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charging batteries with external charger

Patty and Ray

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Even after running my generator through the on-board converter, even for a couple of hours, the batteries never really get fully charged. I have recently been using a separate stand alone automatic 10 amp fast charger that will bring them up. The question is do I need to disconnect the batteries from the internal electrical system while using this outside charger? I have been but is it necessary to shut off the power inside the trailer - my wife hates this.

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Patty and Ray,


First let me welcome you to the forum. We're glad to have you here and hope you can both learn something and contribute.


Generally is is not a good idea to have two charging sources connected at the same time. This is especially true if either of them is a "smart" charger. In you case it probably won't hurt because neither of them likely are.


A better approach would be to consider replacing the crummy converter that many manufacturers install with a good charger. You don't mention how many batteries you have so that will make a difference on the size. Many folks are happy with one of the Progressive Dynamics 9200 series converters that include the Charge Wizzard. It turns a "dumb" charger into a very capable 4 stage charger that works very well. If by chance the converter you have is a 9100 series Progressive Dynamics, you can add the Charge Wizzard to it, but based on your description I suspect it may be another brand.

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Why not just disconnect the RV's charger from the 120 volt side? Much less aggravation for you and wife.


Rif's suggestion is a good one too.

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The only negative to using a battery charger with the RV loads connected would be if that charger has too high a finishing voltage. If the charger is a good one it should not be a problem. You could isolate the batteries to charge them but leave the RV loads connected to the OEM converter for power. For a long term answer, take the advice given by Rif.

Good travelin !...............Kirk

Full-time 11+ years...... Now seasonal travelers.
Kirk & Pam's Great RV Adventure



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...I have recently been using a separate stand alone automatic 10 amp fast charger that will bring them up...

To me this statement implies that the charger shuts down when the batteries are fully charged. I would think if you had the other charger running at the same time the automatic charger might sense the voltage from the other charger and possibly shut down or if it is a smart charger at least reduce its output thinking the battery is nearing full charge.


If your converter is one that can be run without a battery connected, just disconnect the battery and leave the converter on. Your wife will have AC and DC in the trailer if you are connected to shore power or plugged into the generator.

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Rif I like your answer. I've been considering doing this on my motorhome as I dont think the older system I have is as good as the newer ones with the charge wizard and I dont believe my batteries benefit fully from this old system . My problem is getting into it from a technical standpoint as the whole system is a package with converter, and inverter and transfer switch in one box and one wiring harness. The transfer switch and inverter work alright although the inverter is a modified sine wave older style.


To the op a question. Do you have a separate breaker that feeds your existing converter? I did on my fifth wheel and if so you could just turn off the breaker to it rather than power off to the whole RV.

<p>....JIM and LINDA......2001 American Eagle 40 '.towing a GMC Sierra 1500 4X4 with RZR in the rear. 1999 JEEP Cherokee that we tow as well.


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I would check and see what kind of voltage you are getting from you on-board converter/charger at the battery terminals. Even with the stock 30amp, it should be providing a better charge than a stand alone 10amp charger (or am I missing something?). It's possible that your on-board charger is defunct, or your internal wiring is inadequate to provide a sufficient charge.


If your on-board IS providing a charge, then you would want to check output voltage at the charger itself, and then at the battery terminals to ensure an adequate charge is actually reaching your batteries. If not, then no matter how good of a charger you might upgrade to would have it's legs cut out from under it if the wiring is unable to deliver the output.


How are you determining your batteries SOC? What is your normal SOC at the beginning of a charge cycle with your genset/converter/charger? What type and ah capacity are your batteries?

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Patty and Ray, welcome aboard and we will try our best to help. FWIW (Absolutely nothing) here are my thoughts:


"The question is do I need to disconnect the batteries from the internal electrical system while using this outside charger?"


The true answer depends on the internal charger and your external chargers design and quality and output waveform. However, that being said, I can venture a few educated guesses subject to some assumptions.


If the existing internal charger is an older unit, NOT any so called "Smart" 3 or 4 Stage Charger and depending on its size (20,30, 50 amps???) I'm not surprised it doesn't do a good job. Many of the older noisy buzzing heat producing units operate more like a voltage source instead of how a modern smart charger much better regulates. Therefore, it may take longer to charge plus once charged they may not reduce the amperage and could maintain higher battery voltage and potentially overcharge a battery. Some older cheaper units fail to do what a smart charger does by applying higher voltage and amperage initially to bring a battery up but then reduce charging eventually to a float level of around 13.2 volts to reduce overcharging AND EXCESSIVE OUTGASSING OR BOILING OVER.


Next, while there is some very slight risk to super sensitive 12 VDC electronic components (if used while battery is under charge) depending on a chargers design quality and output, generally speaking the battery, being a huge electron bank somewhat similar to a capacitor, acts as a buffer whereby loads are not subjected to any risk posed by a poor quality charger.


While there may not be any problem (due to reasons above), depending on the design and quality of two different chargers (your internal and external) I would not recommend BOTH be operated at the same time even if the risk is small or non existent.


THAT ALL BEING SAID Id consider upgrading to EITHER a so called "Smart" Four Stage (Bulk, Absorption, Float, Equalize) charger that can be used with your existing DC Distribution Panel (My buddy used a Progressive Dynamics in his existing distribution system) ORRRRRRRRRRRRR do as I did and purchase a Progressive Dynamics (or brand of your choice) 9200 series stand alone charger in which case I disabled the old existing "DUMB" charger (so it doesn't conflict with my smart charger) and left the DC distribution all intact. Or else go the combination Inverter/Charger route if you have need for an Inverter. Some of those are extreme high quality and design and even temperature compensated but are pricey and may be more then you need.


SUMMARY While it may not damage anything if two chargers are used at once, SUBJECT to the design and quality and waveform of the two chargers, I still wouldn't use both at once, even though theoretically the battery should act as a buffer and electron bank and capacitor and prevent any harm to LOADS. STILL if the two chargers are in conflict (depends on their design) one or both may not operate as designed for.


NOTE this all depends on your chargers and their design and quality and waveform none of which I have any specs or data, so the above may be way off base, I had to make a few assumptions and take a few educated guesses but just trying to help. It also depends on your battery or batteries and your loads, none of which I have data for SO NO WARRANTY


John T Tooooooooo long retired Electrical Engineer rusty as an old nail so don't bet the farm on the above lol

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I agree with what oldjohnt posted. The very first upgrade to our '93 motor home was a Progressive Dynamics 9200 series charger/converter replacement to the old buzz-kill OEM unit. Easy upgrade. Minimal connections (AC in and DC out). The biggest issue is sometimes just finding the old charger/converter unit. (Listen for the hum...). It is also a cheap upgrade. Here is one for $183:





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2 hours of charging on depleted batteries may not return them to full-charge, depending upon your charger output and battery condition.


2000 Winnebago Ultimate Freedom USQ40JD, ISC 8.3 Cummins 350, Spartan MM Chassis. USA IN 1SG retired;Good Sam Life member,FMCA ." And so, my fellow Americans: ask not what your country can do for you--ask what you can do for your country.  John F. Kennedy 20 Jan 1961


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