Charging battery with an external charger while dry camping?

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Gods Country

Gods Country

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Joined
May 23, 2016
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655
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North Central Pa
Wondering if there are any issues charging the battery on my TT while it is still connected.  No shore power involved.  Just thinking that my battery will charge faster and more efficiently with an external charger plugged into a generator, and jumped to the battery rather then running it through the converter.  Also does the battery need to be disconnected (or isolated) to avoid any problems while this process takes place?  I am going to be dry camping for an extended period and want to run the genny as little as possible.
 
So... use a smaller generator and charger to recharge the batteries instead of using the built in gen and charging system?

Smaller gen and charger:
  1) Might make less noise.
  2) Should use less fuel.
  3) Might not have enough power to keep appliances and lights on.

I'm not sure about having to disconnect the batteries, but that would mean your fridge and water heater would not run, even on propane. The water pump wouldn't work either.

The whole idea of the built in generator and charging system is that the lights and appliances have enough power to still work while the batteries are getting charged.
 
Stephen S. said:
I'm not sure about having to disconnect the batteries, but that would mean your fridge and water heater would not run, even on propane. The water pump wouldn't work either.
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Yes, exactly the problem.  Nothing major but I would like to avoid it.
 
Gods Country said:
Yes, exactly the problem.  Nothing major but I would like to avoid it.
Click to expand...
No problem, go for it. You can charge the batteries from different sources, The larger charger might not go to full charge mode if it is an automatic charger. But what makes you think the external will charge faster? I'd check them both, your converter just may charge the battery faster.
 
Many travel trailers have low quality converter/chargers and they are wired too light.  A good charger connected to a generator might do a much better and quicker charge.  If you have a charger, hook it up at home and see if you have a higher voltage at the battery compared to when the onboard charger is operating.
 
No problem, but the external charger is often not as efficient as the onboard system. Some onboard converter/chargers are excellent and have sophisticated charging algorithms that minimize charge time and preserve battery life. Other models, maybe not so much. Few of the external chargers do as well unless you paid a lot more than for the models typically sold in retail stores. Most of them are single stage (tapered charging) types with a max output of 10A-15A and typical output of half that. Your onboard system is probably 2x or 3x the max charge rate and if its a 3-stage type, actually delivers a fairly high rate of charge when the batteries are discharged.

Overall, though, the time to reach 100% charge is limited more by the batteries themselves than the charger. Once the battery reaches about 80%, the rate at which it accepts charging slows to a trickle and no charger can push the amps in any faster without damaging the battery permanently. That last 20% will take several hours regardless of the charger used. In essence, you will probably be operating the batteries in the 50%-80% range during your stay because you won't ever run the genset long enough to exceed 80% or so.
 
I'm going to have to take a look when I get a chance, but IIRC the converter is on a 15A breaker, and I can't imagine there is more than a 3 or 4 amps for recharging.  I will try to measure the output.  It seems to me that a couple of hours of genny run just tops off the battery whereas my regular charger seems to do a complete charge from about 50% in about 10-12 hours, so the hope is a couple hours of direct charging with yield a better result.
Perhaps marginal at best.  We shall see. 

Anyway my primary concern was back feeding (for lack of a better term) too much voltage into the system if the battery were not disconnected.

Thank you for the replies.
 
Gods Country said:
I'm going to have to take a look when I get a chance, but IIRC the converter is on a 15A breaker, and I can't imagine there is more than a 3 or 4 amps for recharging.
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I think you're confusing 12 volt amps with 120 volt amps.  There's a 10:1 ratio between them, i.e. 50 amps at 12 volts is the same amount of power as 5 amps at 120 volts.

A 10 amp, 12 volt charger will only draw about 1 amp at 120 volts.

In general, unless you have a fairly old RV, the internal converter will do a better job at keeping your batteries charged than almost any external charger.
 
Lou Schneider said:
I think you're confusing 12 volt amps with 120 volt amps.  There's a 10:1 ratio between them, i.e. 50 amps at 12 volts is the same amount of power as 5 amps at 120 volts.

