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Old 02-22-2019, 12:34 PM   #1
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How many are de-rating their Battery Monitor as the batteries age?

Curious if anyone with a battery monitor is de-rating their battery bank capacity as their batteries age. For example a new battery bank with 800 AH capacity will probably be down to 700 AH capacity after a few years. If the battery monitors capacity is left at 800 AH year after year then using 400 AH would show a 50% SOC when it actually is at 43% SOC.
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Old 02-22-2019, 05:37 PM   #2
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It would be more important to know of the resting or idling voltage changed over time at a particular discharge amount. E.G. if 200 AH used to bring the batteries to 12.2 but now it’s 12.0 then you’re losing capacity. At least that’s how I monitor battery performance.
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Old 02-22-2019, 10:36 PM   #3
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Don - Valid question!! And I suspect that many are not doing so, myself included on my last battery bank.

I also suspect that many, like myself on my last battery bank, are not doing yearly testing to check on battery performance. (Lifeline has documentation on checking this yearly, and documenting the health of the batteries in question.).

I would think that the 'Yearly testing.' of battery bank health. Would go hand in hand on the determination of % of total 'Current Health' of a battery bank's AH's capacity(?).

Suspect this is often ignored by owner's, as I had, in their yearly maintenance of many RV items...

Will follow this thread, and see what other's share about battery bank sizing, as well as annual battery testing...

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Old 02-23-2019, 08:36 AM   #4
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Very good point, thought my four 4 year old 8d''s were still producing 1000 a/h's. Moved to lithium and load tested each 8d individually, one dropped 1 1/2 volts lower than the others, so I am guessing I had a lot less than 1000 a/h capacity.
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Old 02-23-2019, 01:52 PM   #5
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Along the same lines if one was not inclined to do an actual capacity test each year then estimating the capacity loss a little higher than it actually is would be beneficial. For example if a 4 year old 800 AH battery bank was estimated to now be capable of delivering 600 AH but was actually capable of 700 AH, then at an indicated 50% SOC the actual SOC would be 57.1%.

Better to err on the high side of decreased capacity than on the low side or worse yet not at all.

I suspect that the battery capacity shown (presently 303AH) for each of my LifeBlue batteries will slowly show a decrease as I get a few hundred cycles on them. Time will tell.
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Old 02-26-2019, 11:58 AM   #6
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Excellent point.

I try to discuss the importance of looking at both voltage and capacity with customers.

The point I try to make with them is that all of the electrical guages are "indicators of what is going on".

It isn't what they really want to talk about.
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Old 03-30-2019, 06:46 AM   #7
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Originally Posted by 757driver View Post
...Better to err on the high side of decreased capacity than on the low side or worse yet not at all...
And risk over charging the batteries?

To the OP:
I wouldn't think this is necessary because your BMK and your selected battery profile for your solar charger should make accommodations based on the BMKs input.
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Old 03-30-2019, 01:03 PM   #8
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And risk over charging the batteries?

To the OP:
I wouldn't think this is necessary because your BMK and your selected battery profile for your solar charger should make accommodations based on the BMKs input.
If ones estimation was way off overcharging could potentially be an issue but far more RV batteries are killed due to undercharging than overcharging.

Any battery monitor that I am familiar with does not know the size of the battery bank it has to be told what it is. Most are capable of (either automatically or user inputs) knowing when the battery bank is at 100% SOC but they do not know how many AH are actually available from that 100% SOC as the batteries age and capacity decreases.

My battery monitor is a stand alone unit that reports SOC etc but it has nothing to do with any charge source solar or otherwise. A proper battery charge profile (solar or shore power) is paramount to proper charging of a battery bank to 100% but it knows nothing of a decreasing battery bank capacity.
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Old 03-30-2019, 02:19 PM   #9
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Aging battery capacity is probably not a major factor is calculation errors for State Of Charge (SOC) percentage.

Simple battery bank SOC monitoring devices don't even have the capability to measure current and don't care what the battery bank capacity is. Stable unloaded and not-on-charge voltage is a good indicator of the battery percent of charge regardless of the underlying battery bank size or stage of life up to a point. The point when self-discharge becomes large screws up voltage SOC, but the battery at that point is a total loss or will be shortly.

The addition of current monitoring to a SOC device adds to the information available to monitor SOC. How that extra information is used depends on what the microprocessor programmer does with it. I have one that measures current out and does nothing with it. It just displays the total watts withdrawn.

A lot more can be done. But simply relying on the current in and current out to report state of charge is going to go wrong sooner or later. The battery monitoring system must ultimately use voltage and other ques to determine status. There are too many unknown variables affecting current totals to make it simple.

What can current tell the BMS to help it determine SOC? Knowing current can tell the microprocessor if the voltage reading is being charged or discharged. When no current is flowing and significant time has passed, it can know by reading the voltage what the SOC is and reset to compensate for variations in the unknow current variables.

Knowing nominal battery bank capacity and other battery bank parameters can help it decide maximum current charge and discharge. It can estimate SOS from current in and current out and apply "fudge" factors, but it must ultimately return to stable voltage to keep from loosing track of where it is.

An intelligent BMS can learn from previous charge discharge cycles and calculate some of the otherwise unknow variables.

Ultimately, the battery needs to be fully 100% no fudge charged. Battery needs this for healthy long life and the BMS can use this to reset is current total accumulations. Stable no or low current for a significant period of time and at specific voltage levels can be used to reset the 100% point.

I am not a BMS design engineer. I am sure there is much more that can be done with the data. I am also sure many BMS's do very little with the data. Ultimately it must come back to stable no-discharge and no-charge voltage measurement.
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Old 03-30-2019, 03:27 PM   #10
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I have a Magnum BMK. It auto adjusts based on the in/out amps every time it reaches 100%.
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Old 03-30-2019, 04:42 PM   #11
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Quote:
Originally Posted by Persistent View Post
Aging battery capacity is probably not a major factor is calculation errors for State Of Charge (SOC) percentage.

