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Old 07-10-2017, 10:57 AM   #1
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Battery usage and running generator

Well I've been camping with the wife for the last week in my 24 ft 2007 dynamax and the coach batteries have died twice. The genny would not start even withe the MC motor running. My question is, how long should I have to run the generator to charge the batteries sufficiently to last the night with minimal usage? I tried to use the MC engine (e450 ford v10) but it charges very slowly. A fellow RVER at the campground jumped my coach batteries enough to start the genny, I then let it run for 2 hours and I plan on running it again for at least 3 hrs tonight just befor bed. Last night I got back to the coach late and only ran the gen for about 15 min. For my set up I have two 6v batteries connected in series. Sorry if this is a new guys question but I'm still learning the ropes.
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Old 07-10-2017, 11:06 AM   #2
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Even running the generator it could take at least two hours of run time to get the house batteries to 80%. Running the main engine MAY allow starting the generator but it might not. Depends on how low the house battery is.
Usually not worth running the generator long enough to get the last 15-20% of charge as the charger SHOULD slow down it's charging rate.
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Old 07-10-2017, 11:07 AM   #3
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How old are the batteries? You should never let them get below 12.2V, that puts them at 50%. 2hrs run time may not be enough time to bring them back to life.
I have a RR frig and I can go about 10hrs before they get to 12.2v then I start my genset and it runs for about 2hrs, 4 (6v) batteries.
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Old 07-10-2017, 11:09 AM   #4
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The gen set powers your RV's inverter/converter/charger, and length of time will depend on it's output.
JHMO: For batteries that low I'm thinking 10 to 12 hours. Same for charging from the alternator 8+ hours of driving .

More info here.

www.marxrv.com/12volt/12volt.htm
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Old 07-10-2017, 11:24 AM   #5
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Typical usage pattern is to run the generator for a couple three hours in the morning while preparing breakfast, this restores from overnight usage. Then run the generator again for a couple three hours during early evening while preparing dinner. Another hour before quiet hours may be needed. YMMV, but this should get you in the ballpark. Adjust for your usage pattern.

Non-LED lights consume quite a bit of power. Biggest consumer if power is the fan in the furnace.
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Old 07-10-2017, 11:41 AM   #6
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Battery's were replaced on apr of 2016. I've been running the fantastic fan while we sleep. Would that kill the batteries
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Old 07-10-2017, 11:46 AM   #7
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Shouldn't. I do it all the time.
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Old 07-10-2017, 12:00 PM   #8
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It's always interesting to see all kinds of different advice about this. The charging characteristics of lead acid batteries are well known if one can define the exact system. Most of us cannot because we do not have the tools and instrumentation to do it. That leaves a lot of room for wide variations in practice.

What we can say is that virtually all systems work by feeding a constant voltage into the summing junction at the battery. Current from the charger is split between charging the batteries and feeding the loads on the system. That results in less than the converters full rated output actually charging the battery.

We also know that in general the charging rate should be in the range of C/5 up to 85% of capacity. For example if you have a 225 AH battery charge it @ 225/5 = 45 amps. If it is empty and you are going to 85% do it for 4.25 hour (225/45 = 5, 5 x0.85= 4.25). The problem is that you should not be at O% to start with. If you are around 50% which is the recommended point for Flooded Lead Acid cells then you need significantly less time but still want around 2 hours. I'll let you do the math if you are interested.

Over 85% if capacity the chemistry slows down and absorbs charge slower. That is where long charge times to get to 100% come in. The good news is that it usually does not matter if you don't get to 100% all the time. It also works out that if you have some solar or are driving that day those charging systems work well to top off the battery over several hours.

That also assumes you have that much power available at the summing junction.

All this leads up to do two things to make your life a lot easier. If you start the genset when you get up it will provide power for breakfast, electric hot water for cleanup and whatever else you need. Give the system 2 hours of charge time and shut down. A little more won't hurt. An hour or two after shutdown measure the battery voltage. That gives the surface charge time to dissipate. Here is a link to State of Charge tables:

https://www.google.com/search?q=12v+...2ABLKBvD8LU4M:

Your voltmeter is not the most accurate way to measure SOC but it is the easiest and good enough. If you are consistently running in the 70-80% range think about running a half hour longer. If your are running in the 90% and above range you can think about less time. If you keep a log of run times and readings you will be able to see patterns.

Repeat the process in the evening to get A/C or heat while doing dinner. A few days of running will let you see what to tweak to keep up in that 85-90% range at the end of the day.

Once you get comfortable you can stop the measurements if you want or install a permanent panel type voltmeter. Do that and you will develop the routine that fits your RV with your people doing things your way. The rest do not matter. They are probably wrong anyway. ;-)
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Old 07-10-2017, 12:09 PM   #9
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do u have any type of battery monitor to show state of charge?
when you changed the batteries, did you check your charge rate on your charger to make sure it is set to the proper rate and type of battery ?

Checked the distilled water level in them ?

if you've brought them down too far, they may be very sick :(
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Old 07-10-2017, 12:23 PM   #10
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Quote:
Originally Posted by JohnBoyToo View Post
do u have any type of battery monitor to show state of charge?
when you changed the batteries, did you check your charge rate on your charger to make sure it is set to the proper rate and type of battery ?

Checked the distilled water level in them ?

if you've brought them down too far, they may be very sick :(
I don't know how to check the charge rate except that the panal read 8.2 before I started the genny, then 12.2 after it was started. I checked the level of the water and the both looked good
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Old 07-10-2017, 12:39 PM   #11
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8.2 ?!?!

HMMM, maybe that's with a load on it...

but if that's the batteries charge now, that's not good- I don't let mine get below 12.2 - that's where I set my AGS to start...

I'm no expert, but I would run the genny all day to see if it will bump it up...

good luck
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Old 07-10-2017, 12:59 PM   #12
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Also a related question. I've heard that I should always put a load on the generator when running. Does this mean that I need to run the AC (ir other item) while running the generator just to charge the batteries, or is the charging creating enough load?
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Old 07-10-2017, 01:05 PM   #13
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The converter/charger in my TT will switch to bulk mode charging only when the battery voltage is < 12.4vdc. If greater, its in float mode and will take forever to reach 90%. Check the charge characteristics of whatever battery charger you have to make sure you start charging in bulk mode, or you may be wasting gen run time.
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Old 07-10-2017, 01:26 PM   #14
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It's not mandatory that the genny have a load to run per se...
just that to properly 'exercise' a genny, they would like for you to run it for an hour or so a month with say the ac on to 'load' it... and make it work... somewhat like my bicep muscles - I can move them, but unless I move some weights around they don't get enough work!

a question... what are you using when the generator is off, not high draw things I hope... like microwave, ac, electric appliances like heaters ???
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