Depth of Discharge: a Contrarian View

[Rusty123]

Originally Posted by sailinglegend

You keep posting links to sites I can't access???? "You have reached a page that is either non-existant or in error!" This your c34.org site with this error message - with a spelling mistake!!!! I can't register, even if I manage to get to the home page - it doesn't work - but I did manage to register a few months ago - maybe I was using a VPN. Don't you want non American locations to see your precious information? I am in Turkey at the moment. Please, please can you fix it?

I don't know what the issue is either, but for reference, I'm able to access without an account - no problems.

[denverd0n]

Originally Posted by Rusty123

...the often cited "don't exceed 50% maximum depth of discharge" guidance...

See, here is where I think you went off the track. No one who knows what they're talking about will ever tell you that you absolutely must not, under any circumstance, or for any reason, ever go below 50% depth of discharge. Yet that seems to be the way that you have interpreted the general rule of thumb.

And that's all it is. A general rule of thumb. And the rule of thumb is, for best battery life, it is preferable not to routinely discharge much below about 50%. That's all. It's not an ultimatum.

And I don't doubt for a minute that there are specific circumstances where routinely discharging more deeply can work out to be more economical. But that's going to be a specific circumstance, not the average situation for the average cruiser. Fine. If that's how it works for you then go for it.

If you truly understand batteries and electricity enough to figure all of this out for yourself, then you probably don't need to make use of a general rule of thumb anyway.

[Maine Sail]

Originally Posted by Rusty123

Please comment or point out any errors in my math or logic.

Russ

Some thoughts...

#1 Unfortunately the data set is based on unrealistic white coat, pencil protector laboratory geekedom that simply does not translate well at all to the real world.. I wish it did because it would mean marine batteries would be lasting coastal cruisers 20+ years, and we know that does not happen, ever...

I have never once seen a battery in a marine application get anywhere near the lab rated cycles.

These cycles are done in a lab and conducted one after the other; discharge, charge, discharge charge all at a controlled temp, optimum charge rates, absorption voltages are higher than most boat owners will accept, and with all kinds of lab geekery going on to "ensure" these batteries hit the absolute max cycles possible. No boat owner I know of can ever hit the same parameters they use in the lab cycle after cycle.....

Unfortunately for us this type of data never translates into the real world. Temps are different we never discharge then immediately re-charge to 100% etc. etc. on and on. Also none of the manufacturers tend to do their cycle testing identically except for that it is done in a lab, which definitely does not ever mimic how we use these batteries in the real world,and especially not in boats.

We also can't forget that it was not very long ago a particular AGM maker claimed 80% DOD was possible with their product in the marine world... They claimed this, like you, to try and show that their approach was cost competitive.. What happened? It 150% backfired on them. 80% DOD on boats was killing these batteries in 8-24 months as a cost of $800.00+++++ per bank. The sulfation was murdering these high tech, high cost AGM batteries yet their lab data never predicted this, only real trials in the real world could. Folks who bought them and paid huge money were NOT happy. Folks like me who bought these claims got burned hard and also were NOT happy.

Today the same exact company that told us 80% DOD in AGM batteries was a better deal than FLA, and doable, is now recommending 50% DOD as the typical safe discharge level for the best cycle life / $$$ equation. Go figure that they now are in concert with Trojan, Deka, Superior, US Battery, Exide, Rolls etc. etc. on the depth of discharge recommendations being not more than 50% for optimal cycles to $$$...

The real world factor throws a monkey wrench into the cost analysis...


#2 Picture yourself driving through a 500 foot bug storm (read 50% DOD). After the 500 feet you can still see out the window enough to drive. You turn on the windshield wipers and get a fair amount of the bugs off, but not all. These 500 foot bug storms repeat over and over and the ones you did not get off the first time remain until the next bug storm. New bugs add to the accumulating visibility issues, over time, until you eventually can't see at all..

Now picture yourself driving through an 800 foot bug storm (read 80% DOD). As you come out the other side the window is totally occluded and you can't see at all. You turn on the wipers and get rid of some but not all, because you started with more bugs. These 800 foot bug storms repeat over and over and the ones you dd not get off the first time remain. New bugs add to the visibility issues until you eventually can't see at all.

Occluded visibility happens much more rapidly in the 800 foot storm than the 500 foot bug storms.

This is similar to what sulfation is like based on depth of discharge. At 80% DOD more material is converted and covering the plates. When you re-charge you never re-convert it all, not at 80% DOD, not at 50% DOD not even at 30% DOD. The difference is that the shallower the cycles the less you have to reconvert and the less sulfation build up you have. The deeper the discharge the faster the sulfation kills the battery. In the real world of marine cycling applications this is MUCH faster than in the lab.

