One last post from me to this thread. I believe these forums serve as a good long-term repository of useful information for readers in the future. So in that spirit, I'll clear up a couple of things and bid my goodbye. I don't care to argue with zealots and trolls. This post will be a bit long. You've been warned. But I feel the need to make some things clear for posterity because there are people on this thread who delight in dogmatic bluster.
TLDR;
- First, don't install a rocker switch where you might kick or otherwise bump it (see below).
- Next, there's absolutely nothing wrong with the design of the DCP switch from Blue Sea Systems. Despite the deranged ranting of internet zealots who will tell you otherwise.
- You must understand the use case, priorities and complete system design in order to fairly evaluate the appropriateness of individual components. Do this on a case-by-case basis. Otherwise you're at risk of being struck by a dogmatic cudgel.
Okay, on to the long form summation for MY USE CASE. YMMV. Read on if you dare.
First, I'll admit that I misstated
how the ACR logic is applied to the open/close of the single relay (I'll go back and correct that post). But I maintain that the I fully understood the end result: Through the comparison of battery voltages, and the sensing of charge current vs. draw current (more on that in my post correction above)
the ACR manages the status of the relay in order to direct charging current where it's needed. I thought is was using something like an H-Bridge when in fact it's much simpler, but enough on that here, see my edits on previous post for details (which I'll correct and explain after this lengthy post).
Having just come from a full and enjoyable weekend on the boat, I can definitely say that my
original post about the
location of the switch was a flawed proposal. It's debatable whether the proposed location needlessly subjects it to potential corrosion/water damage (probably). But what's clear is that I would have likely hit it with my heel a half dozen times this weekend, while getting in and out of the berth. Heel strikes on a rocker switch/breaker are obviously undesirable, so I'll mount the DCP elsewhere. (The
panel in question has both a rotary DCP switch and a 100 amp breaker).
With that cleared up, I'll now address why the DCP is an excellent choice for
my boat and my usage. I took a brief look at Stu's comments here and on other platforms and see that he's a zealot with an axe to grind against a number of things that include this DCP. And he has a fetish for Battery Switches which I find Creepy and Weird as Hell.
More positively,
Scott T-Bird points out the switch can be a good choice for some people with the proviso that there's an understanding of the dangers of ganging a good bank with a "catastrophically damaged" bank. That's 100% correct and happens to be true of traditional A/B/Both switches as well. I have enjoyed a long and very successful career designing and operating broadcast television systems with resiliency and failure mode operations in mind, and I contend that in all but the worst and most urgent scenarios, users should have an
understanding of the nature of a failure BEFORE implementing a recovery step or work around. Anything else is just shooting arrows in the dark. In this case, the question becomes how to identify the nature of your battery failure. I won't get into that except to say that I log my voltages every time I open up or leave the boat (from the meter on my DC distro panel). And once I put in my new house batteries, I'll measure internal impedance using ohm meter at regular intervals and track that over time. Those are health indicators that will establish a baseline.
The DCP (Dual Circuit Plus switches from Blue Sea) is meant to maintain separation between House and Start banks. It happens to also provide a way to gang those together (thus the Plus) - think of that mode as an additive Y-formation - and caries a warning printed in that switch position. This Y-formation is no different from the BOTH position on a traditional A/B/Both. Which is to say, that if you're relying on that position on either type of switch, you'd better understand what conditions have prompted you to do so, and with it what risks you're facing if you gang the banks. Again, see Scott's comments above on that. Keep a multimeter on board and know how to use it.
The way the switches differ is that an A/B switch has the additional ability to direct
either A or B banks
independently to your load, whereas you can't do that with a DCP. This is a
feature of the DCP, not a bug. It certainly isn't applicable or appropriate for all boats or skippers. But it's definitely not a design shortcoming (as asserted by the zealot).
The traditional A/B/Both switch invites a 'try and see' approach to trouble shooting, which can be bad in the wrong hands.
