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Old 12-08-2018, 01:26 PM   #1
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How to Calculate Your Battery Capacity?

We all probably know that the battery capacity will drop over time. How do we know how much the capacity is at a given time?

I had thought about this, and you might have the same curiosity as well. If you have a battery monitor installed, I'm gonna show you a simple way to get it without going through a complete workbench...

At 4:00pm yesterday I had 100% SOC. Since then I had used it for cooking, TV, PC, and a space heater overnight. At 8:00am this morning, I had this on the monitor:

Electricity used: 380ah




SOC: 67.9%



380ah was what had been gone from the battery since it was full, 100 - 67.9 = 32.1 (%), hence 380ah represents 32.1% of my total battery capacity. To get the total battery capacity:

380 / 0.0321 = 1183.8 (ah)

Thus 1183.8ah is my battery capacity. Please be noted, this is a relative number, not very accurate, because the total ah of electricity is relevant to the charge voltage. My lithium cell manufacturers recommended charge at 14.6v, for speed and performance. I chose 14v as I believe the faster charge would shorten the life span of the cells (Technomania has a good write up on this https://www.technomadia.com/2015/02/...attery-update/).

Have fun everyone!
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Old 12-08-2018, 05:12 PM   #2
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Quote:
Originally Posted by CountryFit View Post
We all probably know that the battery capacity will drop over time. How do we know how much the capacity is at a given time?

I had thought about this, and you might have the same curiosity as well. If you have a battery monitor installed, I'm gonna show you a simple way to get it without going through a complete workbench...

At 4:00pm yesterday I had 100% SOC. Since then I had used it for cooking, TV, PC, and a space heater overnight. At 8:00am this morning, I had this on the monitor:

Electricity used: 380ah




SOC: 67.9%



380ah was what had been gone from the battery since it was full, 100 - 67.9 = 32.1 (%), hence 380ah represents 32.1% of my total battery capacity. To get the total battery capacity:

380 / 0.0321 = 1183.8 (ah)

Thus 1183.8ah is my battery capacity. Please be noted, this is a relative number, not very accurate, because the total ah of electricity is relevant to the charge voltage. My lithium cell manufacturers recommended charge at 14.6v, for speed and performance. I chose 14v as I believe the faster charge would shorten the life span of the cells (Technomania has a good write up on this https://www.technomadia.com/2015/02/...attery-update/).

Have fun everyone!
Thanks for the info. Aren't the actual usable amp hours much less than the full battery capacity? I think when you get down to 40% or so of full charge on flooded batteries you are below 12v and basically "dead" and in need of a charge. Lithium can drop lower but still, I don't think the calculated amp hours are all usable amp hours. So, in reality the usable capacity is usually much less than the amp hr capacity of the batteries. Right?
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Old 12-08-2018, 05:29 PM   #3
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This chart is for a particular battery, although all are about the same, but it shows how Depth Of Discharge ( DOD ) effects cycles.

If you have a 100 AH battery, the AH rating us based on a 80% discharge.

It doesn't " hurt " the battery to go down that low but it does give you less cycles.

The 50% DOD is basicly a point where cycle life is at an more economical point.

If you don't have the room for more batteries, run them lower if needed. Sometimes you can't just start charging when at 50% DOD.



Just don't expect the same life as the guy with a bigger battery bank.Click image for larger version

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Old 12-08-2018, 07:23 PM   #4
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If I’d found this post sooner I would have only posted the following here. Have not put a load on them yet but I hope to keep my discharges in the 30% range, not 50%... more cycles. I removed the 4 L16 lead/acid batteries in my Navigator and replaced with AGM...could not go the $$$ for Lion.
Because of where the pos/neg leads come into the top of the battery box I could only get in 10 of the dual purpose Odyssey PC2250 AGM batteries... bought 11. Instead of 500lbs of lead acid batteries (800AH) I now have 860lbs of AGMs... 1260AH. While each battery can handle being charged at 50A and 15.2V, this is not a requirement. Getting the last two batteries in took some doing between the 86 lb weight and the tight fit but D for DONE!

