing very little amperage and the only battery load the alternator/regulator will see will be the trailer/house battery. This is my understanding but it could be flawed. "Managing 12 volts" by Harold Barre is an excellent book.

I appreciate all the responses here. I think I am close to deciding how I will approach the problem.
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Old 10-28-2007, 09:37 PM   #30
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This is another one of those things won't hurt anything, but doesn't do much either. It's like putting a "Y" on the camp site water faucet running two hoses to the trailer and putting another "Y" at the trailer water connection. It doesn't hurt anything, but it don't do much.
I suppose the difference between two hoses and two separate grounds is that if one hose bursts, you won`t get much water from the other but if one ground loses connection, the other ground will still continue functioning for a time until it also corrodes or whatever.......the space in the connector was unused, the wire also sat unused, it wasn`t a big deal to wire it, clipping both ends and wiring it hot, it could serve another light or other circuit....i.e. backup lamps or what ever, so why not?......Benny
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Old 10-28-2007, 10:36 PM   #31
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I suppose the difference between two hoses and two separate grounds is that if one hose bursts, you won`t get much water from the other but if one ground loses connection, the other ground will still continue functioning for a time until it also corrodes or whatever.......the space in the connector was unused, the wire also sat unused, it wasn`t a big deal to wire it, clipping both ends and wiring it hot, it could serve another light or other circuit....i.e. backup lamps or what ever, so why not?......Benny

With the hoses you'll get the same amount of water with one as you do two. With the wires you'll get a very small difference in voltage drop across the wires with two compared to one at any given current. This is, of course if one of the wires is large enough to carry the current. Like I said it won't hurt anything, but don't expect grand results.
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Old 11-25-2007, 09:57 AM   #32
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My wife and I just returned from our first outing with our new (to us) 13' Scamp, which is equiped with the Dometic three-way fridge. After reading several related postings on a teardrop trailer site (our former mode of camping) about this same issue; and also reading on this site about the pros and cons of using propane while enroute, I decided to try powering the fridge from the 12V car system. I ran a fused #10 wire from the battery, along with a #10 ground wire attached near the rear of the car specifically to power the fridge through the trailer's power converter. I installed a Blue Sea ACR (automatic charge relay) Click here! in the TV, which connects the trailer electrics to the TV about a minute after the car is started. This avoids a lot of the voltage transients that I understand can do bad things to power converters. I turned off the fridge at all gas/food stops. So how did it do? After a full day of driving, the fridge was cold, and the Scamp's battery measured 12.3 volts. I think I'll declare success on this one and move on to some other things I want to do to the trailer. I hope this is of interest.

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Old 11-30-2007, 06:53 PM   #33
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A relay which delays the connection of the house battery to the charging circuit seems to be a pretty good idea. The highest demand of the TV battery on the alternator is just after starting the vehicle as the current used in starting is replaced. This could help protect the tow vehicle's charging system when the tow battery has been drawn down below normal levels.

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I installed a Blue Sea ACR (automatic charge relay) Click here! in the TV, which connects the trailer electrics to the TV about a minute after the car is started. This avoids a lot of the voltage transients that I understand can do bad things to power converters.
I am not sure I follow here. The converter is only in operation when you are connected to shore power so it should not be a factor when towing. Glad to see this set-up worked out for you.
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Old 11-30-2007, 10:11 PM   #34
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I am not sure I follow here. The converter is only in operation when you are connected to shore power so it should not be a factor when towing.
Even when not connected to shore power the output of the converter is still connected to the 12 volt trailer system. Therefore if the converter doesn't have the proper surge protection on it's output it's possible to do some damage. The 12 Volt side of the converter needs to meet the same SAE requirements as any other 12 device.
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Old 12-03-2007, 09:16 PM   #35
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Even when not connected to shore power the output of the converter is still connected to the 12 volt trailer system. Therefore if the converter doesn't have the proper surge protection on it's output it's possible to do some damage. The 12 Volt side of the converter needs to meet the same SAE requirements as any other 12 device.
Does this apply if there is a 12 volt (and 120 volt) distribution panel separate from the converter?
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Old 12-03-2007, 11:33 PM   #36
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Does this apply if there is a 12 volt (and 120 volt) distribution panel separate from the converter?
In order for the converter to charge the battery and power the 12 system it has to be connected to the 12 volt system. Therefore the answer is yes. The distribution panel does just that distributes power through individual circuits, it doesn't isolate the converter from the 12 volt system.

Take a look at the Scamp wiring diagram here. You'll see that both the back and the white wire coming from TV and from the battery are connected to the converter.
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Old 12-06-2007, 05:37 AM   #37
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In order for the converter to charge the battery and power the 12 system it has to be connected to the 12 volt system. Therefore the answer is yes. The distribution panel does just that distributes power through individual circuits, it doesn't isolate the converter from the 12 volt system.

