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Old 03-10-2008, 04:26   #1
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Battery Capacity-What is Too Big?

Over the last few months I upgraded my DC system, reading a lot about batteries and charging profiles, struggled to clarify issues that confused me, then when I read back over the many forums, I saw a number of contradictory statements that I feel need to be agreed upon by the more experienced of our fraternity, so that we can help others understand better. (I am also curious if I got it right?)

I hope Rick can guide us thru this but I thought we should first discuss the reality of Cruiser’s use and basic statements from what I have read. (Victron Book) to see if everyone agrees. …(Please use numbers to Agree/disagree or clarify further)


1/..Cruiser’s use has been called “irregular” variation of cyclic where battery bank is charged once or twice a day to recharge but is too brief for full charge unless the yacht motor sails or returns to marina and is plugged in

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.

3/..Stratification
due to only partial charge (#2 above) is a problem specific to Wet batteries with liquid electrolyte and not a problem with Gel or AGM. Guideline with Wet is fully charge after 30 cycles to avoid stratification problems.

4/..Real Working battery capacity
can be calculated for cruisers cycle as Start at 20% depth of discharge (DOD) …maximum DOD should not be more than 70% so (70%-20% = 50%) but with no safety margin so apply 10% leaves us with 40% useable capacity and a DOD of 60%. Then we need to build-in a factor of 0.8 to account for 20% capacity loss when battery ages so 40% x 0.8 = 32%

5/..This means that the battery capacity should be at least 3 times the expected discharge during generator free period.

6/..The higher DOD% a battery experiences the less life cycles it will have. If you look at this graph: http://www.fullriver.com/products/ad...Ccyclelife.pdf you will see that with the above 60% DOD of battery bank you have only 1/3rd the lifecycle as opposed to a battery that only experienced 30% cyclic DOD


Q-1 So if we accept all of the above (or clarify if you don’t) then… what is the optimum battery capacity compared to expected discharge cycle that one should consider to maximize house bank battery life and provide an optimum charging regime for a Cruiser’s cycle?


Minimum 3 times? Or 4 times? 5 times? Or the bigger the better?

For charging purposes and discussion lets use AGM batteries as an example, since I have good factory input about charge voltages and max amps. I can also easily verify with the manufacturer any specific questions that may arise and am busy testing and recording my charging parameters this month.
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Old 03-10-2008, 05:28   #2
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It's like most things as I see it and no one rule works. You really do want to fully recharge when you recharge that is true. The fact that most peopple don't / can't or don't realize it is part of the trade off. If you say go 5 times required then you almost assure you'll be farther away from being recharged without pulgging in regualrly. If you discharge below 50% then you are eating away lifespan. The extra large bank dies faster than the smaller bank assuming you could have supplied the power from the smaller bank. More batteries in a state of dischrge is not an advantage.

I would say you need to have a working avilability of 40% total capacity. That just accepts you normally don't get back above 90% easily. I think having too large abank is mostly money down a rat hole. With the price of AGM batteries this year well into the the painful spectrum you need to get it just right.

As you get into larger banks I also think segmentation becomes the escape to the problem. It's easier to keep a spare bank topped up and then switch it over as the other bank drops down. It depends on how much power you use and how much you can make to recharge. You may over time get the switched out bank back closer to 100% through more charging cycles. This all means you need to have more charging capacity so even that appoach isn't as bright and rosey as it may sound either. A larger AGM bank needs a larger charging source.

Using a battery monitor is where you can do your own research on your own system and see what the trends are telling you.
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Old 03-10-2008, 05:40   #3
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I think you've hit the nail on the head for the battery dilemma. The more batteries you have, the larger the charging source required when at anchor. The ultimate large-charging source is plugging in to the land-based power grid. However, if you're plugging into a land-based power grid, you don't need all of those batteries in the first place! It's quite the conundrum. With all due respect to your cruising-style, you might consider reducing your power requirements and the problem just goes away by itself.

Good-luck, Mate!
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Old 03-10-2008, 05:59   #4
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Quote:
Originally Posted by Pelagic View Post
......
1/..Cruiser’s use has been called “irregular” variation of cyclic where battery bank is charged once or twice a day to recharge but is too brief for full charge unless the yacht motor sails or returns to marina and is plugged in

........
At the risk of stating the obvious, I suggest that as there is no battery really designed for "irregular" variation of cyclic (sic), then using existing battery technology will always be a trade off of one aspect or another even before we fit it.

In fact it is worse than that, because even if we can do everything "right" for a particular battery type, it will still deteriorate and supply less and less until it requires replacing.

So we are faced with a trade off to begin with, then we have to trade off more because it is unlikely we can optimally maintain the battery in a cruising mode.

Almost makes you want to return to oil lamps, sextant and if you must, a hand started diesel.