Here's about as good a write-up as your likely to find on the subject of traditional A/B/Both switches. definitely go read it.
I don't think a DCP is applicable to a bluewater cruiser, or even to yachts with more sophisticated systems and complex needs. But I'll tell you why it's ideal for me, and perhaps for you too - only you can decide what's right for you (much to the consternation of those who would shove dogma down your throat).
My boat (not a yacht) currently lives on a small-ish lake. My overriding interest is the ability to start the engine and get myself home. My second priority is the ability to enjoy a couple of days on the water without ever having to think about how much battery I'm consuming from my house bank. In other words, if I run the house bank way down - that just means it's been a great weekend and it's probably time to head home. Of course, I can run the engine for some bonus time, but that's neither here nor there. The point is, I can enjoy myself, knowing that my start bank is waiting, safely isolated with a full charge. You don't need a DCP to do this. But you do need to manage an A/B/Both switch a bit more carefully in order to revel in this carefree operational mode.
Now, let's look at some failure scenarios - FOR ME, ON MY BOAT. Let's assume that I leave my slip with both banks fully charged.
- The most likely failure scenario on MY BOAT (as it was wired when I got it) was if I left the dock with the A/B/Both switch set to BOTH. This would clearly be my fault - operator error. But it's very easy for this to happen because my shore power charger was wired to the COMMON pole of the A/B switch, (as was my alternator) so the only way to keep both banks on charge was to set the switch to BOTH. That's a common way for these to be wired. Now, there are a few other ways to re-wire this to help me avoid the need to use BOTH for charging, but that was the state of things before I redesigned my electrical system. Obviously, spending the weekend drawing down BOTH would be a boneheaded move. Yes, this can be managed through a series of steps to change the switch position every time I start/stop the engine or leave/return to slip. Failure analysis experience tells me it's a bad idea to require that much fooferaw in order to transition between 'modes of play' - it's appropriate on a flight deck, for example. But I don't need to prove anything to my guests on board (or myself). I choose the K.I.S.S. principle. ADVANTAGE: DCS
- Discharge of one bank. If for any reason, either one of the banks is allowed to discharge below usable voltage, I've got another bank. This is the obvious and simple reasoning behind the A/B/Both switch. Clearly, if it's just a case of having overspent my time on the hook, I should simply be able to switch over to B side and use the start battery to run my Vhf, lights, starter, or anything. Of course, I can do that with the DCP by putting into its own BOTH mode (with the caution symbol on it!). But I'd better be damned sure that I've only got a case of over consumption, and not a failure within the House bank. ADVANTAGE: Toss Up. Both switches carry same benefit and require the same vigilance in this scenario.
- Battery bank failure (not discharge, but actual damage). There are a number of ways a battery bank can be damaged - all of which are well detailed elsewhere and not worth getting into here. But a damaged cell can take down every other cell that's connected to it. Clearly, isolation is key to maintaining a backup solution. DCP together with ACR go a long way to reducing maintaining total isolation. If I'm unaware of a damaged bank (on either side) and decide to charge both banks through the BOTH position of a traditional switch - or if I succumb to the temptation to gang both banks together to get more A/C current through my inverter (I know, don't do this, but if it's there as an option, it's bound to happen some day) I've exposed both banks to this potential failure scenario.
Please note that in scenario #3, if the damage happens to the START bank and I'm using a DCP, I won't be able to just throw the switch over to HOUSE and expect to motor home. Though, by adding a primary disconnect switch for each bank* as described by Scott, Stu and Maine Sail/Rod/Marinehowto, I can use the BOTH position on DCP to send HOUSE only to both outputs (outsmarting the DCP) because I will have disconnected the bad START bank. ADVANTAGE: Toss Up. Potato/Po-tah-to.
