Charging batteries

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Gbhammel

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Coldwater mi
Hello all, the wife and I boondock with our camper. I run a generator to supply power to the camper and use 2 batteries. I am wondering if I can run a extension cord to a 3 stage 55 amp converter off of my generator and charge the batteries while they are supplying power to the rv without hurting the converter in the camper
 

Rene T

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Welcome to the forum.
Why not just run a regular battery charger? I’m far from being a expert so others will chime in.
 

Gbhammel

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Coldwater mi
I use to do that but it takes a long time to charge two batteries one 101 amp hrs the other is 200 amp hrs at 10 amp on a battery charger it takes about 15 hrs if they are at 50%
 

Ex-Calif

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You want to run an extension cord from a house plug or a plug on the generator?

I ran a basic battery charger off the house plug to charge my batteries when my onboard charger failed. Didn't do any harm but it wasn't any quicker than the on-board charger when it was working.

Whether the house A/C is supplied by shore power or generator I don't think the battery charger cares...
 

Mark_K5LXP

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So what's the size of converter in the camper? It's implied it's much smaller than a 55A one you'd operate externally. How much of a boost you'd get from an external one depends on a few variables but they should coexist OK on the same set of batteries. Seems if boondocking is the typical use you might want to optimize the trailer for that, installing a suitable converter in the trailer and perhaps using batteries like AGM or lithium that can accept a charge quickly.

Mark B.
Albuquerque, NM
 

Gary RV_Wizard

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Perfectly ok to run a second external charger, of whatever amp rating and type you like. If the RV isn't also plugged to the genset or shore, its converter/charger won't even be aware of the external one. If it is powered, then the batteries will appear to be already charged, since the external charger is applying voltage. Which ever of the chargers is the most powerful (applies the higher voltage) will win.
 

John From Detroit

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In another thread I mentioned that 12 volt side of life is a perfect example of Commune type Socialism (not to be confused with other types of socialism)
What is Commune type socialism? most famalies at least when the kids are still in school. That aside

The concept is each according to it's needs and abilities
So if you connect an external smart charger while the trailer is not plugged in. THAT device has all the ability and will charge and operate stuff. while the on-board has neither needs nor abilities so it will sit there... Oh the fan may run if the compartment gets hot. that is all. not a sign of anything other than the heat in the compartment.

Same with Solar. no problem adding solar
The only issue is if you have too many sources all with exactly the same output (All but impossible) the batteries may charge too fast.. but .. as I said all but impossible. Normally the highest voltage out even if it's only 0.1 volt will haul the freight .
 

JayArr

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Short answer = Yes, you should not damage the onboard converter.

Some smart chargers (Xantrex as an example) will start a three stage charge cycle with constant current, you may get 55 amps until the battery bank reaches 14.5 volts. This will charge the batteries so much faster than a regular charger at 13.XV. Once the batteries reach 14.5V the charger will switch to constant voltage and hold the output at 14.5V until the amperage being delivered falls below a threshold, like 10A, at that point the charger goes into float mode and effectively shuts off allowing the battery to drift down from 14.5V to the float voltage of 13.6V at which point the charger will produce power on demand to keep the battery bank from falling below this point. None of this should interfere with the old/dumb charger on board, you should be able to fast charge your batteries from the genny.

That being said... why don't you just install the three stage charger in place of the old converter?
 

Gbhammel

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Coldwater mi
Short answer = Yes, you should not damage the onboard converter.

Some smart chargers (Xantrex as an example) will start a three stage charge cycle with constant current, you may get 55 amps until the battery bank reaches 14.5 volts. This will charge the batteries so much faster than a regular charger at 13.XV. Once the batteries reach 14.5V the charger will switch to constant voltage and hold the output at 14.5V until the amperage being delivered falls below a threshold, like 10A, at that point the charger goes into float mode and effectively shuts off allowing the battery to drift down from 14.5V to the float voltage of 13.6V at which point the charger will produce power on demand to keep the battery bank from falling below this point. None of this should interfere with the old/dumb charger on board, you should be able to fast charge your batteries from the genny.

That being said... why don't you just install the three stage charger in place of the old converter?
The converter in the camper is a 2014 model,so I am sure it is also a 3 stage. Just wanting to hook up a other converter to keep the batteries fully charged so when I shut the genny off I have power for 4 to 5 hours
 

Mark_K5LXP

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Trying to connect the dots why an external 3 stage charger would work better than a 3 stage charger already in the trailer.

Mark B.
Albuquerque, NM
 

Gbhammel

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Coldwater mi
Trying to connect the dots why an external 3 stage charger would work better than a 3 stage charger already in the trailer.

Mark B.
Albuquerque, NM
The one in the trailer let's say it's a 45 amp when everything in the camper is mostly dc so you are using 30 to 35 amps off of the rv converter. That only leaves 10 to 15 amps to charge your batteries. So by running a stand alone 55 amp converter to the batteries to charge quicker. Just don't know if it would back feed into the rv converter
 

Gary RV_Wizard

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The one in the trailer let's say it's a 45 amp when everything in the camper is mostly dc so you are using 30 to 35 amps off of the rv converter. That only leaves 10 to 15 amps to charge your batteries
Rare indeed would the other loads consume anywhere near 30-35 amps. Except when the furnace fan is running, lighting is the only large user of DC power and a half-dozen 12v lights is probably no more than 10A; less than that if led bulbs are used. The rest of the 12v demand is circuit boards and that is trivial. But no worries - your use of an auxiliary charger has no drawbacks except your time & effort to hook it up as needed.
 

Mark_K5LXP

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The only flaw in that logic is that whatever trailer loads are present with the onboard converter are also present with an offboard converter. So the difference current in the above example at best would be 10A more to the batteries. Not trivial, but in the scheme of charging a depleted a 300Ah house battery 25A vs 15A is "quicker", but still going to take many hours. Where there'd be a big shift in genset time would be if you isolated the trailer from the batteries, ran the trailer off one converter and charged the batteries from the other. I would bet your draws are nowhere near 30-35A so the lower your actual draw the less of a benefit a larger converter will provide. You don't know what your onboard converter is, so before I'd go through any extraordinary measures to "fix" that I'd get a battery monitor and know what my charge and draw rates were.

Mark B.
Albuquerque, NM
 

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