Battery C20 vs. "Minutes of discharge"?

[Unity]

I'm confused why C20 ratings don't match "minutes of discharge". For example, on a LifeLine AGM GPL-4DA battery (see LifeLine GPL-4DA) they state:

Rated Cap. Amps Hours @ 20hr Rate: 210ah

Minutes of Discharge:
25 Amps: 390 minutes
15 Amps: 680 minutes
8 Amps: 1375 minutes

Using simple math, that says:
25 amps * 6.5 hours (390 minutes) = 162ah capacity
15 amps * 11.3 hours (680 minutes) = 170ah
8 amps * 22.9 hours (1375 minutes) = 183ah

All of these are below the C20 rating of 210ah.

To get closer to comparing apples-to-apples, I converted the C20 rating to an averaged hourly draw, or 210ah / 20 hours = ~10.5 amps per hour. I then used simple linear extrapolation to determine what a 10 amp usage would be based on minutes of discharge, which came out to 179ah. This is 85% of the C20 rating.

Why? And which value should you use for real world comparison? For example, when I do a discharge test, should I compare my tested my battery capacity (using a timed constant current at 10.5 amps) to the C20 rating (210ah) or the minutes discharge (185ah) when attempting to determine relative health of the batteries?

Or do I have some error in my math / understanding that helps explain this 15% difference? Appreciate any insights here...

Chris

[mikedefieslife]

I don't know the maths, but isn't it because it's not linear, and larger loads actually reduce the battery's ability to sustain those loads. Hence different ah figures for 5, 10, 20 hour ratings.

Also bear in mind that greater loads mean greater temperatures, which again prevents the battery from performing to its c20 rating.

[Unity]

Originally Posted by mikedefieslife

I don't know the maths, but isn't it because it's not linear, and larger loads actually reduce the battery's ability to sustain those loads. Hence different ah figures for 5, 10, 20 hour ratings.

Also bear in mind that greater loads mean greater temperatures, which again prevents the battery from performing to its c20 rating.

I assumed since I'm working all from specs from LifeLine that everything is at the "perfect" (consistent) temperature. And while I agree it's not linear, my calculation on a line vs. curve doesn't have much impact. For example, if we use their stated 8 amp instead my (linear calculated) 10 amp, it give 183ah (my calculation at 10 amps was 179a, or only 4 amps different). Even at 183ah, it's still 13% lower (87%) of the C20 rating.

So I'm thinking this difference isn't due to temperature or non-linear consumption...?

[Maine Sail]

Peukert...

[john61ct]

210AH is only C at the 10.4A discharge rate, over 1,200min.

The other rates means of course different time periods to discharge, but also a different AH capacity.

As MS states, that is what Peukert's Law is all about.

> should I compare my tested my battery capacity (using a timed constant current at 10.5 amps) to the C20 rating (210ah) or the minutes discharge (185ah) when attempting to determine relative health of the batteries

Yes, as part of the commissioning process, do a standardized 20-hour test down to 10.5V, then immediately recharging. It may show a higher capacity after the break-in period, go gentle the first few dozen cycles.

Then to determine SoH, run the same test.

Maybe after a year, only more frequently if you haven't been able to give ideal care and suspect a decline.

Once below 80% I would do it quarterly, and plan to replace before 70%.

[sailinglegend]

Originally Posted by Unity

I'm confused why C20 ratings don't match "minutes of discharge".....
Chris

Your observation is interesting, so is Lifeline's suggestion to do a capacity test using the 25 amps at 390 minutes instead of the 20 hour test which is very difficult to do without equipment to maintain the load as the voltage declines.

One possible reason for the difference you have noted is that the 210 Ah rated capacity is at a temperature of 77F whereas reserve capacity is measured at 80F!!!! This might give a 3-4 % difference but not 15%?

I've just done a reserve capacity test on one of my 210 Ah lifelines and it was down to 44% of original capacity. Last year at the beginning of the season a 20 hour capacity tests was down to 55%. Lifeline recommend replace when batteries fall below 50%.

The new Lifelines have been installed and are fantastic - the old ones lasted 14 years.

[Unity]

Originally Posted by john61ct

210AH is only C at the 10.4A discharge rate, over 1,200min.

The other rates means of course different time periods to discharge, but also a different AH capacity.

I totally understand that different discharge rates result in different total capacity. But when I don't get is why the C20 rating, which would be about 10.5a over 20 hours, would be so much more capacity that the minutes discharge (converted to the same 10.5a rate)? Or heck, take the 8 amp rating - in theory, 8 amps should give me more capacity, not less? Yet it still rates substantially below the C/20 @ 10.5a capacity.

Something besides Peukert's law seem to be at play here. Perhaps C/20 is testing down to 10.5V yet minutes discharge is using some higher stopping voltage?

Yes, as part of the commissioning process, do a standardized 20-hour test down to 10.5V, then immediately recharging. It may show a higher capacity after the break-in period, go gentle the first few dozen cycles.

Then to determine SoH, run the same test.

Maybe after a year, only more frequently if you haven't been able to give ideal care and suspect a decline.

Once below 80% I would do it quarterly, and plan to replace before 70%.

That makes sense, and if my batteries were new that is what I would do to baseline them. But they are not new (five @ 4D, four @ G31, 1 @ G27, all in good to excellent health) and so I'd like to try to understand which number to compare them with for that very determination (80%, 70%). If I use the C/20 number on specs, and my test actually gets the minutes discharge, then (in theory) a brand new battery would start at nearly 80% off!

