frank60
Maine
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I have Class B motorhome with one DCG27 90Ah battery that will be Discharged at 45-50AH over a 24hour period. My Converter/Charger is a Parallax 7345 45amp.
My Question Is: Realistically, What size (wattage) generator would I need to re-charge the battery in say 2-4 hours? and what Decibel Level would be appropriate with other campers around? Also, it should be around 50-60#s so I can lift it.
We have never dry camped before and we would like to try it at the state parks here in Maine (maybe 10-15 days a season). I would like to do this without spending alot of money. In my research I see some inexpensive 1200w 2cycle generators & the Sportsman 2000w generator with a 78dB output level which seems to get good reviews. Is 78dB too loud for a campground where you only can run it between certain hours?
Thankyou in advace
Frank
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naturist
Lynchburg, VA
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78 dB is NOT quiet in a campground. The much applauded Hondas run 53 to 58 dB. 78 is about what you get standing next to an idling Powerstroke, I think. With the hood open.
You have 3 requirements in quiet, cheap, and light. Pick one, can't have all three. Cheap will be noisy and heavy. Quiet will be expensive and heavy. Light is going to be both noisy and expensive.
But that's just my opinion.
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pulsar
Lewisville, NC
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Moved from Forum Technical Support,
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Normk
Canada's Wet Coast
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naturist refered to "Cheap, noisy and heavy" why did my ex-wife's face flash into memory....
Recharging is a limited by battery capacity and charging voltage. A high limit for charging a 12 volt battery is in the 14.8 volt range although one might wish to use significantly lower voltage for reasons of battery life and heating.
A "quick and dirty" means of fast charging the battery would be to apply a high charging voltage such that 40 amps or so of charging current is maintained which should recharge the battery at something less than 40 amps per hour. While this will drive a heavy charge into the battery in a short time, it will be far less than ideal. This might be a useful technique if you were operating a U-boat and having to recharge as quickly as possible before aircraft spot you but will horrify anyone who has experience with RV battery operation.
The next limitation, since you suggest a 120 volt AC generator, is the charging rate obtainable by your power converter. You can compare the charge profile of your power converter to battery capacity as easily as I can so will not explore specifics. Please pardon if you already know the information I am attempting to convey because it will be useful background if you do not.
If your power converter is able to provide a (pick a number...say 20 amps) 20 amp peak charging voltage to that battery then the power converter will require a commensurate power supply. Add inefficiency and other loads which will likely mean something in the 400 watt range, give or take. If the requirement is 400 watts, plugging into a 5,000 watt generator will offer not advantage over that of a 400 watt one.
How to charge 90 amp hour in 2 hours into a 90 amp hour battery? In short, forget it. It's not practical. This would mean driving 45 amps into a 90 amp hour battery which is dead and this isn't going to happen. A 90 amp hour battery is not a large sized unit and will be undersized for the recharge rate required for the time you allow.
A low charge state means low specific gravity and so high series resistance, meaning low charging current unless voltage is driven beyond what is reasonable. In the range of 1/2-3/4 charge range the battery may accept 45 amps without overheating but that still will not be good for battery life or those surrounding as it may be gassing in a quite foul manner.
We used to rapidly recharge batteries of this size range by monitoring closely for temperature while boosting voltage beyond what your converter will allow. They recharged very quickly but life was poor.
It may be more practical to use a quiet generator to provide power and recharging for a longer period rather than depending on cycling this battery to such an extent.
What would be the constraints on running the generator for two, four hour periods, for example? This would provide a more reasonable charging rate, break up the charging interval as well as reducing both charging required and discharge.
Hope this does not sound negative.....
On the other hand, someone may know this to be practical although my experience in attempting this kind of battery useage would not seem to support it. Married for 35 years so know how wrong I can be.
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smkettner
Southern California
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78  you may as well bring a pneumatic drill 
For best results I recommend a Honda or Yamaha 1000 and a 40 amp portable Black and Decker charger.
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BFL13
Victoria, BC
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The generator does not charge the battery, a battery charger (such as an RV's converter) does. The charger gets its power (120v) from the generator (or shore power) to make "12v" to charge the battery.
For fast charging you want that charger to do 14+ volts, but the converter you have only does 13.8v so it will take a long time to charge up the battery from being half way discharged. (half way is as low as you should go)
So with only one battery you want say a 25amp charger (the battery will only take so many amps at once) which means the gen will have to supply about 400w, so a small gen will be fine. But soon you will want to have a bigger battery bank and to be charging at 80amps. Now you need more of a generator.
The charging amps cut-off for a 1000w gen is around 45 amps, so charging above that will require a bigger gen, like the 2000w next size up. The usual advice is to get the 2000w up-front and not go through the step of getting the 1000w first and then later wish it were the 2000.
Our rig also has a 7300 converter , which we kept for when on shore power, but we charge the batteries with a fast charger --this one, but there are others --just not as good-- 
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skipnchar
Topeka or somewhere else
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The answer to your question should REALLY be more one of "what size battery charger do I plug into what generator to charge fully in about two hours. I doubt if you will find one that will FULLY charge your batteries that quickly but you SHOULD be able to bring them back to something like 90% in that time.
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frank60
Maine
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thanks for your input but i still have a couple of questions. My battery is located in a compartment Under our bed. To get to it we have to remove a bunch of stuff and frankly a pain.
Question: 1) when using a portable battery charger, do you need to remove/isolate the wires on the battery or can you just clamp the battery charger connectors and start charging? 2)is there a way to connect some sort of pig tail type arrangement to the battery terminals and feed the pig tail outside the MH to make it easier to connect the battery charger?
Should I be posting this message a different way?
Thanks
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steveh27
Grosse Pointe Woods, MI
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I use a portable to recharge my battery when at home. I just clamp on to the posts & do not remove any wires, but I also do not have anything on other than the continual alarm.
I found when boondocking, that running my van engine (class B) charges the battery through the alternator much faster than with my Honda 2000 through the RV converter. I just run the engine for 15 minutes and I'm good for quite some time.
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K Charles
Connecticut
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The only wire you need to find is the shore power and just plug that into your generator and let your converter charge the battery. 78 is loud
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