Depth of Discharge: a Contrarian View

[Rusty123]

(Note: everything that follows is intended to apply to 6V, golf cart type, FLA batteries).

As I've researched replacement batteries for my 28 foot sailboat, it seems almost a mantra on marine sites like this that effective bank capacity should be based on a charge cycle from 50% to 80% (30% effective bank capacity), in order to maximize battery life. Applying this thumbrule to my average daily consumption of 80 AH yields a necessary bank size of 267 AH for daily recharge.

With a Costco just down the street, my preference is to use their 6V golf cart batteries, which appear to be somewhere around 220 AH. So a two battery bank would be not quite large enough for my application (based on the thumbrule), but almost. Unfortunately, I don’t have room for a four battery bank without some extensive surgery. (I also have a small starting battery tucked away, not related to this discussion).

My usage pattern is mostly daysailing, with perhaps 10 - 20 days per year away from shore power, so I won't consume the full daily load very often, but with a 220 AH bank, there will be times when I will go below 50% SOC. So I decided to run some numbers to see what the impact would be.

The worksheet below (click to enlarge) calculates the cost per AH for a two battery bank (with a discharge cycle from 90% SOC to 20% SOC) versus a four battery bank (with a discharge cycle from 90% SOC to 50% SOC). (All figures extracted from graphs on the Trojan battery website, which the exception of cost, which is from Costco. I used a max charge of 90% SOC, since that is what Trojan shows as the "knee" in the bulk charge profile).



Based on this analysis, the cost per 1,000 AH for the two battery bank is $1.27, and $1.48 for the four battery bank. In other words, an “undersized” bank is less expensive, even though the depth of discharge exceeds 50%. This certainly alleviates any concern that I harbored regarding the occasional “excessive” discharging of my two battery bank.

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

Russ

Caveats:

1. The cost per 1,000 AH will of course be higher if you use more expensive batteries (actual Trojans, for example). But the relative difference between a two and four battery bank will remain the same.
2. A four battery bank offers a significant advantage in terms of higher charging rate. However, in my application (and many others with small sailboats), maximum effective charging rate is determined by my engine size (15 HP, so around 60 amps), rather than bank acceptance.
3. Although the cost per 1,000 AH is lower with a two battery bank, the absolute lifespan of the batteries is less. For some (such as those on an extended cruise away from supply channels), this might be more important than cost.

[djmarchand]

I think your cycles to end of life are high, by a factor of 2, particularly at the 20% soc. Somewhere years ago I saw a chart from a major manufacturer, maybe Trojan and my fuzzy recollection is that it was lower.

But otherwise your logic is sound.

David

[nimblemotors]

The 'logic error' is that while the smaller pack costs less, you are in fact getting much less from it, because the larger pack has a MUCH larger reserve capacity. The 50% rule applies to high loads. At low loads, you can discharge much more. So you can run low power things much longer with the bigger pack. Is that worth the extra cost? I think so.

Now if you don't have room, you don't have room.

[SV THIRD DAY]

I suggest you read up a bit on battery sulphanization because it kills more cruising batteries than perhaps any other cause.

Rare is the cruiser (that doesn't live at the dock) that can bring his batteries back to a full state of charge once for every discharge cycles, or call that 10 days on solar or wind alone.

[Cheechako]

Another take might be: Only worth buying ~ the amount of amp capacity that you can recharge in a typical recharge cycle. The rest is uneeded weight and $.

[Rusty123]

Originally Posted by djmarchand

I think your cycles to end of life are high, by a factor of 2, particularly at the 20% soc. Somewhere years ago I saw a chart from a major manufacturer, maybe Trojan and my fuzzy recollection is that it was lower.

But otherwise your logic is sound.

David

Here's where I got my figures:

Cycle life vs. Depth of Discharge (pg 2)

90% knee for bulk charge profile (pg 13)

Originally Posted by nimblemotors

The 'logic error' is that while the smaller pack costs less, you are in fact getting much less from it, because the larger pack has a MUCH larger reserve capacity. The 50% rule applies to high loads. At low loads, you can discharge much more. So you can run low power things much longer with the bigger pack. Is that worth the extra cost? I think so.

Now if you don't have room, you don't have room.

If I understand you correctly, you are saying that a larger bank, discharged only half way, gives more more reserve capacity than a smaller bank. That is very true, and an important consideration, particularly to the cruiser who ventures away from civilization. However, I can't remember ever reading that a bank should be sized to remain above 50% SOC in order to provide plenty of reserve capacity. Instead, what is usually recommended is that a bank be sized to remain above 50% in order to maximize the life of the battery. The former rationale is a matter of risk management, and the latter is a matter of economics. Two entirely different perspectives.

Originally Posted by SV THIRD DAY

I suggest you read up a bit on battery sulphanization because it kills more cruising batteries than perhaps any other cause.

