Oddity with Lifeline Battery Spec

[RossWilliams]

I was looking at the specs for a lifeline 300AH battery.

It gives the following "minutes of discharge"

25 amps 692 miinutes
15 amps 1200 minutes
8 amps 2205 minutes

So at 15 amps you will get 20 hours and the rated 300 AH.
As expected at 25 amps you will get fewer amp hours (25*692/60) 288 AH

What is odd is that at 8 amps you will also get fewer amp hours (8*2205/60) 294 AH. A slower draw down is supposed to result in increased capacity. Here it appears that battery is optimized for 15 amps per hour and you get less capacity if you use more OR less than that.

[GeorgeRa]

You didn’t answer multiple questions on this thread, why folks should answer you this bait like question. http://www.classbforum.com/forums/f2...html#post97613

[markopolo]

Maybe it's a new start. I'll participate if things remain respectful. This could be a good educational topic.

I don't think that there's an accurate formula for converting RC ratings to Ah ratings. There's a lot of guessing on the internet and some simplified formulas but they wouldn't be accurate.

I suspect that it is just coincidence that the 20Hr Ah rating and the 15A Minutes Of Discharge rating are the same in the particular example that Ross posted.

When you start digging into battery ratings you'll often see that the ending volts per cell (vpc) can be 1.67vpc or 10.02V for a 12V battery or 1.75vpc or 10.5V for a 12V battery depending on the manufacturer.

Odyssey seems to use 1.67vpc for Ah ratings and 1.75vpc for RC ratings: https://www.odysseybattery.com/Support/FAQs#7

Q. I do not understand the Ah rating. Please explain.

A. The ampere-hour (Ah) rating defines the capacity of a battery. A typical battery that is rated as a 100Ah battery at the 10 hour rate of discharge is capable of delivering 10A for 10 hours before the terminal voltage drops to a standard value such as 1.67 volts per cell, or 10.02 volts for a 12V battery. Similarly, a 50Ah battery would supply a 5A load for 10 hours. The PC1200 battery is rated at 42Ah, so it can furnish 4.2A for 10 hours.

Q. What is the reserve capacity rating? What does that mean in the industry?

A. The reserve capacity of a battery is defined as the number of minutes that it can support a 25 ampere load at 80°F until its terminal voltage drops to 1.75 volts per cell or 10.50 volts for a 12V battery. Thus a 12V battery that has a reserve capacity rating of 100 signifies that it can be discharged at 25 amps for 100 minutes at 80°F before its voltage drops to 10.75 volts.

[RossWilliams]

"I suspect that it is just coincidence that the 20Hr Ah rating and the 15A Minutes Of Discharge rating are the same in the particular example "

I don't think it is coincidence. As understand it, the industry standard for AH ratings is a steady current for 20 hours at room temperature. So the minutes of discharge rate for the rated capacity would always be 1200 (20 hours).

But apparently Odyssey uses 10 hours, which would result in a somewhat lower AH rating and the minutes of discharge at the rated capacity would by 600 (10 hours).

[markopolo]

Originally Posted by RossWilliams

.......... So the minutes of discharge rate for the rated capacity would always be 1200 (20 hours)..........

I'm not sure exactly what you mean by that. Would you agree that there are three distinct rating standard results shown on the Lifeline page? They are Reserve Capacity (RC) (minutes), CCA and the 20hr Amp Hour rating.

These are all 6V batteries of the same family (I presume):
220Ah GPL-4CT
300Ah GPL-6CT
400Ah GPL-L16T

https://lifelinebatteries.com/produc...eries/gpl-4ct/
https://lifelinebatteries.com/produc...eries/gpl-6ct/
https://lifelinebatteries.com/produc...ries/gpl-l16t/

The ratio of increased minutes available / larger capacity battery shows a consistency that supports my earlier conclusion of coincidence.


Edited: 3 ratings: CCA, RC & AH

[RossWilliams]

Well, they give minutes of discharge for the same amps for all the batteries. So the 15 amps may just coincidentally be 1/20 of 300AH. But that doesn't explain the puzzle of why a lower current produces fewer AH. It doesn't happen for the 220 AH battery. That would be 20 hours at 11 amps, at 8 amps that battery produces 226 Ah and at 15 amps it produces 214. Those are both expected results. Its only the 300 AH battery, where the 15 amp is also the amp rate for the 20 hour AH rating, that the 8 amp AH is actually lower.

