Charging House Batteries

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jagnweiner

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This is probably a stupid question, but electrical items are not my strong suit.  How do I know when my house batteries are fully charged?  And, correspondingly, how do I know when they are about out of juice? 

I have a Powerline EMS that shows DC volts on house or chassis batteries if I press a switch.  I also have a Xantrex ProSine 2.0 inverter/charger.  I tried looking in the manual for the ProSine, but could decipher anything for that.  Not sure if this has any relevance to the answer, but I believe the house batteries are four 6V golf cart batteries (can't tell for sure, because the slideout battery tray is stuck in).

Thanks in advance,
 

mike1245

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Scott
think about putting in an amp hour meter, this shows how many amp hours you have used and then how many you have put back in.  This lets you know at a glance how full or empty your batteries are.
Mike
 

Gary RV_Wizard

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The rough measure is battery voltage. If the battery shows 12.6v or more when at rest (charger not operating and no load), it is pretty much fully charged.  That's not foolproof because there are some conditions that can look ok on voltage but have very little amp-hours available, but that's generally not a problem in day-to-day use.

The remote for the ProSine may have an indicator to say it is in "float charge" mode; that indicates the charger thinks the battery is at or very near full charge.
 

jagnweiner

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Thanks, gentlemen.  One more question for Gary:  If 12.6V is fully charged under those conditions, what is the lowest I would want to allow it to discharge?
 

Just Lou

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Scott, somewhere around 12.2 is considered 50% discharged, and is a good low point threshold.  You will often see a lower voltage displayed when the batteries are under load, so allow them to recover (with no load) for a few minutes before making your measurement and decision.
 

Gary RV_Wizard

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The widely accepted value for 50% charge is 12.1v, but I would not be concerned until it dipped under 12.0v or maybe even a bit less.  That's because the voltage in the charge table is given for a battery at rest, i.e. under zero load.  That isn't what you see on a battery installed in an RV, even if you think "everything is off". Even the tiniest current output will reduce the voltage slightly, so you can typically assume the "resting" voltage would be a bit higher than what is on your meter.

A greater level of discharge would likely shorten the battery lifetime by some unknown and indeterminate amount. So would any of a dozen other things, so it is all but impossible to predict what the effect may be in the real world. Just don't make a habit of deep discharges and you should be fine.
 

Just Lou

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There I go again, showing my ignorance.  Everyone knows... "The widely accepted value for 50% charge is 12.1v", ....not "somewhere around 12.2v." ??? ::) 
 

jagnweiner

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Well, when I visited the RV today, I found both the house and the chassis at around 11.6. Chassis batt would crank the engine a bit, but not enough to start it. I got the genset started and let them charge a while. Then got the engine started and took it for a drive. I was finally able to get my battery trays pulled out to do a visual inspection.  House are almost 5 yrs old and chassis 4.  I thinking I'm going to replace all batteries before our planned trip out west next summer.

I'm thinking I'll replace the chassis now, but wait until after winter to do house so they don't just sit all winter. Does that make sense?
 

Just Lou

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I think your decision to replace them before you embark on a long summer trip is a wise one, but that's not an indictment of all five year old batteries. 
 

Gary RV_Wizard

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There I go again, showing my ignorance.

Lol, Lou. You are anything but ignorant, at least in electrical matters! But there is a widely accepted table of voltage vs State-of-charge and it has been posted here often and is also in the forum Library under "Battery Charging Basics".

http://www.rvforum.net/joomla/index.php?option=com_content&view=article&id=309:battery-charging-basics&catid=39:batteries-and-dc-12v-stuff&Itemid=45
 

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Just Lou

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Gary, I not only found it curious that you would deem it necessary to basically replicate my post, but also to start it with a correction of my ...."around 12.2v" ....to an ....exact 12.1v.
There are several more that DO state that 12.1v is the 50% level, but how many folks consider any of these charts as 'The-Last-Word' on the subject, when the charts state that 11.9v represents "full discharge'? ???
I will, however, 'fact check' any future answers more thoroughly.  It's a waste of everyone's time for me to provide answers that are so erroneous as to require corrections from the staff. 

 

jagnweiner

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Who'd have thought I'd start an argument?  At any rate, the tables are helpful and I now know for sure that <12=bad.
 

n7qvu

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Overgaard, AZ
The Trojan battery website has lots of great info about batteries. They go into painful details regarding charging cycles.

A lot of RVs use Trojan T105s, it applys here.

They state that 50% = 12.1v, and to get max life, don't go below 80% = 12.5v

  http://www.trojanbattery.com/ProductLiterature/GeneralProducts.aspx?tab=0#TabbedPanels1#top

Battery temperature is also a big issue. They love it at 80F, less life above 80F and less power below 80F.

I keep my Stick home solar batteries near 80F with a home made programmer thermo hooked up to 60 watt lites.
 
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