Rare is the cruiser (that doesn't live at the dock) that can bring his batteries back to a full state of charge once for every discharge cycles, or call that 10 days on solar or wind alone.

I am somewhat familiar with sulfation, although I won't pretend to be an expert. However, my understanding (and as you state), is that the biggest contributor to sulfation is chronic undercharging. From that perspective, differently sized banks would be equally subject to sulfation since the maximum state of charge (for FLA) is primarily a function of charging time, not bank size/acceptance or charging capacity. It may also be true that sulfation plays a role in the non-linear relationship between cycle life and depth of discharge. However, that aspect is a "fact of life", and should already be captured in Trojan's Cycle Life vs. DOD chart.

[SV THIRD DAY]

Originally Posted by Rusty123

It may also be true that sulfation plays a role in the non-linear relationship between cycle life and depth of discharge. However, that aspect is a "fact of life", and should already be captured in Trojan's Cycle Life vs. DOD chart.

Oh if only that was true!

The lead and acid pushers assume you WILL follow their recommendations and will not constantly under charge your bank. This is what makes cruising reality different from the battery data tables. Just like the guy in a sad country song losing his girl and dog, a cruiser spending the majority of the time at anchor has battery challanges because almost none of us have enough space for all the solar we really need.

[Rusty123]

Originally Posted by Cheechako

Another take might be: Only worth buying ~ the amount of amp capacity that you can recharge in a typical recharge cycle. The rest is uneeded weight and $.

Yes, that is another way of stating the point. To me, the big eye opener was that the often cited "don't exceed 50% maximum depth of discharge" guidance appears to be baseless for true deep cycle batteries (such as golf cart batteries), and that bank sizing should be based on other things (economics, risk, weight, etc.).

Of course, even deep cycle batteries have a practical DOD limit. Here is a quote from Trojan's website: "Do not discharge your battery more than 80%. This safety factor will eliminate the chance of over-discharging and damaging your battery".

[Rusty123]

Originally Posted by SV THIRD DAY

Oh if only that was true!

The lead and acid pushers assume you WILL follow their recommendations and will not constantly under charge your bank. This is what makes cruising reality different from the battery data tables. Just like the guy in a sad country song losing his girl and dog, a cruiser spending the majority of the time at anchor has battery challanges because almost none of us have enough space for all the solar we really need.

I think you are saying that chronic discharging will reduce the overall performance of batteries. I fully agree with you, but my point is that the level of undercharging in a two battery bank will be essentially the same as a four battery bank, since once absorption is reached, the SOC is a function of time, not battery size.

In fact, if the charging source is the limiting factor (such as solar might be), wouldn't a smaller battery bank actually reach a higher SOC in a given time (and thus be less subject to sulfation?).

[Stu Jackson]

The point you may be missing is this: Your discussion of the 50% to 80% usage is based on NOT beging connected to shorepower and is for cruisers who are on the hook more often than not.

In that case, the 50-80 rule is because it makes little sense, overall energy-wise, to use the engine alternator to replace that last 20% into the batteries BECAUSE of battery acceptance. They take less when closer to full.

Battery Acceptance by Stu Battery Acceptance

In your case, you're daysailing and can recharge completely on a regular basis overnight.

My conclusion is that you're comparing apples to oranges.

If (or when) you start spending more time away from the dock, then your logic makes some sense. Then get a good sized solar panel and/or the next two batteries to increase your house bank size.

That said, the largest bank will ALWAYS last longer because the depth of discharge will always affect the longevity of the bank.

Calder wrote this, often quoted by me and Maine Sail:

stereo-battery connection

[Rusty123]

Originally Posted by Stu Jackson

The point you may be missing is this: Your discussion of the 50% to 80% usage is based on NOT beging connected to shorepower and is for cruisers who are on the hook more often than not.

In that case, the 50-80 rule is because it makes little sense, overall energy-wise, to use the engine alternator to replace that last 20% into the batteries BECAUSE of battery acceptance. They take less when closer to full.

I agree that it makes little sense to charge (with an alternator) past the point that acceptance begins to drop off. I used 90% SOC to describe that point, since that's where Trojan's literature defines it. The "knee" in the curve for other batteries might be lower (perhaps 80% SOC), but I don't believe that would change the spreadsheet numbers dramatically.

Originally Posted by Stu Jackson

In your case, you're daysailing and can recharge completely on a regular basis overnight.

My conclusion is that you're comparing apples to oranges.

I did muddy the waters by describing my usage (primarily daysailing) in the text, but my calculations and subsequent discussions apply only to a "full depth cycle", such as would be experienced at anchor.

Originally Posted by Stu Jackson

That said, the largest bank will ALWAYS last longer because the depth of discharge will always affect the longevity of the bank.