The more material you need to re-convert, there is a lot more at 80% DOD than 50% DOD, the more permanent sulfation remains built up after each cycle. Add on-top of this the non-lab environment and 80% DOD becomes a lot worse than 50% DOD because you have a lot more material to re-convert.. Because you never really get to full this only compounds the issue and exacerbates the decline of the bank in the "real world".... Because of this the plates simply sulfate faster at deeper DOD's in the real world than they do in the labs.

#3 225 AH Bank - At 76Ah / day the average load is about 3.2A. When corrected for Peukert on a 225Ah bank the effective load is about 2.3A over 24 hours. On a 225Ah bank with a 1.27 Peukert and 76Ah's per day your effective bank size is roughly 316Ah. So per day your DOD would really be about 75% SOC when corrected for Peukert.

450Ah Bank - At 76Ah / day the average load is about 3.2A. When corrected for Peukert on a 450Ah bank the effective load is about 1.9A over 24 hours. On a 450Ah bank with a 1.27 Peukert and 76Ah's used per day your effective bank size is roughly 760Ah. So per day your DOD would really be about a 10% DOD, when corrected for Peukert.

The bigger bank wins when you correct for Peukert unless the bank has a very low Peukert exponent like LiFePO4. You can't not correct for Peukert when trying to show a cost efficiency for a smaller bank, in a boat, when deep cycling.

This throws a monkey wrench into the cost analysis....

#4 Voltage sag can be a problem in FLA batteries. At your loads this should not be an issue on a 225Ah bank at 50% DOD but could become an issue at 80% DOD for certain equipment. The drop off in ability for an LA bank to hold voltage diminishes pretty quickly once past about 60% DOD and also when sulfated. They will simply sulfate faster at 80% DOD cycling than at shallower rates.

#5 The 90% "knee" is based on a charge rate of 10-13% of "C" and is how they charge golf cart batteries. No boat owner wants to charge at 10% of "C" unless tied to a dock with ample time on their side.. You will be throwing 20-25% of "C" at an FLA bank on a boat in bulk if you want to be at all efficient. At 20-25% of "C" you will hit absorption considerably earlier. Throw solar at a bank at 5% of "C" and you won't be hitting absorption until the mid to high 90's, throw 25% of "C" at the bank and you hit absorption far earlier...

Absorption voltage, as related to SOC, is entirely current dependent as to where you achieve it in the state of charge. At 20-25% of "C" charging you will be hitting absorption at 70 - 80% SOC..

This also throws a monkey wrench into the cost analysis....

#6 No one has ever said "Don't exceed 50% DOD". What pretty much every battery maker suggests for typical DOD's in a cycling application is 50%. This considers cycle life to costs in a cycling application.. You certainly can cycle to 80% DOD, and it is not going to end the day, but each cycle there will result in more sulfation than the cycles to 50% DOD do especially if not recharging to 100% quickly thereafter.. Heck cycling to just 70% SOC results in even more cycle life so the bigger bank always wins because the cycle depth is shallower and the resulting sulfation is less meaning longer cycle life.. Shallower cycles result in less sulfation, which results in longer cycle life...

These are Trojan's recommendations:

"Discharging batteries is entirely a function of your particular application. However, below is list of helpful items:

1. Shallow discharges will result in a longer battery life.

2. 50% (or less) discharges are recommended."



#7 At the price of Costco batteries none of this really matters. It sounds as if you will mostly be at a dock and only discharging to 80% DOD occasionally. When Peukert corrected, even less often. Recharge to full as often as you can, equalize and suck up the hit in cycle life and all is good.

My biggest concern with deep discharges to the 80% range is always voltage sag on critical items like nav lights, radar, bilge pumps etc.... Less critical items such as fridge compressors and windlasses can also suffer..

Trying and justify this, with lab data, on a cost per Ah basis is not reasonably or easily extrapolated out into use on actual boats... There is no real advantage in discharging to 80% DOD in the real world.

Just use the batteries and replace them when they need it. $160.00 in the whole scheme of boats is a drop in the proverbial bucket. Even if you need to replace the batts every 2-3 years it is still likely less costly than re-vamping the entire boat. If you want a 450Ah bank do it. If you don't, then don't do it.... Either way you will survive and neither will be the end of the world..

It does seem to me that you are the kind of guy who would sleep better at night with the larger bank, rather than with the smaller bank, but that is just my observation from reading many of your posts....

[Rusty123]

Originally Posted by Maine Sail

Some thoughts...

But...but...but...