Apart from that, I hadn't realized that stratification (#3) applied more to FLA than Gel / AGM. I had thought they all suffered equally in this respect.
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Old 03-10-2008, 08:23   #5
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Quote:
you might consider reducing your power requirements and the problem just goes away by itself.
At low power levels wind and solar become quite real sources of power. When you have a large need for power these alternative sources can't do the job well at all. That leaves engine driven alternators to make up the gap. Once you are doing that then you might as well forget alternative sources except in times of passages where you might go without refrigeration and be able to run the boat with alternatives alone.

There are multiple scenarios like that where because of what you are doing your power use can change and thus open up more alternatives. If your power consumption is always the same then it's easy to compute when you will run out of battery and you require a much more regular charging program.
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Old 03-10-2008, 08:28   #6
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I imagine the following would might also affect the size of batter bank you choose.

- charging system output (non-shore power, i.e. alternator). Dont want a bank so big that it a long time to charge from the alternator.

- number of physical cells. Constructing a bank out of paralell 8Ds instead of series/paralell golf carts might be better. If one battery goes bad or is 'marginal' the whole bank could be pulled down to lesser efficiency. With an 8D system (in this example) failure of one 8D might be more obvious than the same capacity made of golf carts. Though space constraints usually dictates battery size/configuration choice, but if you have the space...
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Old 03-10-2008, 08:38   #7
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IMHO bigger is better given the trade offs of charging, weight and use. For Example on avg most full time cruisers we talked with burn about 200-250 Ahrs/day [12v] [not budget cruisers folks with boats 40-55 ft or so]. Given that almost all carried somewhere between 750 - 1100 amp/hrs of capacity. All had multiple charge sources.

As an example we carrier 820 Ahrs in a Rolls bank. To charge it the main engine alternator is 270 amps controlled down to 220 with a Link 2000R. Trust me when we need to put amps back this does it. We also have a second alternator [135 amp] on the engine ready to take over should we have a failure[it normally tops off the start bank]. Then we also have a 100 amp charger which is driven off the generator. So for us I do bulk charge on the engine till output is less than 100 amps then turn on the genset and do the rest to maximize fuel burn. FWIW we ran between 5-85% for week then took up to full charge 1X/week and equalized 1x/month to maximize performance and battery life.

On a big boat there numbers can get larger but then you are probably running 24v which changes all the numbers.
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Old 03-10-2008, 08:44   #8
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While I agree with much/most of the above, there is another consideration: battery capacity = longevity when no charging is available.

I awoke at 0430 yesterday morning, anchored some 42 miles downriver from my home berth in Washington, DC. Plan was to catch the morning tide and carry it all the way home.

Started the engine at 0435. For the first time ever, my Balmar 100A alternator and MC-612 regulator failed to put out a charge....any charge.

It was dark. Cold (45F). I needed to get going to catch the tide. Battery voltage was 12.4 under load of anchor light and frig....less than 25% depletion of a 675AH Trojan house battery bank.

I quickly did some easy troubleshooting of the regulator which is located in my aft cabin and easily reachable. Nothing amiss. Further troubleshooting would require moving heavy stuff from a side locker (anchors, etc.), and crawling into the engine space to get at the alternator itself. No way, Jose.

I needed to get going. Did a quick calculation....I would need the following at a minimum:

a. running lights (sidelights, sternlight, steaming light) for 2 hours before dawn;

b. compass light, two GPSs, laptop w/charting application -- Offshore Navigator; and

c. bilge pumps.

If necessary, I could cut off the frig (saving 6A), and didn't have to use the autopilot.

At 0510 I got underway in the dark, after clearing a 15' long pole from the anchor and chain.

Watched the voltage closely.

No problems at all. Actually, after 2 hours underway the voltage was showing a bit higher, alternating between 12.4 and 12.5 on the digital voltmeter!

Could have gone all the way with this arrangement, if need be.

OK, I had a backup....an onboard generator which could charge the batteries at up to 100A rate through a Victron MultiPlus inverter/charger. And, later on, just for the heck of it, I did charge them for 45 mins or so.

But, the point is, that setup with some minimal on-the-fly conservation (no frig, autopilot, radar, running lights, etc.) could EASILY have taken me the 6 hours to home. As others have said, the bigger the better...up to a point!

In the event, a brisk and building wind helped most of the way. My masthead anemometer recorded 33 knots off Rosier Bluff south of the Wilson Bridge. Taking spray over the bow in the narrow Potomac is a real kick in the butt :-)

Bill
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Old 03-10-2008, 08:48   #9
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I think you've hit the nail on the head for the battery dilemma. The more batteries you have, the larger the charging source required when at anchor.
That's actually not true. The charging system needs to replace the power you use. If you had a very large (ok HUGE) battery bank and only cycled it between 95% and 95% full you would actually not be able to use a large charging system because the batteries wouldn't accept a high charge rate when they are that full. Even if you make the bad decision to discharge the huge bank a lot, having a smaller charging system only means you have to run it longer to charge the batteies.

