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Battery readings should be, what?


Steven P
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Last year, I replaced the PO's 2 12V (bad) deep cycle marine batteries w 4: new, 6V Trojan t105 AGMS.   They have been great compared the the bad 12Vs.  We don't boondock much at all and have been living in our coach during a house remodel.  My question is, what should the batteries read when I unhook feom shore power and no genny running,, no engine running, and they are fully charged.  I notice they read  from 12.3 to 12.6 once unhooked with only the res fridge and a few lights on.  From reading different posts, I feel this should be above 13V.  It float charges at 13.6V. And I feel I set up the Magnum remote properly when I did this last year.   All these readings are from the Magnum inverter remote.   Thank you. 

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"A few lights" are pretty insignificant, but if the res fridge compressor is running it will draw 10 - 20A from your Trojans. 

Chemically, a flooded lead acid happy place is 2.05V/cell, or 12.6V.  Float charging is good at 2.25-2.27V/cell, or 13.5V.

A good idea is to compare voltage readings at the battery terminals with a good voltmeter (not Harbor Freight) with the Magnum readings.  Take note and adjust for  discrepancies. 

Wikipedia and batteryuniversity.com give you more reading. 

- bob

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Duplicated your conditions but with 15A  of inverting. Voltage dropped from 13.6 to 12.4 once I turned the solar off and 12.6 once I turned the inverter off. My 700 AH of AGMs are probably down to 60% of their capacity and will replace with Lithiums assuming the cells I ordered show up… have cleared US Customs.

Would say your readings are normal.

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By tightening ever 12V cable connection I could find and cleaning a couple that looked suspicious, I decrease the difference in battery voltage and display  by almost 1 V (1.2 to only 0.4 with 100A load and 0.2 difference at 40A load). Without a load there isn’t any difference in the display and at the batteries.

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The most accurate way to to read battery voltage is at the battery with all loads disconnected and about an hour after being fully charge and the charger turned off. Lead acid batteries will hold what's called a surface charge for short time after charging and this is not accurate. A fully charged 12v or 2 6v batteries (flooded lead acid) in good condition should be at 12.6 or above. Battery voltage can tell you the state of charge but it can also indicate the condition of the battery. If you know the battery has charged for a sufficient amount of time to be full and sat long enough to remove the surface charge and your readings are much below 12.6 volts then your battery is in poor condition and probably nearing the end of its life. Here is a state of charge chart for flooded lead acid batteries:

324453609_Batteryvoltage.jpeg.f98cb4395e567af0d0d8192d6ccefcc9.jpeg324453609_Batteryvoltage.jpeg.f98cb4395e567af0d0d8192d6ccefcc9.jpeg324453609_Batteryvoltage.jpeg.f98cb4395e567af0d0d8192d6ccefcc9.jpeg

Also, when using a voltmeter to read state of charge you'll need to wait a bit after any large discharge for the voltage to normalize. I don't like to take my batteries below 50% but when I use the inverter all night with a heavy load they will get down to 11.9 volts. If I turn the inverter off for a half hour the voltage will come back to 12.1 which is the more accurate reading.

I hope this is helpful.

Sorry I somehow enter the chart 3 times.

Edited by Tom Wallis
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The most accurate way to measure state of charge is by using a shunt. If using voltage you have to remove all load and let the battery rest for up to 30 min. Read about a Victron BMV 712.

Tim 

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The OP has AGM batteries that complicates things…you really need to find a SOC chart for your particular AGM batteries, not a generic one and definitely not a lead acid flooded although adding 0.25V gets you close.

I like the below chart as it a range of voltages for each SOC acknowledging different AGM batteries have slightly different voltages. Downside it this chart is the voltage is after resting 12 hours further complicating things.

*I did find a chart for the OPs Trogen AGM batteries… 2nd chart.

0048F3F9-3EE8-442B-9D5B-AC8B09B2B202.png

8218A4D4-A7B5-46E1-8557-8B3FD5E6D8B4.png

Edited by Ivylog
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Thanks all.  This is all good info and I'll look into everything.  I'm hopingbto have some time over the weekend for testing if the rain goes away here.  I do have a Klein multimeter and the batteries and their connections all look good, but an unhooking and cleaning won't hurt.  

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Depending on what coach you have and what electronic devices you have in your coach, there are electronic devices such as flat screen TV's and other devices that you may thing you have turned off but are not.  Many electric devices, especially in newer coaches when turned off still draw current, although minor, unless completely unplugged from their power source.  Those are called "parasitic current draws".  Any kind of battery memory and other devices will do the same.  When boon docking, all our TV's and associated amplifiers and satellite receivers are plugged into power strips so they can be completely isolated from any power source and will not draw power when not needed.  So just turning off TV's and the lights will not stop parasitic draws.   I switched from typical acid type batteries to all AGM.  My inverter could work with AGM batteries and they hold their charge longer and no topping off because they are called "maintenance free".  All batteries require some kind of maintenance but not having to worry about battery acid works for us.  Stay safe, Stay well and Happy Holidays.

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I put in a dual induction cooktop and was surprised how much power it draws waiting to do something…added a power strip to take it offline. Instead of multiple power strips I use the bottom circuit breaker in the inverter box to shut off everything at night. Does require using a 12V outlet to charge the phones.

Agree AGMs are worth the extra $$… been using for 15+ years BUT the price of Lithium cells have come down to where I’m building 600AH for $600. This puts them in the AGM price range plus they should give 3X the cycles.

Edited by Ivylog
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