I can't argue with any of this - very clearly stated. But I do have a couple questions/comments:

*It is clear that a larger bank "wins" in terms of lifespan. What isn't clear is whether the win is worth the cost (in various scenarios, not just mine), nor how to make that calculation. In my profession, this would be analyzed through some sort of Return on Investment (ROI) calculation. But as you illuminate, using manufacturer's data to perform that calculation has problems. It's too bad that some sort of third party (Practical Sailor, for example) doesn't test batteries in this manner, like they do with bottom paint (or maybe they have, and I just haven't seen it). Probably not sexy enough to sell subscriptions though.

*In paragraph #3, it appears that you adjusted the effective bank capacity upwards to account for Peukert, and also adjusted the effective load downwards. It seems to me that you should adjust one or the other, but not both. No?

Originally Posted by Maine Sail

It does seem to me that you are the kind of guy who would sleep better at night with the larger bank, rather than with the smaller bank, but that is just my observation from reading many of your posts....

I don't know how you came to that through reading my posts, but you're absolutely right. If I had a nickel for everytime I bought or built something, and then later wished I had aimed higher...

[Rusty123]

I don't know about anyone else reading or participating in this thread, but this has been a very educational process for me. I've learned more about batteries in the last few days, either through reading the comments, or doing research to support my own hypotheses, than in all the years prior.

Probably the biggest takeaway for me is that before any -- any -- discussion of alternative scenarios, one really needs to adjust for Peukert. A few days ago, I was convinced that I would need to run my alternator twice a day to maintain adequate battery charge, and now I know that once will be enough, all without spending a dime.

Thanks to all for the thoughtful comments.

[Maine Sail]

Originally Posted by Rusty123

But...but...but...

I can't argue with any of this - very clearly stated. But I do have a couple questions/comments:

*It is clear that a larger bank "wins" in terms of lifespan. What isn't clear is whether the win is worth the cost (in various scenarios, not just mine), nor how to make that calculation. In my profession, this would be analyzed through some sort of Return on Investment (ROI) calculation. But as you illuminate, using manufacturer's data to perform that calculation has problems. It's too bad that some sort of third party (Practical Sailor, for example) doesn't test batteries in this manner, like they do with bottom paint (or maybe they have, and I just haven't seen it). Probably not sexy enough to sell subscriptions though.

The problem is that every situation is different, in the real world, so no type of testing by PS could replicate that. The lab data is as accurate as it can be for that type of testing but with batteries the variables are far to broad to fully capture and apply across the board.

What could be done is an A/B test where each battery is treated equally in cycle testing. This would tell us which batteries win in that particular type of test. Unfortunately it may not at all be reflective of how the battery will perform in the real world.

I do however work in that environment every day, as does Charlie J and Btrayfors among others. When we tend to quantify what we see out in the real world the n=X is much larger than the single boat owner so we can draw conclusions, based on actual hands on data across many vessels, that most simply do not have access to..

All things being equal, and they never are, I have yet to see a larger bank not win out every time over a smaller one that had the same loads applied. Shallower discharges mean longer life even when a large bank and small bank are charged exactly the same way. Because of Peukert the larger the bank gets beyond the average loads the DOD becomes even shallower....

Originally Posted by Rusty123

*In paragraph #3, it appears that you adjusted the effective bank capacity upwards to account for Peukert, and also adjusted the effective load downwards. It seems to me that you should adjust one or the other, but not both. No?

That is just how it works with Peukert. A larger bank will see a smaller "effective load" than a smaller bank will making it more efficient at driving the same load..... It is not the actual load but the Peukert corrected load the battery sees when you draw current out at below or well below the 20 hour rate. Most FLA batteries have fairly high Peukert exponenets. The numbers below span above and below the 20 hour rate of the battery using a 3.2A load and it may help give it some perspective..

PCBS = Peukert Corrected Bank Size - at load

PCL = Peukert Corrected Load


25Ah Battery, 3.2A Load, Peukert 1.27, PCL = 4.1A / PCBS = 19.5Ah

50Ah Battery, 3.2A Load, Peukert 1.27, PCL = 3.4A / PCBS = 47Ah

64Ah Battery, 3.2A Load, Peukert 1.27, PCL = 3.2A / PCBS = 64Ah

100Ah Battery, 3.2A Load, Peukert 1.27, PCL = 2.8A / PCBS = 113Ah

225Ah Battery, 3.2A Load, Peukert 1.27, PCL = 2.3A / PCBS = 316Ah

450Ah Battery, 3.2A Load, Peukert 1.27, PCL = 1.9A / PCBS = 762Ah

675Ah Battery, 3.2A Load, Peukert 1.27, PCL = 1.7A / PCBS = 1275Ah

Originally Posted by Rusty123

I don't know how you came to that through reading my posts, but you're absolutely right. If I had a nickel for everytime I bought or built something, and then later wished I had aimed higher...

I go through this sort of thing with customers every day so call it a lucky guess....