In system design, there are a couple of engineering terms, "cohesion" which refers to the degree to which elements of a module belong together and "coupling", the degree by which a system module depends on each one of the other modules of the system. Ultimately one works towards architecting a system whereby the modules are highly cohesive, meaning the components of each system are highly related to one another; and loosely coupled, in other words, each system module can do it's job independently of the others. If a defect is found in one module it's impact is cushioned behind it's own system boundaries. This simplifies the complexity of the sytem as a whole, which simplifies maintainability as a side benefit among other things.
I'm relatively new to boating, having been an owner for two years, first a cabin cruiser and for just over a year now a 36 foot sailboat which my family and I have been living aboard and refitting for the past 6 months. We're in the later stages of converting to electric with an Electric Yacht 360ibl and a 48v AGM bank. As I've been devouring all the information I can find on the various systems on board one thing strikes me as odd: it appears all of the literature proposes a tight coupling of all of the vessels system with a single battery bank with a separate bank occassionally waiting in reserve. The vessel's plumbing, navigation, safety, propulsion, etc are all tied to a single point of failure, the battery bank, either primary or reserve. Many of the systems have non-electric manual backups for redundancy, manual water pumps, for example. For those of us moving to electric propulsion it seems we are still proposing a similar setup for our house loads, either running it directly off the propulsion bank, or in some cases charging the house bank from the propulsion bank. This remains analagous to charging from the alternator of an ICE. In other words, our gas tank is providing the charge for the house bank and loads. To refill our metaphoric gas tank when away from shore, we use wind, solar, to a lesser extent water regen and the occasional ICE generator to top it off but the tank still provides both propulsion and electrical capabilities to the overall system.
Given my concerns about range once we start cruising full time, it just feels wrong tapping into my propulsion system to power and charge everything else. Having multiple systems draining my bank which contains precious energy which I may need to power away from weather or in rough seas goes against my logic. Any shorted circuits introducing parasitic battery loads which go unnoticed would affect the power source of all systems and reduce immediate propulsion range since everything is tightly coupled to the battery bank. This setup was a no brainer when energy came from cheap petrol. Now with wind, solar and water regen while off the grid, refilling the tank will take longer and be less predictable than grabbing a spare jerry can of fuel.
I'm contemplating refactoring the overall system to have a small low Ah AGM tied to each cohesive system based on it's consumption needs which are already currently segregated by circuit in the electrical panel (nav, safety, instruments, etc). I suppose I'd need multiple battery switches to be ABYC compliant and since all of my charging methods will be 48v, wiring won't need to be too large to accomodate voltage drop over the smaller round trip distances of my 36 footer from the solar and wind chargers or I could have a small panel near each system's battery. However I would need to feed all the charging sources into one or more chargers capable of handling multiple banks. Alternatively for some small batteries, a small trickle solar panel would suffice which would make the circuit completely independent while still retaining the ability to tap into another system's battery or charging source if the critical need arose. I could tap into another battery if need be should a particular system's battery fail, or I could just leave it be until I replace the battery in some cases (for example I won't need the electric head or windlass while I'm under way). For these simple systems I won't have to worry about a bad battery in series contaminating the rest of the overall battery bank. And my propulsion bank can remain used only for propulsion. I believe it would be cheaper to replace a small AGM if needed instead of a large one used as a series component of my propulsion bank. And because of the reduced capacity of each battery, charging will be quicker and chargers should be cheaper and thus easier to replace and maintain an inventory of backups.
This has been a thinking out loud post and if you've read this far through my ramblings I do have some questions:
Anything obviously wrong with attempting this design pattern?
Any recommendations for chargers for multiple small banks?
Is there a clean way to combine multiple 48v charging sources into a module of multiple-bank capable chargers? What might that look like?
Anybody done something like this who can share their experience?
Thanks for reading and good luck with your own projects!
Roland
s/v Miss Teak
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