Take a look at the Scamp wiring diagram here. You'll see that both the back and the white wire coming from TV and from the battery are connected to the converter.
Thanks Byron. I was thinking of Parker's example of running a line from the tow vehicle directly to the converter (I assume to the distribution panel that is combined with the converter) and from there to the fridge. I thought that the fuse at the panel should protect the converter but I suppose fuses don't protect equipment from voltage transients since fuses are primarily over-current devices. This is an issue I have not read about. I have a Progressive Dynamics 9200 (not yet installed) and I can find no info on built-in protection from transients. There is protection for reverse polarity if the battery is hooked up incorrectly but transients are short duration spikes which have a cumulative effect.

I plan to have a separate line from the tow vehicle to the fridge with no connection to the battery and converter but I also plan to run a charge line to the battery which will also be wired to the converter. What effect would the house battery have on voltage transients that originated with the alternator? I know that some older converters require a battery to be used even when run from shore power to smooth out the voltage. In any case, it looks like the Blue Sea ACR may be a good option for my battery charge line. Pretty inexpensive as well. I appreciate all the good responses I have received on this thread.
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Old 12-06-2007, 10:29 AM   #38
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Thanks Byron. I was thinking of Parker's example of running a line from the tow vehicle directly to the converter (I assume to the distribution panel that is combined with the converter) and from there to the fridge. I thought that the fuse at the panel should protect the converter but I suppose fuses don't protect equipment from voltage transients since fuses are primarily over-current devices. This is an issue I have not read about. I have a Progressive Dynamics 9200 (not yet installed) and I can find no info on built-in protection from transients. There is protection for reverse polarity if the battery is hooked up incorrectly but transients are short duration spikes which have a cumulative effect.
Cumulative effect - True for ESD events (very short thousands of volts spikes), unlikely for power transients (600 volt max)

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I plan to have a separate line from the tow vehicle to the fridge with no connection to the battery and converter but I also plan to run a charge line to the battery which will also be wired to the converter.
I know people have claimed to do things like this and have it work, but I wonder about it. From what I see it appears that the limiting factor is the ability of tow vehicle to produce the kind of power needed.

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What effect would the house battery have on voltage transients that originated with the alternator? I know that some older converters require a battery to be used even when run from shore power to smooth out the voltage. In any case, it looks like the Blue Sea ACR may be a good option for my battery charge line. Pretty inexpensive as well. I appreciate all the good responses I have received on this thread.
Transients won't have any noticeable effect on the house battery. Most large vehicle transients occur when there's a loose connection to the vehicle battery. The alternator is capable of putting out 60 to 90 volts without the batter connected and in some cases there can be as much as a 600 volt spike.

I haven't checked what happens with my Blazer with the fridge running and not. Since I'm changing my TV to my Dakota, that's all I've checked. With it there's enough alternator output to charge the battery (about 2 amps) and run the fridge. From the few times I've run the fridge when towing with the Blazer, it doesn't appear to do both. It appeared that I was drawing some current from the house battery along with the Blazer's system. The Dakota has the tow package which includes a very large battery and I think a larger alternator.

Good Luck
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Old 12-07-2007, 03:00 AM   #39
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Transients won't have any noticeable effect on the house battery. Most large vehicle transients occur when there's a loose connection to the vehicle battery. The alternator is capable of putting out 60 to 90 volts without the batter connected and in some cases there can be as much as a 600 volt spike.
My concern wasn't for the battery so much as the converter since, as you pointed out, it is connected to the battery. The only reference I could find to voltage transients and mobile electronics was this site. They have a product to sell. I haven't noticed posts by anyone complaining of their converter being fried by their alternator while towing but a shortened life caused by transients over time would be harder to detect. I'm not sure if this is something I should be concerned with.

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Old 12-07-2007, 10:01 AM   #40
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My concern wasn't for the battery so much as the converter since, as you pointed out, it is connected to the battery. The only reference I could find to voltage transients and mobile electronics was this site. They have a product to sell. I haven't noticed posts by anyone complaining of their converter being fried by their alternator while towing but a shortened life caused by transients over time would be harder to detect. I'm not sure if this is something I should be concerned with.
That site really talking about what SAE (Society of Automobile Eningeers) refers to as "battery disconnect". Those types of transients are always possible, however if you're house battery is connected to the TV electrical system, it's unlikely. That said I doubt that any modern converter would NOT have transient protection. If you had transient voltage caused failure it wouldn't be accumulative, it would be instantly a "catastrophic". As you said, there's not been any reports of they type of failure, which also indicates that there's probably transient protection in most if not all converters. So not to worry.

Transients causing shortened lifer over time. That's a concern mostly in the electronic manufacturing industry or in the electronic repair industry. What "accumulative effect" is talking about is "Electro Static Discharge"(ESD) events. Those are what happens when shuffle your feet on carpet and draw a spark from your finger. There's a big difference between "can" and "do". ESD events can cause accumulative problems. NOTE "can" not "do". The entry is NOT through the power lines, either output or input power lines. It's possible if you open up a piece of electronic equipment and poke around to have an event that could damage something, but from the outside just not gonna happen. So again, not to worry.

