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Old 01-03-2009, 10:59 AM   #1
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I'll be doing some dry camping using the generator during the day as needed, but at night will be running off the coach batteries.How low can I let the voltage get before I will damage them? I have a Sears digital multimeter that I think is petty accurate and have made up a plug that I can put into the dash 12v outlet ( runs off the coach batteries ) to get real time voltage reading.
Thanks for your help.
Greg
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Old 01-03-2009, 10:59 AM   #2
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I'll be doing some dry camping using the generator during the day as needed, but at night will be running off the coach batteries.How low can I let the voltage get before I will damage them? I have a Sears digital multimeter that I think is petty accurate and have made up a plug that I can put into the dash 12v outlet ( runs off the coach batteries ) to get real time voltage reading.
Thanks for your help.
Greg
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Old 01-03-2009, 11:21 AM   #3
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Voltage State of Charge
12.6+ 100%
12.5 90%
12.42 80%
12.32 70%
12.20 60%
12.06 50%
11.9 40%
11.75 30%
11.58 20%
11.31 10%
10.5 0%

Avoid discharging the battery below the 40% level whenever possible!
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Old 01-03-2009, 11:39 AM   #4
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">I have a Sears digital multimeter that I think is petty accurate and have made up a plug that I can put into the dash 12v outlet ( runs off the coach batteries ) to get real time voltage reading. </div></BLOCKQUOTE>

Greg, are you sure your dash 12v outlet runs off the coach batteies? I always assumed that mine worked off the chassis battery, but I never actually checked it out.

Ron
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Old 01-03-2009, 01:25 PM   #5
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Dave: thanks for the chart, I'll print it out.


Ron: I was kind of surprised too that the "cigarette lighter plug" ran off the coach batteries but when I turn off the 12v. kill switch by the door, the plug is dead.
This is on an '03 Sightseer 30-B


We plan to go down to Everglades National Park to watch the Super Bowl and spend the night. All we will need to power is a 19" tube type TV and the satellite receiver after the 8:00 PM no generator quiet time.

Greg
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Old 01-03-2009, 01:35 PM   #6
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note that measurements of voltage (or even specific gravity) are for a resting battery and the charts make assumptions about temperature and other things.

Things such as cycle to cycle variance, temperature, age, and use profile can each make a 10% or greater impact on state of charge calculations. You fool yourself if you think you can get a really accurate measure of what's left in your battery.

For RV use, a good rule of thumb is to try to keep the battery above 12.0v as measured after it has been sitting without any significant charging or loads on it for a half hour or more.
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Old 01-03-2009, 03:57 PM   #7
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The best way is to try to calculate your hourly use in amps. If you have an ammeter you can measure it. The easiest way if you don't have a shunt in your battery lead, and you probably don't, is to use the clamp on type available at Sears for $60. If you have 1 12 vdc deep cycle battery, you can probably go 4 to 6 hrs for your tv satellite receiver and a light or two. If you have two batteries, twice as long. Lights will draw the most current. Try to use less than half the battery capacity, which is probably about 225 amp/hrs.
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Old 01-04-2009, 01:36 PM   #8
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Dave from MN ...

Thanks for your table ... it was immediately useful to me.

I had just purchased new coach batteries ... they would not charge above 11.6 volts after about 2 weeks.

I took them back and asked them to check them out ... sure enough two of them were "marginal" (their words ... "defective" my words) ... after a short discussion they decided to replace all three batteries ... now they charge to about 12.8 or 12.9

At first I was thinking there was something wrong with my inverter / charger ... but after driving for 6 hours and seeing the alternator charging them at 13+ volts I knew there was something wrong with the batteries themselves ...

You helped solve my problem ... and I got a good nights sleep last night ...

Thanks
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Old 01-04-2009, 01:42 PM   #9
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Dave from MN- That chart is great and very useful. Thanks for the taking the time to post it.
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Old 01-04-2009, 01:50 PM   #10
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Just to make sure everyone is informed, I strongly suggest you follow this LINK and understand the situations involved with our batteries and 12 volt systems.
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Old 01-05-2009, 01:12 PM   #11
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I agree with John Healley that the only accurate way to measure battery usage is measuring amp hours. This can be done with a amp hour meter using a shunt. If you do a lot of dry camping I recommend installing one. I have a Xantrex and use it not only for amp hours but for all battery information.

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Old 01-05-2009, 05:29 PM   #12
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while Dry camping and running lets say the coffe maker in the morning, I see the voltage drop to lets say 10.2 or lower during the coffee brewing then return to a higher setting.
Is the voltage in the chart a no load voltage or a loaded voltage? I think I know the answer but just to fine tune the discussion.
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Old 01-06-2009, 12:10 AM   #13
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The battery or bank will be a rest (no load) and surface charge has been removed.
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Old 01-06-2009, 09:25 AM   #14
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Okay if measuring amp hour usage is the best way to determine battery condition why is there no cumulative way to keep a running total of draw down for the batteries?
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Old 01-06-2009, 10:45 AM   #15
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As charles tuit mentioned in his post above they are available.
See here for one.

If you Google "amp hour meter" you will see a number of them.
Not real cheap though which is probably why you don't see them used a lot by the ordinary RVer.
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Old 01-07-2009, 02:04 PM   #16
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Seems to me we are looking for absolute accuracy when approximations are adequate. AMP-hour meters have got to be pricy!!

I have learned to look at battery power as a certain number of hours of useage by the most power hungry loads. IOW, 2-coffees, 4-hours-TV, and 1-overnight heat with outdoor temp ~45.

YMMV. Run some loads as normal and check the battery voltage afterwards to see how much is left.

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Old 01-08-2009, 11:52 AM   #17
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Okay if measuring amp hour usage is the best way to determine battery condition why is there no cumulative way to keep a running total of draw down for the batteries? </div></BLOCKQUOTE>
counting electrons in and out is not a good way to determine battery condition much less a best way.

The electron counters hide the assumptions. Yes you can get a fairly accurate measure of the energy put into and the energy taken out of a battery by measuring current and voltage over time. The meters that do this are getting somewhat smarter so they can also calculate the contributions of use profile (i.e. Peukert considerations) and, sometimes, even temperature.

But they all need to know the energy capacity of the battery and that is not something you can know with precision. And they cannot predict the future, which is what you really want to know.

Having an expensive meter can be a comfort but it may not be a solution.

You can determine battery status. The smartgauge does this by evaluating battery behavior. Conductance testers can also be used. Battery status is only useful when considered with battery capacity and that is meaningful only when future use is estimated.
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