Battery Capacity-What is Too Big?

[Wotname]

Pel,
Just thinking aloud, wouldn't a small genset well installed and soundproofed possibly be more cost effective for the big load periods rather than large battery capacity. I haven't the pleasure of being aboard a 60 ft mono but I would have thought room won't be an issue.

[Pelagic]

Your right Wotname, room is not an issue, but size of the gen does become one (unless you have space for 2 of different size as powerboats often do that)

Generators are your portable shore power and are normally sized to handle all the conveniences (like 3 air-con units) dive compressor, hot water tank, at the same time, so you are forced to have a pretty big one (10 to 12Kw).

The biggest problem with Gens is that it is bad for them to be run at light or no loads, so you need to keep them working while charging the bank back up to 80% SOC (state of charge).

That is why, 2 to 3 hours a day running your Gen going from 70% to 80% suggests to me that with a big bank/big chargers you will keep a steady load on the generator, if other stuff cycles out.

I am doing tests at different DOD to see at what SOC the chargers dramatically dials down their delivery and that will be the hump where I would shut down the Gen..(80-85% is my guess?)

This is where we get into charge voltage settings of 29.8V during the bulk charge and is something not exactly clear to me, but I remember Rick explaining it on one Thread but haven’t found it…..Anybody know?

Not sure about costs anymore with today’s diesel prices. Spending 2 to 3000USD on a large battery bank that allows you to run everything off batteries (including water-maker) for 21 hours of the day rather than only 16 hours should I think easily pay for itself over 6 years.

I have not checked but would have to add up all the battery and charger costs and compare 3 hr vs 8 hr daily Gen operating costs for fuel and gen maintenance to know

But to be honest, the biggest factor for me is that even the best Gens soft mounted in good sound enclosures with a wet exhaust separator still set up harmonics (especially in a steel hull) so you will always feel it, if not hear it.

I like my quiet and only would use the Gen on a hot steamy night, when bugs or hard rain close everything up and we need the air con!

[GordMay]

Originally Posted by Pelagic

... This is where we get into charge voltage settings of 29.8V during the bulk charge and is something not exactly clear to me, but I remember Rick explaining it on one Thread but haven’t found it…..Anybody know? ...

Rick has made several excellent contributions on battery charging, including (but not limited to):

How "fast" you can reliably charge your battery
http://www.cruisersforum.com/forums/...tery-1432.html

Understanding a model for battery charge acceptance
http://www.cruisersforum.com/forums/...ance-1442.html

Search Rick’s profile for many other expert contributions:
http://www.cruisersforum.com/forums/...earchid=374166

[Jon D]

Originally Posted by Pelagic

Great Input from a number of sources with a couple of key points we need to agree on before continuing as it will impact on charging solutions.

2/..Batteries do not need to be fully recharged after every discharge. It is acceptable to recharge to only 80-90% as long as you do a full recharge at least once a month for Wet batteries. From what I have read Paul, that is an accepted practice by AGM battery manufacturers because as you and GreatKetch pointed out before, once you get to that absorption phase of the last 10-20%, that is when you put in very little amps for the generator time spent….so why bother and just increase battery capacity by 20% to avoid that issue. That is the philosophy I am exploring when calculating capacity

Jon D has in my opinion described a real scenario of a live aboard cruiser enjoying his creature comforts, with multiple charge options for large Ah deficits. Also his charge profile for Wet batteries that confirms my #2 (I think he meant 75 to 85%)

So for the sake of this discussion if we use JD’s numbers and say 1000Ah with 200Ah deficit in 24 hours that works out to 5 times battery Capacity

Paul has suggested 4 times, which would be 800Ah

If we compare Life cycles (at the 80% Start)

Paul 4x =800Ah bank Start at 640 -200ah/24 =440Ah remaining = 45% DOD cycle

Jon D 5x = 1000Ah bank Start at 800 -200ah/24 =600Ah remaining = 40% DOD cycle

Theory 8x = 1600Ah bank Start at 1280 -200ah/24 =1080 remaining = 32.5% DOD cycle


Also as Btrayfors notes it gives you charging flexibility and emergency reserve?

What am I missing here?

Actually I did mean 50-85%. On real deep cycle wet cells they are designed to go down to 50% and not significantly impact life cycle. In normal practice we lived between 65-85% cause I ran the genset to make hot water etc. every day or so.

I spent a lot of time talking with the Surette/Rolls folks and my conclusions after those conversations was they work better with active charge/discharge cycles and last longer. Also full charge 1X week not per month greatly extends life cycle. Same with equalizing 1X per month using a hydrometer etc.