Just as side note, what you feel when you're the victim of an ESD event is not shock, it's a burn. Some places where they have cold, dry winters, people carry keys in their hands that they tap against a piece of metal when possible. The ESD event happens and they don't get burned because the arc is to the key not their finger.

Hope this helps clearify a bit.
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Old 12-07-2007, 03:34 PM   #41
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I am reading this whole thing and a few things occur to me.

Most of the time when talking about surge or transient voltage I think we are referring to the input or 120VAC side of the converter.
Transients here could cause problems in the converter and as a result at the 12vdc output of the converter as well.

When there are voltage swings in the 12vdc side the battery certainly acts to help filter or absorb these mostly and therefore helps provide protection on the 12vdc side of the converter.

I am guessing when the question was asked about the battery helping to protect the system this is really what was being asked?

It is very common in 12vdc applications to use a large battery to help provide filtering from a noisy power supply and the battery is very effective in that role.

I suspect that when there is a load on the 12vdc side (like when running the fridge) and the battery is being charged too(like from the T.V) that the output side of the converter is isolated anyway from the rest of the converter.
Diodes in the converter output would prevent current from flowing back and causing problems just as they always would when using other forms of battery charging when the converter is powered.

I can compare it directly to running a genset or Solar charging.Each is charging the battery and not bothering the converter.

As for the fuses in the distribution panel,there are different ways they can be connected and for the life of me I understannd some of them like I understand the convention of using the Black wire as the Positive connection.......not at all.

In my new Scamp I discovered that the battery is just connected to the whole trailer thru one of the fused output circuits of the converter.
This makes no sense to me at all and when I asked Scamp to explain it to me,they could not. "It is just how we do it" thats all.

I figured this out when trying to determine which fuses controlled what loads and I could not disconnect one of the circuits even with the fuse out and discovered this is the one the battery is connected to.

This also tells me that the converter in this Scamp can have no actual charging circuit at all but that it just charges depending on the load throughout the entire trailer and the status of the battery.

I am astounded by the lack of science involved here and I have to wonder how many of us have trailers wired this way and don't know?

Anyway there is much to consider here if we want to understand it but the bottem line I think is that these 12vdc systems are just as resilient as the condition of the battery lets them be.
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Old 12-07-2007, 04:19 PM   #42
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I am reading this whole thing and a few things occur to me.

Most of the time when talking about surge or transient voltage I think we are referring to the input or 120VAC side of the converter.
Transients here could cause problems in the converter and as a result at the 12vdc output of the converter as well.

When there are voltage swings in the 12vdc side the battery certainly acts to help filter or absorb these mostly and therefore helps provide protection on the 12vdc side of the converter.

I am guessing when the question was asked about the battery helping to protect the system this is really what was being asked?

It is very common in 12vdc applications to use a large battery to help provide filtering from a noisy power supply and the battery is very effective in that role.

I suspect that when there is a load on the 12vdc side (like when running the fridge) and the battery is being charged too(like from the T.V) that the output side of the converter is isolated anyway from the rest of the converter.
Diodes in the converter output would prevent current from flowing back and causing problems just as they always would when using other forms of battery charging when the converter is powered.

I can compare it directly to running a genset or Solar charging.Each is charging the battery and not bothering the converter.

As for the fuses in the distribution panel,there are different ways they can be connected and for the life of me I understannd some of them like I understand the convention of using the Black wire as the Positive connection.......not at all.

In my new Scamp I discovered that the battery is just connected to the whole trailer thru one of the fused output circuits of the converter.
This makes no sense to me at all and when I asked Scamp to explain it to me,they could not. "It is just how we do it" thats all.

I figured this out when trying to determine which fuses controlled what loads and I could not disconnect one of the circuits even with the fuse out and discovered this is the one the battery is connected to.

This also tells me that the converter in this Scamp can have no actual charging circuit at all but that it just charges depending on the load throughout the entire trailer and the status of the battery.

I am astounded by the lack of science involved here and I have to wonder how many of us have trailers wired this way and don't know?

Anyway there is much to consider here if we want to understand it but the bottem line I think is that these 12vdc systems are just as resilient as the condition of the battery lets them be.

Unless there's a mechanical relay in the converter to disconnect the output, then the output is connected all the time to the 12 Volt system.

I can only think of one case where you could get transients into the converter output. If remove the house battery from the trailer and have a poor connection to the battery in TV then transients are quite possible. Even with a relay and no other protection it would be possible to damage the output circuits of the converter. Like I said earlier not to worry. My guess is that they all meet SAE specifications. Since I believe this to be the case the rest is academic.

Side note: It's kind of fun to play with high voltage load dumps and watch is cause the smoke to be let out. But, then it's back to the drawing board to fix so it doesn't happen.
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