*I'll note that I'm more likely to forego adding two switches and just use a 1/2" nut driver to remove the positive wire from a failed bank if needed. This is because my battery installation is somewhat cramped, but very neat. Mounting two switches and adding the requisite 1/0 battery wire is going to make it even more crowded in that lazarette for a tiny convenience of throwing a switch in a very low probability situation. I can way more easily reach for a nut driver and pull the wire off the terminal. The other reason I'm likely to do it this was because this particular boat has my name on the Title.
There are likely other failure modes that I'm not considering here. I'm sailing around a lake, not setting off for Perth or Penzance. Similarly, across all aspects of my life, I don't own anything that could be considered to be, or is marketed with the word "Tactical" since I don't intend to invade any countries or plan to do any LARPING. If a troll on the internet is telling you there's only one method for all use cases, they're probably wrong and probably a zealot.
There are compounding issues to be considered. The most glaring one to me is the possibility of damaging my alternator in scenario #1 above. If I've got the engine running with the intention of charging banks, I'd better have started it with the switch in the appropriate position. There are certainly steps I can take to remedy my mistake, but the concatenation of mistakes tends to ratchet up the probability of further mistakes, leading to higher stakes with each iteration.
Of course, the DCP works best in combination with an ACR. I know and understand why Maine Sail is a proponent of keeping the Alternator on the HOUSE bank when designing a dual circuit system. However, I would counter that with two assertions:
- In retrofitting scenarios, that requires separating the Alternator from the Starter, as they're commonly wired together on a single branch on many factory harnesses. I will likely rewire that part of the harness some day, but for now I prefer to leave them together.
- Once the ACR has bridged the two circuits, a charge current on either side is as good as one on the other side. In other words, if that relay is closed, the electricity does not differentiate between HOUSE and START banks. The hungrier bank (that is, the one presenting lower impedance to the charge source) will draw proportionally higher charge current. If you need something more sophisticated, you're likely running LiPO batteries and a candidate for a true Battery Management System that can regulate charge currents and voltages. None of that is applicable in my installation.
All of this is a very lengthy treatise on things that Shouldn't happen. And situations that Should be manageable.
But there's one last point I want to stress about why I made very well informed choice to design my system for my boat so that I can manage it the way I please. My boat happens to have come with two DC distribution panels. There's the factory panel from Hunter, with 10 DC breakers, a fused position and janky SPST switch someone threw in (soon to be removed!). But there's also a Blue Sea panel with 5 more DC breakers and volt meter. And with my professional background, I recognized a design opportunity that suits me very nicely.
I have designated that 5 position panel as my Critical Loads Distribution Panel. I'll move all nav lights, my instruments and my VHF to that panel. The factory panel will have all cabin lighting and convenience loads and perhaps refrigeration, some day on it. It also happens to have A/C distribution on its other half too. So, if there were ever a problem on that panel that can't be solved with the throw of a breaker, I am not pressed to dive into it. I can take that whole panel off line and keep my Critical Loads powered via a separate path, either on their default House Bank, or they can be directed to the Start Bank with a simple transfer switch. Now.... would I have designed it that way from a blank sheet of paper? No, probably not. But since I have the two panels, that's the way I'm going to use them. With this design, I don't HAVE to be able to direct my START bank over to the HOUSE side in order to make use of VHF and Nav lights. I certainly CAN do that, with the precaution stipulated above. But I also have the further option of EASILY directing the Critical Loads to the START Bank if necessary. That's a very different paradigm that results in a different way to reach the same desirable outcome.
I haven't gone to lengths here to tell you what you should do on your boat. I'll leave that to the zealots. I have tried to describe here ONE way of doing things that is electrically correct, and offers some options to this skipper of this boat. As with any tool, you must know how to use it and understand the dangers inherent in its use. Don't let others prescribe solutions for you based on dogma and zealotry. Inform yourself. Learn. Understand. And consider your needs as only you can. Then take the best of everything you've learned and make some informed choices about what's right for you.