Also the comment about how do you use the power??
Refrigeration in the tropics, pc usage, lights, radio, watermaker, pressure water, cooking etc all add up quickly. At full output our refer burns 8-9 amps. Cycles on/off and on avg burns <100 amp/hrs in Maine for a day but in FL in the summer 150 or so. Running the laptop is 7 amps or so... Watermaker 22 amps. If you are living with comfort you need the power, camping less so.

Bill - that's the reason for the dual alternators. If I have a failure on the primary, it's a switch to engage the secondary to all banks. Only had to do it once out cruising where a pressure switch caused a failure in the regulator on/off.

[camaraderie]

Pelagic...I think you have a good handle on the tradeoffs but I would caution you about using fullriver's dod chart and assuming it is accurate across all designs and mfrs.
Certainly the Odyssey batteries and the "soon to be released" fireflys have quite different properties bot in charge acceptance and DOD capabilities which might cause you to rethink this entire exercise.
For exaple...if there were a battery that would provide the same life cycles as a fullriver BUT do so at 100% discharge...then you might consider just a 200amphour battery bank. We are not quite there yet but you get the point I hope. Things are changing!

FWIW...I have 1100amphours of AGM's serviced by a 135amp charger and we had house use of typically 150ah's a day. Worked out well and we would charge for a couple of hours every other day with the genny. Topped off once every 2 weeks...usually when we went to a dock...but occassionally when we needed the AC running anyway!

[Roy M]

Therapy, this isn't B.S. My neighbors, who had installed these units years ago, still ask me when they should change their battery banks. Either we are living in some space warp where the laws of physics are inverted, or possibly, the concept works as described, still according to standard physics and electrochemistry. It is very difficult for folks to accept new technology (even when it's been around for almost a couple decades). "Lead-acid batteries only lasted 3-4 years for my grandfather, and that's all they can last today......". It might be worth a small trial examination. That's how I started using them, on discarded batteries at my yacht club. When the electrolyte returned to a density of 12.75-12.8, that comes only from sulfates being reintroduced into solution after having been plated out on the lead. If it were bogus, why would the equalization cycle be a standard part of modern battery charging? These things accomplish the same result without heating up the plates with higher voltage and current, and potentially damaging them from heat and warping. Just apply a teeny bit more oomph (voltage) to the returning electrons and they can make the jump into the outer valence ring and float off into the electrolyte with a smile (ion humor).

[Pelagic]

Thanks Gord (again!)

This is what I was looking for in one of Rick's papers

"No, I am not suggesting a higher voltage during bulk charging because, by definition, bulk charging is current limited with an ever increasing voltage until the voltage limits out to the acceptance value (which I DO claim can be higher than 14.4V @ 70 deg. F)."


My Fullriver batteries state that you can use an acceptance value up to 14.9V or 29.8 for 24V

What I am trying to understand is; Does this higher voltage acceptance mean you can achieve a deeper re-charge quicker?

[Therapy]

Originally Posted by Roy M

Therapy, this isn't B.S. My neighbors, who had installed these units years ago, still ask me when they should change their battery banks. Either we are living in some space warp where the laws of physics are inverted, or possibly, the concept works as described, still according to standard physics and electrochemistry. It is very difficult for folks to accept new technology (even when it's been around for almost a couple decades). "Lead-acid batteries only lasted 3-4 years for my grandfather, and that's all they can last today......". It might be worth a small trial examination. That's how I started using them, on discarded batteries at my yacht club. When the electrolyte returned to a density of 12.75-12.8, that comes only from sulfates being reintroduced into solution after having been plated out on the lead. If it were bogus, why would the equalization cycle be a standard part of modern battery charging? These things accomplish the same result without heating up the plates with higher voltage and current, and potentially damaging them from heat and warping. Just apply a teeny bit more oomph (voltage) to the returning electrons and they can make the jump into the outer valence ring and float off into the electrolyte with a smile (ion humor).

I am open minded and know most do not believe in the pulse technology. I have only one small AGM for use with my Jon boat at the moment. It cannot be charged fully any more (cheap charger too). It may just be the way the website appears.
I don't see whey it would not work though.