A 10 amp, 12 volt charger will only draw about 1 amp at 120 volts.

In general, unless you have a fairly old RV, the internal converter will do a better job at keeping your batteries charged than almost any external charger.
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Never considered that. :))
 
It seems to me that a couple of hours of genny run just tops off the battery whereas my regular charger seems to do a complete charge from about 50% in about 10-12 hours, so the hope is a couple hours of direct charging with yield a better result.
Click to expand...

Until you can get down to the specifics of what internal (house) charger you have vs what external charger you would use instead, there is no way to determine which is likely to be more efficient. However, I am highly skeptical of your perception that the external is faster or better. Both chargers will [eventually] achieve 100% charge and the total time is likely to be fairly close unless the external charger is really small, e.g. 5A or less. The gauge on the external charger may appear to show a full charge sooner, but that's not a very reliable indication.  You need some fairly accurate standard of measurement to compare, e.g. a battery hydrometer.

If your onboard charger is any model newer than the old Magnetec 63xx, I would place my bet on the onboard system for both time and power efficiency, whether your goal is 100% SOC or a more reasonable 80% SOC.

It takes a certain amount of time to push back in xx amp-hours of power. In general, the charger with the highest max amp rate will be a bit faster, pumping in more amps when the battery is low (discharged). As the SOC increases and the charge rates slows, the larger charger no longer has a rate advantage and the time to reach 100% tends to even out.
 
As Gary said, most late model RV converters are hard to beat. Often times the WFCO which is common will only charge at 13.6 volts. This isn't bad if you are on shore power but if you want to charge off your generator with short run times upgrade to a converter that does 14.4 v or better yet 14.8.

I ran on gen our first year and upgraded to a Progressive Dynamics 4655 which is fine but if doing it over I would go with 14.8 v model. I also bought an actual 12 volt battery charger which was $400+. If you get into chargers that go over 15 v during equalization you may want to check your rv devices as to being able to handle that voltage.  When I was running four GC2 I would run the battery charger and the converter.  Some chargers, solar controllers etc wont play together or one just drops out, sometimes they both contribute as mine did with about 70 amps going in.

Some auto type chargers have low amp output and yet sometimes reach too high of a voltage. Some people do use that high voltage characteristic to perform equalization when needed. So, different products, different results and uses.
 
Also as far as efficient charging goes, you may want to check you voltage drop from the converter to the battery. Some rigs have a long distance between the two and a considerable voltage drop can occur.  It's also common for the set screw terminals to loosen at the converter causing heat and voltage drop. Measure v at the converter terminals and at the battery terminals to check voltage drop. It's common for builders to use 6 gauge and people often upgrade to 4 or as I did to 2g. Again if you are on shore power all the time it doest matter, if you are on generator you want high efficiency and short run times.

My sons FW bunk had the converter at the back and the battery at the front 30+ feet away so I moved the new Boondocker 80 amp converter up next to the battery. It's more efficient to run 120 volts through romex to the converter than it is to run it through 12 volts. Also I believe Gary mentioned above you have 120 v ac divided by 12 dc = 10.  My .7x amp ac residential refrigerator pulls 7 amps DC (running amps not starting amps).
 
In that case, your best bet is to pull out the converter/charger section and replace it with a more modern charger.

The Progressive Dynamics 4600 series converter will drop right in when you remove the existing converter from the power center.  It has the Charge Wizard built-in but lacks the ability to select the different charging modes manually.

Or you could get a PD9100 Series Converter or a PD9200 Series converter and mount it closer to the batteries so the charging rate isn't choked off by the existing wiring between the battery and the power center.  This will also give you the ability to manually boost the charging voltage to 14.1 volts for a fast charge when you're using a generator.

The difference between the 9100 and 9200 is the 9200 has the Charge Wizard circuitry built into the converter with the option of adding an external pendant to manually select the operating modes while the 9100 has the Charge Wizard circuitry in the external pendant.  Six of one a half dozen of the other.
 
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