My speculation would be that the AH was determined based on a rigorous test and the test for the other numbers used a different procedure. When the results for 15 amps for the 300 ah battery didn't match the rating test, they plugged in the number from the official rating test. No one noticed that the other numbers were no longer in sync relative to the official AH rating. And, yes, that is just speculation.

[markopolo]

I don't think you can accurately convert RC to AH ratings so you can't take an RC data point and turn it into an Ah rating. They're meant to show different capacities or capabilities.
--------------------------------------

Lifeline often mentions "mil spec" and on Concorde's site there's a reference to both 1.5vpc and 1.67vpc mil specs. Concorde Battery - AGM Aircraft Batteries

It would be nice to know what ending voltage Lifeline is using.

I had previously thought that 1.75vpc was the cut-off for RC and that 1.67vpc was the cut-off for AH ratings but Trojan seems to use 1.75vpc termination for both RC and AH ratings.

I've confirmed 1.75vpc as a BCI standard for RC ratings but haven't found an authoritative source for the AH rating test termination voltage. It might be an IEC standard but I don't know for sure.


Edit: added can't

[RossWilliams]

I thought RC was reserve capacity. The "Rating Cap." listed on the batteries is expressed in AH. But this link explains how to convert from RC to AH and says this.

"The reserve capacity of a battery is the number of minutes for which it can run at 25 amps of current without its voltage dropping below 10.5 volts. It ... technically specifies the battery's charge capacity. ... Ampere-hours is a different unit for describing the same quantity."

[markopolo]

Yes, there's that formula and the simple way of just dividing 25A RC minutes by 2.

It's easy to test both. Here's the data for a Trojan T-105: https://www.trojanbattery.com/product/t-105/

RC @25A 447 minutes & 20hr AH 225

First lets try the formula in the link you posted.
447x60x25/3600=186 way off!

How about simply dividing by 2:
447/2=223.5 pretty close!

I wouldn't describe either as accurate. Might as well use the simple divide by two for a quick estimate though as it is easier to remember.

The actual definition of RC Rating from BCI (Battery Council International):

RESERVE CAPACITY RATING — The time in minutes that
a new, fully charged battery will deliver 25 amperes at 27°C
(80°F) and maintain a terminal voltage equal to, or higher than,
1.75 volts per cell. This rating represents the time the battery
will continue to operate essential accessories if the alternator or generator of a vehicle fails.

This site - Battery Reserve Capacity Defined | All About Lead Acid Batteries - does a good job of explaining the difference between RC and AH ratings.

[RossWilliams]

The formula gives you AH at a 25 amp draw for 447 minutes. It is way too complicated. You can simply divide 447/60 to get the hours and multiply that by the amps. It gives you the same number

The AH rating from the company is, in this case, more like 11 amps (225AH/20 hours) at 20 hours.

RC is going to be more accurate if you are putting a heavy load on the battery for a short time. AH is going to be more accurate for long slower draws. As you say, neither one is going to be accurate for real usage. And either one will allow you to compare the capacity of batteries of the same voltage under the same conditions. There may be slight variations in how batteries compare using the two different standards based on their peukert constant.

[markopolo]

Originally Posted by RossWilliams

The formula gives you AH at a 25 amp draw for 447 minutes. ...................

Specifically, what formula are you referring to?

Maybe pick a few batteries with published RC and AH ratings and demonstrate how the formula can be proved.

I picked the Trojan T-105: https://www.trojanbattery.com/product/t-105/ and the formula was wrong.

[RossWilliams]

No, the formula wasn't "wrong". The formula accurately provided the AH from the battery when discharged at 25 amps for 447 minutes(7:27). Simple multiplication of amps times hours tells you it was correct. The company's AH rating was for the AH at approximately 11 amps for 20 hours (1200 minutes). You would not expect those two numbers to be the same.

You really can't get from RC to AH or vice-versa without knowing the battery's peukert constant. I suspect that is why both ratings are provided. As people running AC have discovered, the battery's AH rating is very misleading when you have heavy draws over short periods. The RC would be equally misleading if you are drawing down the battery over a long period. Different ratings for different types of use.

further edit:

Its interesting to consider how RC could be used to help determine battery capacity needed for AC in an RV. Using your example battery, the battery would generate 300 watts of power(25 ampsX12 volts) for 7.5 hours. If your AC draws 1200 watts you will need 4 batteries to run that long before the batteries will be fully discharged. If you only want to discharge to 50%, you will need 8 batteries.

