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Old 07-05-2009, 10:22 PM   #1
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how long Generator to charge batteries ?

hope this isn't an open ended question, but.....Basically...I have a 5K onan generator in my '08 Southwind M?H. I have 4 - 6 volt batteries.... how long should it take for the generator to charge the batteries, with a minimum load draw during the charging time (no a/c, micro, etc). OR, is it faster to start the M/H to charge the coach batteries?

seems that i can run the generator for 4 hrs before bed time, and barely get enough battery charge to keep the frig from flashing a "lo DC" code during the night, (again no furnace running, no a/c, etc) ???? I do have a 2500 watt inverter that stays on, but it is only hooked up to 1 TV, dvd player, & a tuner, and 2 oulets, which all the appliances where turned off.
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Old 07-06-2009, 01:20 AM   #2
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It is not possible to answer your question because there are too many variables in your question. Is the generator powering the converter which is then charging the batts? You need to know how many amps the converter will deliver to the batteies. You need to known how many amp hours each of your four batteries is. Trying to charge from the engine alternator is a losing proposition. You'll have to run the engine for something like 5-6 hours to get a meaningful charge.

Much of this is simple math. If you have four 100 amp/hr batteries, and you discharge them to 50% (which is all the lower you should take the batteries) you need to put back 1/2 of the original 400 amp/hrs, or 200 amp/hrs. You also need to factor in some losses because charging is never 100% efficient.

So, if you need to put back say, 215 amp/hrs. you divide the output of your converter into the 215 amp/hrs. need and you'll have a rough idea. Example: Assume you have 4 100 amp/hr. batts discharged 50%. Assuming some more, that you converter will put an honest 40 amps into the batteries, you'll need 5.375 hrs. to fully recharge the batts. The smarter way to do it is to work between a floor of 40% discharge, recharging to 90%. It takes a loooooong time to get the last 10% of charge. Be careful of those posts (if any) that claim to recharge in 1/2 and hour by using the truck engine at idle. That's sheer lunacy. The physics simply do not allow such a thing with stock alternators and typical batteries. Even if you installed a special, high-output alternator, you'd damage or even melt batteries trying to recharge that fast.

One legit way to decrease charge time is to replace your converter with a higher amp unit. If your generator is powerful enough to push a larger converter, you could consider repleacing your coach batteries with AGM batteries like a Lifeline. That type of battery can accept a charge at just about any rate your converter is likely to produce. The drawback, as usual, is cost.

I have had several different kinds of batteries, chargers and generators over the years. I have yet to see a "50% to full" charge in less that 3.5 hrs.
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Old 07-06-2009, 03:40 AM   #3
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ATVr has it right You drain a battery faster than You charge think this way watch TV with the antenna bouster and make a pot of coffee say TV for 4hours and a light is needed also and Your charge is also at say 10 amps I would bet You need 10 hours to fully charge back up after all think what You have drained out over the use time.A lower amp charge also is better for the batt. than a fast high rate of charge so or so most people claim I use LED lighting and recharge with a small Honda and a external batt.charger in day time watch the news at the same time and use the extra power the Honda has to offer for it is running any way...Bushman
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Old 07-06-2009, 07:12 AM   #4
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There is no magic formula as to how long to recharge a battery...depends on how low it was discharged and also the charge rate from the charger circuit.

You will recharge faster if you use a separate charger like on an auto battery that is plugged into a 120 volt source. The chargers from a generator have a pretty low charge rate.

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Old 07-06-2009, 07:36 AM   #5
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Have you checked the water level in your batteries? Could be the generator is doing its job but you may have a bad battery.
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Old 07-06-2009, 07:40 AM   #6
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Hi,5151.It is open end on how long it takes to recharge batterys.If you are dry camping one should watch high and low loads.( high loads) We run the gen. with inverter off to make cofe,run micro,hair dryer,and air cond.(low loads)Inverter off if not using it, power ant. off when not in use,even with this off there are a lot of small loads on.,clocks 12 v for LP,Smoke and LP,CO dectors.As stated above ,at 50% down it takes 4 to 5 hrs to full,and thes small loads can pull it down faster then you think because they are on 24-7.
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Old 07-06-2009, 08:10 AM   #7
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Much of this is simple math. If you have four 100 amp/hr batteries, and you discharge them to 50% (which is all the lower you should take the batteries) you need to put back 1/2 of the original 400 amp/hrs, or 200 amp/hrs. You also need to factor in some losses because charging is never 100% efficient.
The OP noted that he has four 6 volt batteries. Most 6 volt GC batteries are about 225 AH. When you total capacity for 6 volt batteries in a 12 volt system, since the batteries are in series/parallel the correct total capacity is one half the total battery capacity. Thus four 225 AH batteries give you 450 AH of use.
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Old 07-06-2009, 11:41 AM   #8
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Your question left out an important fact.