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Old 12-08-2018, 08:22 PM   #5
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Thanks for the info. Aren't the actual usable amp hours much less than the full battery capacity? I think when you get down to 40% or so of full charge on flooded batteries you are below 12v and basically "dead" and in need of a charge. Lithium can drop lower but still, I don't think the calculated amp hours are all usable amp hours. So, in reality the usable capacity is usually much less than the amp hr capacity of the batteries. Right?
You are correct - the usable capacity is usually much less than the amp hr capacity of the batteries. My calculated capacity is TOTAL CAPACITY.

Per the type of the battery, the recommended set point varies. In general, flooded battery is 50%, while lithium is 20%. However, just like twinboat stated, you may go further lower but the life span/cycles would be significantly affected.

Take lithium as an example, 2000 cycles is considered standard when max discharge is to 20% of total capacity each time. If one retains above 80% during the charge/discharge cycles, the cycle count could be reliably in 5000 - 6000 range. The rule is - the less discharge the longer the life span.
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Old 12-08-2018, 08:35 PM   #6
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If I’d found this post sooner I would have only posted the following here. Have not put a load on them yet but I hope to keep my discharges in the 30% range, not 50%... more cycles. I removed the 4 L16 lead/acid batteries in my Navigator and replaced with AGM...could not go the $$$ for Lion.
Because of where the pos/neg leads come into the top of the battery box I could only get in 10 of the dual purpose Odyssey PC2250 AGM batteries... bought 11. Instead of 500lbs of lead acid batteries (800AH) I now have 860lbs of AGMs... 1260AH. While each battery can handle being charged at 50A and 15.2V, this is not a requirement. Getting the last two batteries in took some doing between the 86 lb weight and the tight fit but D for DONE!
Nicely done . That would give you many more years of service when you keep it above 70% of SOC.

One small caution though - probably you have already known - fasten well the batteries toward the center of the coach. The centrifugal force could be huge when you are driving in high speed and making a sudden left turn. don't rely on the door to hold them.
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Old 12-08-2018, 09:20 PM   #7
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I think as far lithium (LFP) goes the difference is minimal when you start putting #s to it.

Going by the chart in previous post...
700 cycles at 80% dod or 80ah = 56,000ah
3,000 cycles at 20% dod or 20ah = 60,000ah
Not that big a difference it's just that you're using the battery faster.

If I go by what GBS claims for longevity in my lfp bank it looks like this.
(100ah battery)
2,000 cycles at 80% dod or 80ah = 160,000ah
3,000 cycles at 70% dod or 70ah = 210,000ah
So having 500ah bank (5 -100ah batteries) at 70% dod x 210,000 = 1,050,000ah.
We're averaging 30% dod or 150ah+/- daily.

Going a step further using recorded data since putting the bank in service 31+ months ago of full timing with 930+ days living off of the batteries.
365 days x 150ah = 54,750ah (average), that could be 19 years of use in a fulltime living setting. I'm sure it will age out or something before hand, only time will tell.
All in a 147# package.
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Old 12-08-2018, 11:18 PM   #8
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Itinerant - I have never used the accumulated ah's to project the longivity, but it makes sense. 19 years is a looooong time .
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Old 12-09-2018, 12:06 PM   #9
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Nicely done .
One small caution though - probably you have already known - fasten well the batteries toward the center of the coach. The centrifugal force could be huge when you are driving in high speed and making a sudden left turn. don't rely on the door to hold them.
Thanks, getting the last two batteries in was a chore as the bottom 3 are behind the metal lip at the bottom as are the 2 at the top with only 1/2” headroom. You can see some of the wooden blocks that I added to keep them from moving around. I had to cut some of the handles off to make them fit. Since taking the picture I’ve added some more for the top 2 batteries. Agree the door would not hold them in place.


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Old 12-10-2018, 06:21 PM   #10
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Just a word of caution. Here was our experience with overloading our compartment with AGM batteries. We fixed it and reinforced it and then all was fine but it was a learning experience.
http://walkaboutwithwheels.blogspot....appen.html?m=1
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Old 12-10-2018, 08:05 PM   #11
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Just a word of caution. Here was our experience with overloading our compartment with AGM batteries. We fixed it and reinforced it and then all was fine but it was a learning experience.
Walkabout With Wheels Blog: On the Moon, This Wouldn't Happen...
Good warning. I looked at your battery bay, it appeared no cross bars under the floor from the manufacturer. Some do some don't. Glad you had it beefed up.

