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Old 14-05-2023, 06:24   #61
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Re: Battery Bank Size?

Quote:
Originally Posted by Dockhead View Post
This is all very good theory, but . . .

sizing of the battery bank is not actually a function of the capacity of charging sources. They are really kind of separate problems, and if anything, I think one would be more likely to make changes to charging sources to suit possible battery bank size, and not the other way around.

In my opinion, the battery bank should be as big as reasonably possible and practical. Other than cost and weight, a larger bank is all good. I don't think there is an "optimum" size, other than bigger.

If your charging sources can't put in an efficient amount of power (at least 10%, better 15% of C), then they should be increased. It's relatively cheap and simple to install a larger alternator, or better, a second alternator. A large-frame school bus alternator is a tremendously valuable charging source, and they come in very large sizes, large enough to be 10% of any imaginable battery bank (actually, since main engine alternators often work at speeds where they produce less than all of their output, you generally want to size them bigger than 10% of C). Of course you have to take care with the belt drives (double belts are required on the bigger ones) and might have to change some pulleys on the engine.

If you don't have an alternator that can put out enough power to feed a battery bank which you could otherwise afford in terms of weight and cost, then you should change it. That means it's a bottleneck in your system. 5 hours a day of running your main engine is ridiculous, in my opinion. That would not be pleasant cruising. That's just a too small alternator.

To give you an example from my usage -- I have a 110 amps (* 24v) school bus alternator, which is theoretically more than 25% of C for me. If I run the engine at a fast idle, I reckon I'm getting 80 amps (has a very steep charge curve -- made for school busses ), which is a bit less than 20%. And indeed, as the theory would suggest, I can get my batts from 50% to 80% in less than two hours of fast idling, whereupon I have about 100 amp/hours of juice to play with until the next chargin session. I stop at about 80% when I'm off-grid, because the batteries start accepting less and less power, so run time versus stored power falls off.

Remember it takes a combination of charging power and bank capacity to store a given amount power in a given amount of time -- you are limited by the lesser of acceptance rate of your bank or the charging capacity of your charging source. So to reduce your charging sessions to tolerable periods of time -- and I suggest that 5 hours is way too much, especially if you're using the main engine -- you need to consider both of these factors.

If you're using a generator, there is another factor to consider -- charging batteries can be combined with other activities which require the use of the generator. Hence a lot of cruisers will try to group together their AC power tasks and do it all at once, and the batteries are getting charged incidentally at the same time. This improves the efficiency of the whole process, and also, if you have a lengthy job requiring AC power, like washing and drying clothes, you can take the chance to get your batts above 80% for once, which they need from time to time for proper health. Obviously you also need a large enough battery charger to use your generator runs effectively -- better between 15% and 20% of C (I have 17%). And a large enough generator to run that charger. My charger at 70 amps and 24 volts nominal, needs about 2.5kW of input power when it's charging at maximum rate.

My charger takes about two hours to get the batts to 80% from 50%; then another hour to get them from 80% to absorption phase, 85% or 90%.
Hi . Just reading through this old thread, and imho it's worth adding some information about Treating good quality AGM Batteries (e.g. 99.9% pure Lead Plates) well enough to only discharge them to a certain Discharge level, such that you get 3500 recharge Cycles Flat lining, and below which, you start losing Recharge Cycles. That Critical State of Charge not to go elow, is 80% if Capacity. So the issue then becomes. if you have say 800 Watts of Solar in a Sunny place, and for getting hot water for Showers /Washing up, plus decent alternator output say for an hour before Dawn, with the auto pilot on when making Breakfast, and an hour after Sunset making the evening Meal, 20% of what Bank size do you need to keep above 80% State of Charge during the hours of Darkness? Since I found out about this giving an absolute minimum of 20 years AGM Service Life (confirmed by someone who had the batteries damaged by accident after 20 plus years ) I've ditched all Plans to Test Lifepo4 Batteries I was offered for free to test, as right now there is no Data available to set up a proper Lifepo4 proper Regime for a long Service Life, other than Discharge below 50% and you're losing recharge Cycles, and add in - Mobile Phone and Tablet practice of not charging above 80% to greatly extend the Life of the Battery ! Does that apply to Lifepo4 batteries too ? Maybe, is about the best answer I can presently get. Lead likes being charged to 100%, and to get back there ASAP. using a max of 20% keeps them healthy. Lifepo4 Healthy Capacity bandwidth looks to be 30% of Bank capacity as possible "Healthy usage Bandwidth which is 50% more than AGM, but Maybe, that is too optimistic? but time will tell just how to treat Lifepo4 right. I'm not prepared to be a .lab Ratvattempting to determine that , even with a free Battery Bank. If I'm in the Middle of the Ocean, well I want Tried and tested thanks, plus hot water from the heat exchanger etc, as a bonus from running the engine for 2 x 1 hour runs, and perhaps a Quiet Wind Generator to help while at Anchor. Hopefully it's a bit of food for thought . Best Wishes and Fair Winds. 👍⛵️✨️✨️✨️
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