Now, use the AH rating. Your AC still draws 1200 watts, 100 amps at 12 volts. So for 7.5 hours of time you are going to need 750AH, 1500 if you only want to draw the batteries down to 50%. 7 batteries will give you 1575 AH, more than enough.

Same batteries, using the RC tells us we need 8 batteries to stay above 50%. If we mistakenly used the AH rating, we would end up drawing the batteries well below 50%. Not really a problem, but if you sized your system to draw down to 80%, the difference would likely result in warm cabin in the morning and potentially damaged batteries.

[markopolo]

Originally Posted by RossWilliams

..... But this link explains how to convert from RC to AH and says this.........

Is that the formula you are referring to?

[RossWilliams]

Yes, the one you used to get 186 amp hours. If you multiply 25 amps by 447 minutes and convert to hours you get the same result.

[markopolo]

I still have to read the further edit but just want to stay with the 186ah to see if I'm missing something.

Trojan says the 20hr rating is 225Ah but the conversion formula comes up with 186Ah so I'd say that's a wrong answer.

How is it not wrong?

[markopolo]

Originally Posted by RossWilliams

further edit:

Its interesting to consider how RC could be used to help determine battery capacity needed for AC in an RV. Using your example battery, the battery would generate 300 watts of power(25 ampsX12 volts) for 7.5 hours. If your AC draws 1200 watts you will need 4 batteries to run that long before the batteries will be fully discharged. If you only want to discharge to 50%, you will need 8 batteries.

Now, use the AH rating. Your AC still draws 1200 watts, 100 amps at 12 volts. So for 7.5 hours of time you are going to need 750AH, 1500 if you only want to draw the batteries down to 50%. 7 batteries will give you 1575 AH, more than enough.

Same batteries, using the RC tells us we need 8 batteries to stay above 50%. If we mistakenly used the AH rating, we would end up drawing the batteries well below 50%. Not really a problem, but if you sized your system to draw down to 80%, the difference would likely result in warm cabin in the morning and potentially damaged batteries.

You didn't notice that was a 6V battery. I come to the same basic conclusions as you though. The RC 25A rating results are closest to being correct because we happen to need 100A in this example using 8 6V batteries. The end user would be disappointed with the run-time if expectations were based on the AH 20-Hr Rate.

T-105 100A DC air conditioner current 100% duty cycle using 80% battery capacity (not sure if applying 80% to time is OK)

Using 1.5kWh rating x 80% = 1.2kWh 8 batteries = 8 hrs.

Using 25A RC 447 minutes x 80% = 358 needs 8 batteries for 100A at 12V = 6 hrs

Using 20-Hr Rate 225 x 80% = 180Ah per pair so 8 batteries = 7 hours

Using 5-Hr Rate 185 x 80% = 148Ah per pair so 8 batteries = 6 hours

Using 50%

Using 1.5kWh rating x 50% = .75kWh (need 1.2kWh) 8 batteries = 5 hrs.

Using 25A RC 447 minutes x 50% = 224 needs 8 batteries for 100A at 12V = 3.73 hrs

Using 20-Hr Rate 225 x 50% = 113Ah per pair so 8 batteries = 4.5 hours

Using 5-Hr Rate 185 x 50% = 93Ah per pair so 8 batteries = 3.72 hours

Hopefully all the math is correct!

-----------------------------------

Back to the 186ah to see if I'm missing something.

Trojan says the 20hr rating is 225Ah but the conversion formula comes up with 186Ah so I'd say that's a wrong answer.

How is it not wrong?

[RossWilliams]

Originally Posted by markopolo

I still have to read the further edit but just want to stay with the 186ah to see if I'm missing something.

Trojan says the 20hr rating is 225Ah but the conversion formula comes up with 186Ah so I'd say that's a wrong answer.

How is it not wrong?

The answer is right, you are asking the wrong question. The formula provides you with the battery's actual AH output at 25 amps. You want to know the manufacturers AH rating. The formula does not answer that question, its the wrong formula for that purpose. Which is my fault, I didn't read it carefully when I suggested it.

[markopolo]

Got it. ..

[RossWilliams]

Here is a not so simple formula for converting RC to AH:

C = I * H * (t/H)^(1/k)
(C=I*H*POWER(t/H;1/k) as a spreadsheet formula )

C is the battery AH rating

I is the amperage used to determine the RC - the standard is 25 amps
H is the hours used to determine the AH - the standard is 20 hours
t is RC time of discharge in hours
k is the battery's peukert constant

[markopolo]

That's beyond me.
I'll stick with dividing by 2!