[Rick]

A.
The 14 year battery longevity revealed by ROY M is not unusual at all yet it may be a mistake to conclude that any pulse-type charger is the reason. Tests have shown for decades (since before 1930) that 20 year lead-acid longevity with deep-discharge cycling can be achieved only if a proper acceptance voltage is reached during charging for a sufficiently long time. Note that the equipment that ROY M is stated to use does have an acceptance cycle. Tests using so-called desulfation pulse chargers without having a sufficiently high acceptance charge voltage on recharge do not result in anything other than short longevity. I claim that if ROY M removes his pulse charger he will not notice any difference as long as he keeps using a good acceptance cycle. I realize that this subject is almost like arguing religion.
B.
The Amp-hour-law defines the safe charging current that may by used to recover a deep-discharge battery all the way up to 100%. If a large bank is discharged 200 Amp-hours representing only a 5% depth-of-discharge then it can charge accept 200 Amps (initially) with a decreasing charge current (along the curve of epsilon) all the way up to zero Amps at 100% state of charge. The Amp-hour-law removes any conjecture or rules-of-thumb for safe charge currents that may be used for any lead-acid battery (yes AGM and gel-cells are lead-acid, just not flooded).

C.
Stratification is not a factor with AGM and gel-cell batteries.

D.
The 50% maximum depth-of-discharge limit is an economic optimum in terms of the cost per unit of energy derived over the life of the battery. Yes, some particular batteries may exhibit a different value in terms of that optimum yet it is a relatively valid "target" when attempting to minimise the battery bank size for a given Amp-hour cyclical design.

E.
The minimum acceptance voltage for flooded lead-acid batteries is 14.4V @ 20 deg C. This is a minimum, not maximum. If the Amp-hour law charging is used one may notice a practical value for charging to be in the rane of 14.7 to 15V for flooded, AGM and gel-cell batteries.

[Pelagic]

Thank Rick for confirming some key points as it helps me understand each piece of the puzzle.

Actual scenarios help me even better, so would appreciate your comment on this exercise:

1/…That this is a cyclic cruising scenario, mostly on the hook, where recharging is done by running the generator to power one or two large battery chargers, and the goal is to minimize generator hours by putting bulk amps in as fast as is practical until acceptance voltage is reached. …That we have intelligent re-charging and good battery monitoring capabilities

Q1…Does this favor setting voltage level to maximum allowable by manufacturer (12.9 for Fullriver AGM DC or 29.8V for 24v)?

2/…That we use a House Bank of AGM’s so that “Stratification” is not an issue.

Q2…Does this favor recharging only up to where the return on Gen recharging declines significantly (say 75 to 80% SOC)?

3/..Assume: We have enough space and size that minimizing Battery Capacity is not an issue and initial purchase cost is secondary:

Q3 what is the optimum size of Battery Bank? 3x, 4x, 5x, 8x? … for an average 200Ah deficit between recharges for this kind of application?

Are there any other issues to consider?

Thanks for any comments

[camaraderie]

Rick...with respect...I am going to disagree with you on point B above.
The newer AGM technologies place no such limits on charging amperage.
Nigel Calder had verified that at 50% DOD, the Odysseys can be safely recharged at SIX times their rated 20hr. capacity. To be clear...a 100ah group 31 can be charged at 600 amps!! Ten minutes and you are recharged if you can find a 600amp source!
My little 135amp charger would put back the missing 50ah's in less than 30 minutes!

Firefly with their foam technology looks for even better performance but has no consumer products just yet...just opened a new plant to supply the trucking industry and the military which will be their first customers. Their batteries are also HALF the weight!
All of this and more is covered by Calder in the Feb/Mar 08 of Professional Boatbuiilder. Professional BoatBuilder - February/March 2008
Your further thoughts after looking at this would be appreciated.

I know that if I were putting together a cruising charging/battery system today, I would be thinking pretty differently than I did just a few years ago.

[GordMay]

Originally Posted by Rick

D. The 50% maximum depth-of-discharge limit is an economic optimum in terms of the cost per unit of energy derived over the life of the battery. Yes, some particular batteries may exhibit a different value in terms of that optimum yet it is a relatively valid "target" when attempting to minimise the battery bank size for a given Amp-hour cyclical design.

RICK: Please Comment

According to Glacier Bay*< Glacier Bay - Support >

The Typical Battery Cycle Life (100 amp/hr Trojan Deep Cycle Battery)

Depth of discharge x Number of cycles = Total Amp-Hours provided
during service life

10% DoD (100 A-H x 0.10) x 6,200 cycles = 62,000 Amp-Hours (lifetime)
20% x 5,200 = 104,000
30% x 4,400 = 132,000
40% x 3,700 = 148,000
50% x 2,900 = 145,000
60% x 2,400 = 144,000
70% x 2,000 = 140,000
80% x 1,700 = 136,000

Suggesting that much higher depths of discharge (40 - 80% discharge) provide a higher battery energy output over a “life-time” than the lower DoD’s (30 - 50% discharge) typically recommended.

