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Old 07-21-2010, 03:12 PM   #1
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Safe Battery Drop Before Recharging?

I'm playing around with my recently installed Onan auto gen start. I have it set to start the generator when the battery amperage drops to 11.9 amps for 60 seconds. How much lower can I go on a consistant basis without damaging the batteries? I have four new Trojan 125's on my inverter and I'm trying to determine how long I can run my refrigerator before the generator whould kick on.

Expert opinions are welcome!
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Old 07-21-2010, 03:56 PM   #2
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Most articles I have read recommend no more that 50% discharge before recharging in order to not shorten battery life. 50% discharge corresponds to about 12.2 resting volts (no current draw). Since you have the refrigerator as a current draw, the voltage would be somewhat lower.
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Old 07-21-2010, 04:12 PM   #3
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Your generator user's guide will give you charge times vs. battery size and depth of discharge. Paz is correct that the rule of thumb is 50% discharge. This gives the best use from the batteries and still provide a fairly good charge cycle life span.
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Old 07-21-2010, 08:09 PM   #4
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Quote:
Your generator user's guide will give you charge times vs. battery size and depth of discharge.
Since an RV generator doesn't directly charge the batteries in most RVs, how could the generator manual give charging times? It doesn't know the size of the charger or the charging algorithm in use, so it has no way to estimate charge time. I just checked the manual for my Onan QD7500 and it says nothing at all about charge times.

Standalone generators with a 12v output circuit may have a charge time estimate, but that assumes that the genset's own charge function is in use. It would be incorrect if using the RV's own charger, as most people do.
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Old 07-21-2010, 08:47 PM   #5
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I'm certainly no expert on the topic but I did attend a seminar on batteries today at The Rally in Louisville... kind of sounds like one of those Holiday Inn Express commercials I guess.

Anyway, just to add further validation to the 50%/ 12.1 or 12.2volt input... that's what the guy was preaching today in the seminar. I think he would say the 11.9 was a bit lower than you'd want to go.
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Old 07-21-2010, 11:13 PM   #6
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Originally Posted by RV Roamer [Gary] View Post
Since an RV generator doesn't directly charge the batteries in most RVs, how could the generator manual give charging times? It doesn't know the size of the charger or the charging algorithm in use, so it has no way to estimate charge time. I just checked the manual for my Onan QD7500 and it says nothing at all about charge times.

Standalone generators with a 12v output circuit may have a charge time estimate, but that assumes that the genset's own charge function is in use. It would be incorrect if using the RV's own charger, as most people do.
Look here, it's under Managing Electrical Loads pages 6 and 7. Maybe you should be more thorough before you post.
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Old 07-22-2010, 01:21 AM   #7
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I have a Onan AGS (EC30) system and I have mine set to start at 11.8 volts and run 2 hours. I determined this to be a good compromise as I can also measure the amount of amp hours used from my batteries and this volttage was close to a 50% discharge under load. The load is the variable here (I used a 12 amp load to check this, which is average usage for me at night. My batteries are 4 years old and going strong using this setting. Two hours of generator run time gets me back to about an 80% charged state again. To get to a 100% recharged state would probably take about 6 hours (440 amp batteries and a 100 amp charger).
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Old 07-22-2010, 05:47 AM   #8
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I have a Onan AGS (EC30) system and I have mine set to start at 11.8 volts and run 2 hours. I determined this to be a good compromise as I can also measure the amount of amp hours used from my batteries and this volttage was close to a 50% discharge under load. The load is the variable here (I used a 12 amp load to check this, which is average usage for me at night. My batteries are 4 years old and going strong using this setting. Two hours of generator run time gets me back to about an 80% charged state again. To get to a 100% recharged state would probably take about 6 hours (440 amp batteries and a 100 amp charger).
Harry-
Thanks, I just installed the same system. Let me ask another question. If you're dry camping and run your batteries down to around 12 volts, then drive away in the morning, do you use your generator in addition to your alternator charging going down the road? Also, do you have an all electric refrigerator that you are using?
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Old 07-22-2010, 10:01 AM   #9
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Dick... I usually run the gen. in the AM (it will do so automatically anyway) to go through the bulk charge cycle, so as not to put the entire load for recharging the batteries on the alternator when driving later in the day.
This is just hearsay but makes some sense that many alternator failures are due to overload trying to keep up with both chassis and house battery demand.

I have the standard Norcold 1201 fridge but it runs off the inverter while on the road. It draws about 550 watts AC running off the inverter, so that is also part of the load on the alternator.
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Old 07-22-2010, 12:14 PM   #10
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Dick... I usually run the gen. in the AM (it will do so automatically anyway) to go through the bulk charge cycle, so as not to put the entire load for recharging the batteries on the alternator when driving later in the day.
This is just hearsay but makes some sense that many alternator failures are due to overload trying to keep up with both chassis and house battery demand.

I have the standard Norcold 1201 fridge but it runs off the inverter while on the road. It draws about 550 watts AC running off the inverter, so that is also part of the load on the alternator.
Thanks Harry, I have a full size GE all electric residential refrigerator and ice maker in my MH. I have an inverter bank consisting of 4 Trojan 125's, a seperate 2 battery 12V system for the coach 12V stuff, and 2 chassis batteries. After using the inverter bank for a peiod of time and also the 12V stuff, that would be a lot of load on the alternator.
I did some experimenting yesterday and after using the inverter for 9 hours, my voltage on the hand held remote read 12.3 volts when I turned it off and went back to charge from shore power. I believe this has satisfied my mind that the refrigerator is going to be alright and permit me to overnight without running the generator, except when we have the weather we're having now and need airconditioning.

Thanks also to all the other contributers and their advice!
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Old 07-23-2010, 08:25 AM   #11
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Tarheel,
What coach do you have? Mine has the residential Fridge and 8 house batteries. I keep my AGS set to start gen at 11.8 but I seem to only be able to get about 4 to 5 hours dry camp time before my gen has to start to recharge.
I've tried turning the fridge off to see how long it will stay cold so that I might could leave it off while we sleep so that the gen wouldn't have to start in the middle of the night but that doesn't appear to be a viable option since in one 3 hour test during the day at 70 degrees outside, the freezer temp went for -6 to 27.
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Old 07-23-2010, 10:23 AM   #12
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Tarheel,
What coach do you have? Mine has the residential Fridge and 8 house batteries. I keep my AGS set to start gen at 11.8 but I seem to only be able to get about 4 to 5 hours dry camp time before my gen has to start to recharge.
I've tried turning the fridge off to see how long it will stay cold so that I might could leave it off while we sleep so that the gen wouldn't have to start in the middle of the night but that doesn't appear to be a viable option since in one 3 hour test during the day at 70 degrees outside, the freezer temp went for -6 to 27.
Millsmax -
I have a 43' Gulfstream Tourmaster. I have 4 batteries connected to my inverter, 2 12V batteries for my 12V stuff, and 2 12V chassis batteries. I changed my inverter batteries to Trojan 125's and running the inverter in my driveway the last few days, I got up to 12 hours and the voltage was only down to 12.2 when I shut it down.

I'm pulling out in an hour and headed west, so everything will get a workout and I'll know for sure in a couple of days how everything works. I'll let you know hte results. I also have a basement freezer and it is going now also.

I have a full size GE home style all electric refrigerator. So I hope things go well, I would hate to run my generator constantly.

Dick
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