2 pair of six volt batteries is around 450 amp hours. if the batteries are half full that means you need to replace about 220-230 amp hours, plus 10% for loss so figure 250 amp hours

If your converter is a Progressive Dynamics 9260 that will take around 4 hours to get 'em up to 90 percent and another 4 hours to fill 'em up.. IF it's a pd 9280, it will take about 1 hour less, (3 to get 'em to 90 percent and 4 to fill 'em up.

NOTE: that "4 to fill 'em up" will never change

A 100 amp Xantrex Prosine 2.0 will get 'em to 90 % in about 2.4 hours. Then 4 to fill 'em up.

You left out the speed of the converter.. The generator needs to be big enough to power the converter (plus whatever else it needs) A traditional generator with a 1,000 watt rating WILL run a PD 9180 with charge wizard (or 9280) a 1,000 watt inverter generator will not (9260 yes, 9280 no)

You can gessustimate the generator needs from there.
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Old 07-06-2009, 12:06 PM   #9
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I posed a similar question to one of the Motorhome mag techs, plus some others. In a nutshell, here are the answers:
  1. Your genset is not really a battery charger; trickle at best.
  2. A battery charger plugged into any 120vac is better, even your own generator, but has some drawbacks; use in emergency.
  3. The engine alternator is far and away the best battery charger for all of the batteries in your coach.
Other more knowledgeable folks' answers, not just my opinions. The proof? As I see it, what else brings your dead car battery back as fast and safe as the alternator, once you jump it and get it started?

Hope this helped.

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Old 07-09-2009, 01:35 PM   #10
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Im no expert but doesnt it draw alot of power having an inverter turned on even if it isnt running anything? Our inverter turns on and off from the outside which is very inconvenient so we dont use it.
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Old 07-09-2009, 02:06 PM   #11
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Define "a lot of power." It is true that all inverters use at least some power when on, even if nothing is turned on. However, a lot of models have some form of "search" mode that reduces the current draw if there is no load. (you may have to enable this feature) Some send out a pulse to see if anything is on. My particular brand pulls back to a very strange looking, almost msw, type output. This reduces the current use while idling. When you turn something on, it returns to full sine-wave.

Some "search" ideas are better than others. Mine happens to be one of the more power hungry designs and will use around 8-10 watts or so. I've seen some that are as low as 3-4 watts, but I have never heard of or seen any inverter that doesn't use a little at idle. They have to. It is the only way they can know when you turn something on. 8 watts works out to less than 1 amp. A typical incandescent RV lightbulb is 1.3 amps.

Now, whether that is a "lot" of power is up to you to decide. In my use, there is something being drawn from the inverter nearly anytime in a 24 hr. period. I don't worry about wasted power at idle because it hardly ever is idling. On a hot day, I may be pulling 130 amps for the AC, in the evening I will have the TV on, the micro and such. At night it's the furball heating pad and the elec. blankets.
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Old 07-09-2009, 02:49 PM   #12
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Im no expert but doesnt it draw alot of power having an inverter turned on even if it isnt running anything? Our inverter turns on and off from the outside which is very inconvenient so we dont use it.
Depends a lot on the inverter but the "Idle" Current can be less than 1 amp (at 12 volts) per kilowatt, so no, it does not draw that much to have an inverter on.

And if you have a automatic inverter like a Xantrex prosine 2.0... IN the presence of shore or generator power it draws nothing.. It just turns itself off. (Well, it draws power to charge the batteries. but the inverter is off)
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Old 07-09-2009, 03:28 PM   #13
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hmmm
i have a few questions now
my inverter is a "AS NEEDED" if you leave it in auto and it detects a loss of shore or genny voltage (120) it will take up the slack, and i only recently discovered it is wired to run the fridge outlet, which will zapp the batteries quick,
we found out last weekend we were out visiting friends and power shut off due to a storm, so for about 30 minutes the coach was running on inverter only and my batteries got the 11 volt stage.

anywho....
when running down the road the engine alt only charges the chassis batteries, why i dont know, maybe its wired that way, i would think it should be set up to charge all the batteries when the main engine is engaged.???
any ideas
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Old 07-09-2009, 04:50 PM   #14
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hmmm
i have a few questions now
my inverter is a "AS NEEDED" if you leave it in auto and it detects a loss of shore or genny voltage (120) it will take up the slack, and i only recently discovered it is wired to run the fridge outlet, which will zapp the batteries quick,
we found out last weekend we were out visiting friends and power shut off due to a storm, so for about 30 minutes the coach was running on inverter only and my batteries got the 11 volt stage.

anywho....
when running down the road the engine alt only charges the chassis batteries, why i dont know, maybe its wired that way, i would think it should be set up to charge all the batteries when the main engine is engaged.???
any ideas
Your battery isolator solenoid has probably failed, a common problem.
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