* I’m unable to independantly confirm these figures on Trojan’s website.

[Rick]

Gord,
The Glacier Bay "formula" is overly simplistic in that too many other missing factors affect the accuracy. The major missing factor is the average time spent in the average state of discharge. Also an assumption is made that charging begins almost immediately when the maximum DOD is reached and that the cycles are repeated indentically. Average temperature and full state of charge electrolyte specific gravity are also missing. It is assumed that the charging regimen is "proper" for recovering lost capacity on a cycle-by-cycle basis.

Camaradarie,
I don't disagree with your comments regarding the fact that some lead-acid batteries can be recharged much faster than what the Amp-hour law states. The Amp-hour "law" is of course not actually a law as rigourous as Lenz's Law, for example, it is the statement of a formula which, when followed, guarantees that the battery will not gas excessively or overheat excessively. Consider this; using a conventional 3-step charging regimen having an acceptance voltage limit of 14.4V a dead battery may be recharged in 5 hours or more. Using a higher acceptance limit of 15V the same 3-step process may yield a dead-to-full time in about 3.5 hours, a value close to using a charger that can replicate the Amp-hour law regimen. To be sure, Hawker batteries have been recharged in 45 minutes yet even they will live longer if 3 hours are used. Long has it been known that both AGM and gel-cell batteries can be recharged at xC, where C is the rated capacity and x is an integer between 1 and about 6 or so. The larger the integer the lower the expected lifetime, especially in warm environs.

Pelagic,
It is not a particular battery manufacturer's electrochemist who advertises an upper voltage limit it is someone concerned with making good on warranty returns. They are not famaliar with the fact that 3-step charging is merely a first approximation to the Amp-hour law. As Comaraderie correctly pointed out some manufacturers are aware that fast charging can be accommodated if done with care. I have charged Fullriver batteries at 15V many times over several years and love the results. Obviously one must apply a float voltage at an appropriate time.

What is frustrating is that affordable 300Amp and above chargers are not available (yet) to minimize genset run times yet these chargers WILL come. Switch-mode charger designs are going to make that difference.

AGM and gel-cel batteries can easily be recharged to at least 90% state-of-charge with reasonably short genset runtimes if you use elevated acceptance voltages (at least 14.65V).

For your case of a 200 Amp-hour cyclical use I recommend that a 600 Amp-hour AGM will be close to "optimum" for you. This includes some "slop-factor" for realistic aging. There are a few good AGM brands which will actually deliver on their advertised capacities. Keep in mind that for using lights, bilge pump, microwave oven, etc, that uses an actual 100 Amp-hours with a flooded battery will use LESS than that with an AGM (banks of same Amp-hour rating for comparison) because the true energy delivered by the AGM will be MORE for 100 Amp-hours than the true energy delivered by a flooded battery delivering 100 Amp-hours. This is because the AGM will deliver that energy at a higher voltage for the same current. Another considerably important issue to consider is that AGM and gel-cell batteries deliver current "through" a much lower internal cell resistance at all levels of current. This means that a microwave oven might cook a baked potato in 8 minutes with an AGM and 10 minutes with a flooded-cell battery.

Regards,
Rick

[Pelagic]

Very interesting numbers Gord.

But then how do we apply that to our scenario of TBD Ah House bank with daily 200Ah Deficit and the reality that cruisers rarely re-charge back up to 100% if living onboard at anchor?

Does it make more sense to let the batteries go from say DOD 20% to 40% before recharging back to 20% ?

Or do we accept the 40% differential as optimum and go with a 20% to 60% re-charging profile?
I cant quite figure out the math his morning

10% DoD (100 A-H x 0.10) x 6,200 cycles = 62,000 Amp-Hours (lifetime)
20% x 5,200 = 104,000
30% x 4,400 = 132,000
40% x 3,700 = 148,000
50% x 2,900 = 145,000
60% x 2,400 = 144,000
70% x 2,000 = 140,000
80% x 1,700 = 136,000

[forsailbyowner]

We spend most of our time at anchor. Our alt energy usually gets the voltage up to 14+ volts on a good sunny day. If we have some cloudy days in a row we simply get a couple of bags of ice and shut the fridge off for a while. Since we also cruise we end up running the engine often enough to make up any extra amps needed. If the gravity gets low I do an equalize charge and use Ice instead of reefer for a while and its seemed to work well so far. Not so scientific but it works for us.