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Old 11-09-2018, 02:37 AM   #1
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Default Some battery questions

Ok. These are going to seem obvious to most of you but they are things that I have wondered about for a year or so. My situation is a single 12v AGM cabin battery that I supplement with a 100w folding solar.

1. Don't go below 50% on the battery to not damage and extend life. How do you determine 50%?

2. What do you do if your battery is hitting 50% in, say, the middle of the night? Shut everything down?

3. My solar controller monitors battery voltage but it's only active when I connect up solar. Should I have a volt meter? Suggestions?

4. When I put the RV in storage, I disconnect the ground to the cabin battery. Then as I'm prepping for a trip, I plug the RV into shore power at my home. This should charge the battery, right? I have a spare battery tender. Would it be better to use that to get the battery up to full charge or use the convertor/charger that's built into the van?

Thanks!

Thanks!
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Old 11-09-2018, 02:48 AM   #2
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Ok. These are going to seem obvious to most of you but they are things that I have wondered about for a year or so. My situation is a single 12v AGM cabin battery that I supplement with a 100w folding solar.

1. Don't go below 50% on the battery to not damage and extend life. How do you determine 50%?

2. What do you do if your battery is hitting 50% in, say, the middle of the night? Shut everything down?

3. My solar controller monitors battery voltage but it's only active when I connect up solar. Should I have a volt meter? Suggestions?

4. When I put the RV in storage, I disconnect the ground to the cabin battery. Then as I'm prepping for a trip, I plug the RV into shore power at my home. This should charge the battery, right? I have a spare battery tender. Would it be better to use that to get the battery up to full charge or use the convertor/charger that's built into the van?

Thanks!

Thanks!

You need to get a battery monitor like a Trimetric if you want to actually know for sure what your state of charge is all the time. Well worth the cost and effort to install IMO.


You may also want to search here and find some the discussions about the 50% rule. I think you will find that going under 50% once in a while would not appreciably change the life of you battery. You are much, much, more likely to shorten you battery life because of poor charging that gives either over or under charging. A battery monitor will very quickly show you how good, or bad, your charging systems are working to take good care of your battery.
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Old 11-09-2018, 02:03 PM   #3
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Booster has been doing his best over the years to get us to think of battery output in terms of ENERGY and to stop worrying about CYCLES.

Take a look at the scary cycle life chart on this 6V AGM battery data sheet: https://www.amstron.com/content/AP-G...tion_Sheet.pdf

read between the lines.JPG

If you read between the lines you see that the energy that you get out of the battery at the different levels of discharge isn't scary at all!

When I read between the lines I see:

2 x GC2 210Ah 6V batteries (210Ah @ 12V)
500 cycles at 100% DOD = 12v X 210Ah (100% DOD) X 500 = 1,260 kWh total output until replacement
675 cycles at 80% DOD = 12v X 168Ah (80% DOD) X 675 = 1,360 kWh total output until replacement
1100 cycles at 50% DOD = 12v X 105Ah (50% DOD) X 1100 = 1,386 kWh total output until replacement

The difference between 50% and 80% DOD is so minor that it is not worth worrying about. Most people fail to make the connection that deeper discharges mean that you used more energy. Energy output is what you purchase a battery for.

If you have 10 candy bars (energy supply) and eat 1 per day (Depth of Discharge) you'll have candy bars (some energy) for 10 days. If you eat 2 candy bars per day (greater DOD, more energy consumed) your candy bar supply will only last 5 days.

Boosters suggestion of adding a monitor to know the state of charge is the way to go. It is really the only way to know that you've fully charged the battery. Getting the battery full is critical to lead acid battery life.

You can try to manage the battery by monitoring voltage. It is no where near as effective as using a SOC monitor. To check voltage you have to first turn off any loads like lights, furnace or fan etc.

Phillips State of Charge chart flooded and agm.JPG
interstate chart.JPG
Never go below 12.0V resting voltage could be one of your basic rules.
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Old 11-09-2018, 02:28 PM   #4
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...................

4. When I put the RV in storage, I disconnect the ground to the cabin battery. Then as I'm prepping for a trip, I plug the RV into shore power at my home. This should charge the battery, right? I have a spare battery tender. Would it be better to use that to get the battery up to full charge or use the convertor/charger that's built into the van? ............................
It's unlikely that the converter (unless newer 3-stage) or the trickle charger gets the battery full. The same applies to alternator charging particularly if a diode type isolator is in use.

It's quite possible that you are continually in partial state of charge cycling that results in shortened battery life.
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Old 11-09-2018, 02:55 PM   #5
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All that Marko said is right on, especially on the very high probability that you are essentially never getting your battery fully charged.
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Old 11-09-2018, 02:57 PM   #6
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Thanks booster and markopolo.

So I've seen voltage monitors for RV applications that range from $18 to $300. Is the difference quality/accuracy or the range of things that the monitor does?

Also, markopolo, you mention that it's possible to be in a constant state of charging with the onboard or trickle chargers. What is the solution?

Thanks.
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Old 11-09-2018, 03:03 PM   #7
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Thanks booster and markopolo.

So I've seen voltage monitors for RV applications that range from $18 to $300. Is the difference quality/accuracy or the range of things that the monitor does?

Also, markopolo, you mention that it's possible to be in a constant state of charging with the onboard or trickle chargers. What is the solution?

Thanks.

You don't want a voltage only gauge for battery monitoring. You need a true battery monitor that keeps track of the power you use, and the power you put back into the battery. This is one, but shop around.


https://www.amazon.com/TriMetric-TM-...monitor+system


You would need better charging equipment to do a better job of getting the battery fully charged.
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Old 11-09-2018, 03:22 PM   #8
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..................Also, markopolo, you mention that it's possible to be in a constant state of charging with the onboard or trickle chargers. What is the solution? ..................
I was trying to explain that you're likely always at a partial state of charge and cycling the battery in that state. You might get to 90% when plugging in before a trip. Then you use the battery and it gets down to 40%. Then your solar charges it back up to 80% and that night you take it down to 50% SOC. You might never be getting that battery full. Not getting the battery back to 100% SOC allows more lead sulfate to form on the battery plates. More lead sulfate = earlier loss of capacity & earlier battery failure.

https://www.practical-sailor.com/iss...s_11691-1.html

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A partial state of charge cycle test is testing in which the battery is discharged and partially recharged but does not return to 100 percent before being cycled again. This mimics what happens when we’re at anchor or when we store our boat on a mooring or in a marina with no electricity. In these cases, partial state of charge operation is a fact of life—a fact of life that can be murder to batteries.
Fortunately with RV's we're better able to care for the batteries because we have more opportunities to charge the batteries than boaters get.
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Old 11-09-2018, 04:15 PM   #9
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It's unlikely that the converter (unless newer 3-stage) or the trickle charger gets the battery full. The same applies to alternator charging particularly if a diode type isolator is in use.
I think modern vehicle computers examine battery voltage and adjust alternator output voltage to compensate for diode or other losses.
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Old 11-09-2018, 04:21 PM   #10
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I think modern vehicle computers examine battery voltage and adjust alternator output voltage to compensate for diode or other losses.

They will only do that if you move the voltage sensing wire to the coach batteries instead of the starting battery. There is also a risk to it in that the voltage the van and starting battery can see will be high enough to cause in some cases. Think and alternator stabilizing at 14.5 volts, plus add on a volt for isolator, and you would be putting 15.5 volts on the van electronics.



The moving the sense wire was pretty common a decade ago, a hangover from when alternators usually ran at about 13.8v in the further past.
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