Ivylog, check under your battery floor if you see cross bars. It's important. Yours weighs nearly 900 lbs (batteries, cables and miscellaneous stuff). That's a lot, got to be strong there.
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Old 12-10-2018, 09:05 PM   #12
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This chart is for a particular battery, although all are about the same, but it shows how Depth Of Discharge ( DOD ) effects cycles.

If you have a 100 AH battery, the AH rating us based on a 80% discharge.

It doesn't " hurt " the battery to go down that low but it does give you less cycles.

The 50% DOD is basicly a point where cycle life is at an more economical point.

If you don't have the room for more batteries, run them lower if needed. Sometimes you can't just start charging when at 50% DOD.



Just don't expect the same life as the guy with a bigger battery bank.Attachment 228444
Twinboat
I know you have made this point before but putting it with the life cycle chart finally brought if home for me.
I've seen these curves before but had the impression deeper DOD created a penalty when in reality when I run the #s the cum capacity of any batty /bank is relatively constant.
AH capacity x % DOD x expected # cycles is essentially the same for all cases.
I guess the only real loss is if one runs them beyond 80% DOD repeatedly and truly damages the batty.
I now have a different perspective
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Old 12-14-2018, 05:30 PM   #13
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Originally Posted by twinboat View Post
This chart is for a particular battery, although all are about the same, but it shows how Depth Of Discharge ( DOD ) effects cycles.

If you have a 100 AH battery, the AH rating us based on a 80% discharge.

It doesn't " hurt " the battery to go down that low but it does give you less cycles.

The 50% DOD is basicly a point where cycle life is at an more economical point.

If you don't have the room for more batteries, run them lower if needed. Sometimes you can't just start charging when at 50% DOD.



Just don't expect the same life as the guy with a bigger battery bank.Attachment 228444

I think we have to be careful here. The referenced curve indicates that a deeper discharge does no harm, above 20% at least. If this curve is for an LFP battery then it's fairly accurate. But, the curve for a lead acid battery should be very different. If you google "agm depth of discharge versus life" and select images, you will see a number of images such as the one linked below. Note the 50% DOD curve; it takes the battery to 60% capacity after about 650 cycles. If instead, you discharge only to 30%, you get a bit over 1600 cycles. So, about 2.5 times more cycles. Iif you add 67% more battery capacity so that a 30% discharge provides the same AH as a 50% discharge, you will get 2.5 times more cycles. Clearly Lead-acid battery life suffers greatly as DOD is increased. The same curves show the disastrous impact of going below 50% as well.


https://www.bing.com/images/search?v...=20&ajaxhist=0
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Old 12-14-2018, 06:45 PM   #14
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I think we have to be careful here. The referenced curve indicates that a deeper discharge does no harm, above 20% at least. If this curve is for an LFP battery then it's fairly accurate. But, the curve for a lead acid battery should be very different. If you google "agm depth of discharge versus life" and select images, you will see a number of images such as the one linked below. Note the 50% DOD curve; it takes the battery to 60% capacity after about 650 cycles. If instead, you discharge only to 30%, you get a bit over 1600 cycles. So, about 2.5 times more cycles. Iif you add 67% more battery capacity so that a 30% discharge provides the same AH as a 50% discharge, you will get 2.5 times more cycles. Clearly Lead-acid battery life suffers greatly as DOD is increased. The same curves show the disastrous impact of going below 50% as well.


https://www.bing.com/images/search?v...=20&ajaxhist=0
You have to consider the amps provided not only the # cycles.
Multiply batty capacity x %DOD X # CYCLES To compare amps delivered.
If only look at # cycles you should size a bank to only discharge 0-5% but that's not practical big banks cost more discharge less and last more cycles.
Small banks cost less discharge more and don't last as long.
Your choice when figured on a $/ cum amps delivered not a big difference
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