Calder wrote this, often quoted by me and Maine Sail:

This might be the key distinction: in that Calder (and many others) are measuring the life of a bank in terms of cycles, while I am proposing that the life of a bank be measured in terms of total amp hours (and dollars). Doubling the size of a bank provides more cycles, for sure, but it doesn't double the number of cycles (1200 vs 800 in my example). It does, however, double the cost to purchase and/or replace the bank.

So as long as you have the capability to drive to the store and buy new batteries when needed (not the case in the middle of the Pacific, I'll grant), then it appears to be cheaper to use a smaller bank, and discharge it more deeply (to a point).

To reiterate, I'm not necessarily saying that a smaller bank is better than a larger bank. There are lots of other factors at play, including reserve capacity, charging acceptance, etc. What I am saying is that the "don't go below 50% SOC" guidance that is almost carved in stone seems a bit arbitrary.

[noelex 77]

I have not had a look at your figures closely, but the cheapest deep cycle battery bank is generally with an average discharge close to 50%.

For a boat that is depending on generator, or alternator charging as the primary source a much bigger battery bank is often justified because of the higher charge acceptance. This means a larger battery bank is often more cost effective because it reduces expensive generator, or engine run time.

However, this thinking seems to have extended to boats that use solar/wind as their primary charge sorce. It is not unusual for these boats to have an average DOD of under 20%. This is often a source of pride for the owners, but the drawback is that their battery cost is significantly higher per year.

As always there are compromises, but as a general rule those boats that get most of their power replacment from a generator fit a battery bank that is too small. Those that get most of their power from solar/wind fit a battery bank that is too big.

[Rusty123]

Originally Posted by noelex 77

Those that get most of their power from solar/wind fit a battery bank that is too big.

And perhaps those with small alternators as well, as I alluded to in caveat number 2 of my OP.

[btrayfors]

Interesting discussion. Sorry I don't have time now for a full comment, and I don't want to opine on the OP's theory re: "smaller is cheaper...sometimes", but I would offer the following up for consideration.

1. Stu is right about sulfation. Batteries will sulfate if chronically undercharged. They will also continue to sulfate even when "fully charged" and floated at, say, 13.2-13.4VDC, albeit at a slower rate.

2. The average load is important in these calculations. If you have, say, a 100AH per day usage rate and you have a 225AH rated capacity battery bank, you're drawing an average of 4.2 amps over 24 hours. That represents just over 1/3 of the 20-hour rated capacity. Peukert comes into play here. You don't have a 225AH bank....with this average draw you have an effective capacity which is more like, say, 250 or 260AH.

If you have the same average draw and a 450AH rated capacity battery bank, you're only drawing 1/6 of the 20-hour capacity rating. So you don't have a 450AH capacity, you really have a capacity over 500AH.

3. Battery recharging cost is capital. Calder has documented the REAL costs of battery charging via different methods. The mechanical means...engine, generator, even wind generator....are incredibly expensive. Much more than you'd think. No question that solar is the way to go, if you have the real estate for the necessary panels.

I think you should have enough batteries aboard to easily get thru a couple of days with no charging. More is better, IMHO. And, re: COSTCO batteries, why not install 2 more if you have the room. They only cost $70 each, and you'll spend more than $140 worrying about getting along with a smaller bank :-)

One final thought re: small engines. There's no reason you couldn't fit a large alternator to a small engine. Even a 15HP engine can turn a 100A alternator at anchor. Think of it as a genset, not a propulsion engine.

Need it for propulsion? Just fit a good external regulator, like the Balmar MC-612 or 614, and fit the optional toggle switch. This cuts alternator output in half...at will. And, you can "de-rate" the alternator to any desired size.

Say you "de-rate" the 100A alternator by 20%. Now you have an 80A alternator which can run for extended periods. Need to power? Flip the switch and you now have a 40A alternator on your little engine.

Bill

[Stu Jackson]

What Bill just described (a simple switch) is what the regulator folks call Small Engine Mode, like this -

Small Engine Mode - discussion with link to the picture of the toggle switch: Alternator heat, Regulator Controls, Small Engine Mode

Please note that this is DIFFERENT than Amp Manager, which he also included.

I agree with Bill, for $140, go get two more batteries, even I could afford that!

I think you're over analyzing an issue that has been pretty much resolved over the years. There's reinventing the wheel, and there's also ctting hairs finer than necessary.

The point of solar with too large a bank MAY be true, but one has to consider this:

If you go straight 12V, then three batteries of 130 ah each works just fine. BUT, if you go 6V, then you are, BY DEFINITION, forced to go in 225 ah INCREMENTS, so two batteries is 225 ah, and four is 450 ah and you can't use three.

This proves a simple law of reality: What's available on the marketplace often makes engineering, a pretty exact science, sometimes overcome by what material is being used.

Good luck. Fun discussion.