Simple battery bank SOC monitoring devices don't even have the capability to measure current and don't care what the battery bank capacity is. Stable unloaded and not-on-charge voltage is a good indicator of the battery percent of charge regardless of the underlying battery bank size or stage of life up to a point. The point when self-discharge becomes large screws up voltage SOC, but the battery at that point is a total loss or will be shortly.

The addition of current monitoring to a SOC device adds to the information available to monitor SOC. How that extra information is used depends on what the microprocessor programmer does with it. I have one that measures current out and does nothing with it. It just displays the total watts withdrawn.

A lot more can be done. But simply relying on the current in and current out to report state of charge is going to go wrong sooner or later. The battery monitoring system must ultimately use voltage and other ques to determine status. There are too many unknown variables affecting current totals to make it simple.

What can current tell the BMS to help it determine SOC? Knowing current can tell the microprocessor if the voltage reading is being charged or discharged. When no current is flowing and significant time has passed, it can know by reading the voltage what the SOC is and reset to compensate for variations in the unknow current variables.

Knowing nominal battery bank capacity and other battery bank parameters can help it decide maximum current charge and discharge. It can estimate SOS from current in and current out and apply "fudge" factors, but it must ultimately return to stable voltage to keep from loosing track of where it is.

An intelligent BMS can learn from previous charge discharge cycles and calculate some of the otherwise unknow variables.

Ultimately, the battery needs to be fully 100% no fudge charged. Battery needs this for healthy long life and the BMS can use this to reset is current total accumulations. Stable no or low current for a significant period of time and at specific voltage levels can be used to reset the 100% point.

I am not a BMS design engineer. I am sure there is much more that can be done with the data. I am also sure many BMS's do very little with the data. Ultimately it must come back to stable no-discharge and no-charge voltage measurement.
I disagree. As batteries age and loose capacity it is very important in calculating SOC. If a 5 year old 800 AH (when new) battery bank is now only capable of supplying 500 AH a reading of 50% SOC on the battery monitor will actually be 20% SOC. A very significant difference.

Using an at rest voltage to determine SOC is not practical while an RV is in use. I do not know of anyone that shuts down their 12v system and disconnects the battery lead and leave it sit for a few hours in order to determine SOC with voltage.

Here is a good white paper on programming a battery monitor.

https://marinehowto.com/programming-a-battery-monitor/
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Old 03-30-2019, 04:45 PM   #12
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I have a Magnum BMK. It auto adjusts based on the in/out amps every time it reaches 100%.
I do not have a Magnum BMK but from what I have read in their manual you still have to tell the BMK the size in AH of your battery bank. I think the auto adjusting you are referring to is the charge efficiency setting.
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Old 03-31-2019, 10:57 AM   #13
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I disagree. As batteries age and loose capacity it is very important in calculating SOC. If a 5 year old 800 AH (when new) battery bank is now only capable of supplying 500 AH a reading of 50% SOC on the battery monitor will actually be 20% SOC. A very significant difference.

Using an at rest voltage to determine SOC is not practical while an RV is in use. I do not know of anyone that shuts down their 12v system and disconnects the battery lead and leave it sit for a few hours in order to determine SOC with voltage.

Here is a good white paper on programming a battery monitor.

https://marinehowto.com/programming-a-battery-monitor/
I started reading the above article. The writer seems accurate and his writing is good. His technical knowledge is better than mine. I agree with his assessment of the value of measuring current in and out and the accuracy of calculating state of charge from the current measurements.

He and I agree it is good information to have initially. It helps to understand what the appliances consume in a general way. It is a great help in learning what your solar is producing and how weather and nearby forest cover is affecting output. It is overly complicated to use it to calculate with high precision what the SOC is because precision SOC does not matter for RV and TT purposes.

It does not matter whether the SOC is 40% or 50% when your generator starts. What matters is that you periodically charge for more than 16 hours to restore the plate chemistry. If your battery bank capacity is 100% of spec or 85% of spec matters a little, but it doesn't matter much until it drops below 80% because that indicates failure is near.

Measuring the battery voltage with no current in or out is all that is needed to determine SOC. When you have to cut a planned 5 day dry camp short, you need the current measuring to figure out why and if you need new batteries or not.

Relax and enjoy! Don't obsess.
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Old 03-31-2019, 03:29 PM   #14
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I started reading the above article. The writer seems accurate and his writing is good. His technical knowledge is better than mine. I agree with his assessment of the value of measuring current in and out and the accuracy of calculating state of charge from the current measurements.

He and I agree it is good information to have initially. It helps to understand what the appliances consume in a general way. It is a great help in learning what your solar is producing and how weather and nearby forest cover is affecting output. It is overly complicated to use it to calculate with high precision what the SOC is because precision SOC does not matter for RV and TT purposes.

It does not matter whether the SOC is 40% or 50% when your generator starts. What matters is that you periodically charge for more than 16 hours to restore the plate chemistry. If your battery bank capacity is 100% of spec or 85% of spec matters a little, but it doesn't matter much until it drops below 80% because that indicates failure is near.

Measuring the battery voltage with no current in or out is all that is needed to determine SOC. When you have to cut a planned 5 day dry camp short, you need the current measuring to figure out why and if you need new batteries or not.

Relax and enjoy! Don't obsess.
Obsess, never!! I think that if someone is going to go the trouble and expense of adding a battery monitor it might as well be as accurate as reasonably possible so that they can make more informed decisions about their batteries.
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