So still don't understand … I thought I'd removed Peukert's law from the equation by using very similar amp draw rates and yet minutes discharge and C/20 (using about same amp draw) result in very markedly different values.

[Unity]

Originally Posted by sailinglegend

I've just done a reserve capacity test on one of my 210 Ah lifelines and it was down to 44% of original capacity. Last year at the beginning of the season a 20 hour capacity tests was down to 55%. Lifeline recommend replace when batteries fall below 50%.

The new Lifelines have been installed and are fantastic - the old ones lasted 14 years.

Did you by chance test your brand new LifeLine? If so, it would be wonderful if you could share the results. What current level to you test at (and was it constant)? How long did it run? Was it also a 210 Ah battery (the GPL-4DL)?

[sailinglegend]

Originally Posted by john61ct

210AH is only C at the 10.4A discharge rate, over 1,200min.

The other rates means of course different time periods to discharge, but also a different AH capacity.

As MS states, that is what Peukert's Law is all about....

Peukerts to you too!!! Sorry only a joke!

Peukerts is not why the 8 amp 'reserve capacity' is lower that 210 Ah. Peukerts says that the capacity at a lower amperage that the C20 rate would mean the actual available capacity would be higher.

So MaineSail can you explain that one?

[Unity]

FYI - I've submitted the question to LifeLine as well, including a link to this discussion in case they want to participate. Will include their comments here should I hear back from them directly.

[sailinglegend]

Originally Posted by Unity

Did you by chance test your brand new LifeLine? If so, it would be wonderful if you could share the results....

You should NOT do a capacity test with brand new batteries, they need several discharge cycles before the reach their full capacity - maybe as many as 50 some sources suggest.

I will do a C20 test in about six weeks when we finish sailing down to about 50 percent and plot this on a graph. At the end of each season I can re-test each battery and put on the same graph and any reduction, or increase, will show.

Doing a C20 test down to only 50% is not accurate as the actual voltage at 50% changes with battery age and from battery to battery. C20 down to 10.5v is the only accurate way to do that - but who wants to flatten their new batteries!!!! Doing that with old batteries and immediately recharging may actually do some good.

[john61ct]

Breaking in lead batteries properly, including initial "commissioning charging" is critical to getting max longevity, and will often increase their capacity.

This process for lead banks does involve a very similar process to load testing, and done properly, and infrequently over the bank's lifetime, neither causes any damage.

But yes, capacity will likely further rise **after** the breaking in period, so do one before and after, and use the better result as your benchmark for later on.

If not done then, your test must be compared to the mfg rating, which with quality makers in the US, is the 20-hour AH rating.

Obviously the other specs are for a different purpose, so IMO just ignore them.

_______
protocol:

The first few dozen cycles, go easy, avoid high*current*rate discharges and charges.

First step, really fully charge, as per mfg spec endAmps,

then, manually follow mfg equalizing instructions.

Then discharge at .05C or 5A per 100AH, for about 20 hours. The battery voltage should then be resting around 10.5 Volts.

Then recharge at ~.15C or lower for the rest of the break-in period, say two months if regularly cycling, or 50 cycles over time, trying to avoid huge draw currents as well.

http://www.amplepower.com/primer/break-in/index.html

https://www.catalina36.org/comment/51915#comment-51915

http://www.sailnet.com/forums/showthread.php?p=3779746

[CaptainSteven]

I found using the C20 value gives misleading data. I found if I use peukerts formula with 72 hours instead of 20 I get data that more closely represents my true capacity with a constant of 1.2.

Here is the formula I use:

T = C/(I/(C/R))^n X (RXC)

[sailinglegend]

Originally Posted by john61ct

Breaking in lead batteries properly, including initial "commissioning charging" is critical to getting max longevity, and will often increase their capacity.

.........Then discharge at .05C or 5A per 100AH, for about 20 hours. The battery voltage should then be resting around 10.5 Volts......

Your information on breaking in new batteries is excellent.

But your C20 discharge test suggests that you have never ever done one because your quote ..."The battery voltage should then be resting around 10.5 Volts"... is nonsense.
The fall in voltage on a graph is almost vertical you can see the meter going down every second 10.52v, 10.51v, 10.50v, 10.49v, - so you must be watching carefully to record the hours or minutes to the point it reaches 10.5v.

[john61ct]

Originally Posted by sailinglegend

Your information on breaking in new batteries is excellent.

But your C20 discharge test suggests that you have never ever done one because your quote ..."The battery voltage should then be resting around 10.5 Volts"... is nonsense.
The fall in voltage on a graph is almost vertical you can see the meter going down every second 10.52v, 10.51v, 10.50v, 10.49v, - so you must be watching carefully to record the hours or minutes to the point it reaches 10.5v.

Yes it can be tedious, but stopping at precisely 10.50V and recording hours-minutes elapsed is a requirement for an accurate load test.

Equipment can be set up to automate that process, I believe Maine Sail has suggested ways to use apps on a smartphone to allow you to step away for at least parts of it.

Obviously if just doing the commissioning protocol, such accuracy is not needed, but the lower the Volts go, and the longer you leave the batt that low, the greater the likelihood you do more harm rather than good.

An adjustable LVD would be a good investment.