In the simple world, there is no question that if you want a longer life span, the larger the battery the better. Of course battereis are heavy and expensive which limits the practical size.
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Old 03-10-2008, 09:48   #10
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In my case, the weight-carrying capacity of a multihull helps resolve the dilemma. As with an airplane, I can only devote so much to the operational needs of my craft, and try to maximize the potential for crew, provisions, spares and toys. That said, I have tried to save weight in areas such as materials, anchoring systems, water storage strategies, non-wind propulsion, refrigeration and similar factors. Those things have base levels of electrical need, fixing the minimum battery capacity I can afford to carry. Having low weight alternative generation of electricity is another factor that enters into my considerations. It is a sort of blessing to have these constraints since it avoids the likelihood of being seduced into carrying bigger, heavier loads that other designs might be able to accomodate (at, what I consider, a loss in performance for my expectations). I have a forty-foot trimaran, weighing about 12,000 pounds, loaded for local (not long-range) cruising. I use two identical banks of series-connected 6 volt Surrette 4D flooded cell batteries, and a Balmar alternator designed for my Yanmar 3GM30F engine (about 75 amps). I am waiting until the last moment to purchase the latest generation of photovoltaic panels and the least irritating wind power system. My regulator is a multi-stage unit, all wiring is new and sized for the projected maximum loads to reduce voltage drop and waste heat. The freezer/reefer is super-insulated with a high capacity compressor and coldplate to reduce run times to a minimum. Most lighting units are LED or cold cathode flourescent. And I have used PulseTech battery conditioners for many years to reduce sulfation, resulting in my last set of batteries lasting over fourteen years.
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Old 03-10-2008, 14:24   #11
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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

The forward thinking that RoyM discusses for low weight solutions and intelligent efficient consumers is great and is the future that we all aspire to, but in many of our (monohull) cases, we have older less efficient consumers that will eventually be swapped out for better ones… Luckily (for us dinosaurs) weight is not a critical issue so it is back to battery capacity and charging profile.

Wotname, your point is well taken about aging and diminished capacity of batteries which is why Victron in their book advise a 20% allowance for this factor, when computing initial 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

JohnD 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

When you look at the Fullriver Life cycle graph again: http://www.fullriver.com/products/ad...Ccyclelife.pdf you see that for double the cost of going from 4x to 8x you get about 3 times the Life cycle.

If space, weight and initial outlay is not an issue and you have an intelligent charging system suitable for the AGMs does this not make economic sense to go with as big as you can up to 8x?

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

What am I missing here?
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Old 03-10-2008, 15:11   #12
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It depends on your situation, for us we were able to half our battery bank by installing a large (700 watts) solar array. We are then simply compensating for night time draw or approximately 70 - 90 amps instead of the typical 210-220, and each day we get 100% charged on the batteries by 11:30am on a sunny day, it could be well into the late afternoon on a cloudy day. It also means we're typically at float charging for up to 7 hours. We have 2 100 amp alternators on our two 3JH3E engines just in case. This allows us to operate a 130L refrigerator, 110L freezer and lights, laptop, etc. All lighting is either flourescent or LED. Our battery bank is 230 Amp hours with golf cart batteries.
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Old 03-10-2008, 16:09   #13
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And I have used PulseTech battery conditioners for many years to reduce sulfation, resulting in my last set of batteries lasting over fourteen years.
WOW!!!!!!!!!!!!!!
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Old 03-10-2008, 21:57   #14
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Wow!

Reading wht people consider normal for electrical consumption ( 200 AH or more) per day leaves me a bit speechless.

I have a 40 foot boat and consider myself incredibly profligate if I "spend" 100 AH in a day. We are on very different planets as far as usage is concerned and maybe this isn't the thread I should be giving advice on!

I can't even think of what kind of toys I would WANT to add that would use that much power.
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Old 03-10-2008, 22:45   #15
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Your right Greatketch, it does seem large, but a mono live aboard in the 60ft range (50ft cat) will have all the amenities, fridges/Freezers/icemaker, electric kettle, microwave, larger stereo system, office equipment as well as all the Nav-com stuff and autopilot.

Not much different than a small house or apartment… so it can be a big load!

Larger boats are then needing 5kWh per day and usually switch to a 24v system.

Depending if the watermaker and dishwasher are added to a washing machine it can climb up to 12kWh per day and can get a bit silly

Usually around 400Ah at 24v or 800Ah at 12v and a 12Kw Genset to recharge everything.

Solar panels wont easily keep up with that, so it usually is up to an onboard gen to recharge at anchor and a large case alternator when motorsailing.

I don't believe in roughing it when living on a sailboat, so i like to be self contained away from marinas and on the hook with the blender and lots of ice for those sunsets.

The gen goes on in the morning when we are doing all our chores and then that is it till next morning. Hence the large house Bank
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