Unfortunately I'm overseas behind a firewall right now.
Are you serving the country? If so, thank you.
I don't understand the concept of a 'house' battery yet, but I'm sure the link will explain it to me.
Mark,
"House" battery(ies) are, in most cases, any batteries that are not your starting battery(ies). What it really refers to is "house power," as opposed to "shore power." Since you are seeking an understanding of the "concept" of a house battery, I'm going to give you an explanation that might be broader than your needs...
Let's forget about boats for a moment...no, really. Let's say you have a generator at your house. The power goes out, and you fire it up. You flip a big switch that disconnects the power to your home from the utility (power company) and connects your generator to your home. You are now running on "house power" as opposed to the power you would normally get from the utility. It is house power because it is coming from your house, and not some outside source.
Back on the boat, but, let's go back in time a few decades, before the smart relays and smart inverters hit the scene. Say, it's 1965 and you are aboard your 40' downeast lobster boat. You have a white beard and say "arrr" a lot. People always think you are winking at them, because you squint in the sun and think sunglasses are fruity.
All throughout the cabin, you've got power outlets. Not one at each location, you've got three! One is labeled "shore power" the next says "house power" and the third says "DC." Before any of your land lubber guests or green crew plug in their new fangled Mr Coffee or hairdryers, they have to talk to you. Says you, "arrr, don't use that one labeled DC. It's not for you. You can only plug into shore power. And if'n we ain't n'shore, you ain't got n'power."
Why three outlets? Shore power outlets are wired to your shore power connector, which you power via the pylon at the dock. Once you are connected, you've got plenty of power, as much as the dock and your boat are wired to handle. Time to turn on the fridge and battery chargers.
House power outlets are the power you get from your fancy "rotating inverter" which runs off a belt from your engine. If the engine isn't running, no power. And even when it is, it doesn't have much capacity for power, so you have to budget it. You know the power consumption of everything on board, and you have to choose what is on and what is off.
DC is wired to your house batteries. As long as they are charged, DC power is available, for items that run on DC.
Now we move forward 20 years or so, and you've got a nice solid state inverter on board. This boat only has two outlets in each location: house power, and shore power. The DC outlets are gone. They weren't that useful, as most people don't own many appliances that run on DC, and thanks to your inverter, you've got house power whether the engine is running or not. That inverter is wired to your house batteries. Why the "house batteries" and not just "batteries?"
Maybe it's the rum, or the good fishing, or perhaps just the riveting conversation but, more than once, out on anchor, with the radio playing and the cocktail lights on, the blender whirring and the VHF chattering away, you ran the batteries down, and you couldn't start the engine. You get towed back to the marina and tell your electrician, "look, I can't have that happen again."
Five and a half weeks later, you discover the electrician has put a big red switch on your boat that says "ALL, 1, 2, NONE." Hanging from the switch are two notes, the first is the bill, which causes blindness. Later, your vision comes back and notice the second says, "always switch when motor is off, never when motor is on, switch to 1 when the engine is going to be off, switch to ALL when the engine is going to be on, switch to 2 if you just want to charge the starting battery, bilge on own circuit." Makes no sense, especially the bilge part but, you play by the rules and never have a problem again (because you obey the note, and, actually replace your batteries when they are shot, unlike all of your electrician's other customers). Okay, you have one problem, over the first winter, all your batteries fail. You call your electrician and he says, "pull the drain plug and it won't happen again." Sounds dumb but, come the second winter, when you lay her up on the hard, you pull that garboard drain, and come spring, the batteries are fine.
Why the rules? Let's look at them:
A. "Always switch when the motor is off, never when the motor is on." When you move the switch, as it goes from one position to the next, in the time between two switch positions, nothing is connected. The alternator needs a battery connected to ballast the system. If none is connected, the voltage can go too high, and burn up the rectifier. This is called "load dump" and generally only occurs when the alternator is supplying a large load right before the battery is disconnected. A large load could be something like topping up a battery that just started the engine so, one would be wise to mind this rule. There are special diodes that can be connected to prevent it but, the most dependable ones are sacrificial (work only once).
B. "Switch to 1 when the motor is going to be off." This switch position has your house batteries connected to your electrical system, but not your starting battery. This allows you to relax at anchor, lights ablaze and stereo singing, and no worries of running the batteries down and not starting, because the starting battery is not connected.
C. "Switch to ALL when motor is going to be on." This switch position has your house batteries and your starter battery connected to your system. This allows your engines' alternator to charge all batteries simultaneously.
D. "Switch to 2 if you just want to charge the starting battery." This switch position has only your starting battery connected to your system. Your alternator can charge one battery much quicker than a whole bank.
E. "Bilge on own circuit." With out getting into too many tricks involving diodes and better, the electrician has wired the automatic bilge pump before the main battery switch. He wants to be sure, when you leave the boat, and decide to switch to "OFF," to prevent the possibility that you left something aboard turned on, and it may over-discharge your batteries, that your automatic bilge pump still functions.
And as for that drain plug when the boat is laid up on the hard? If you leave it in, and the autobilge is wired the way your electrician did it (the right way), every time it rains, rather than the rain running out the drain, it gets pumped out by the bilge. Since the motor isn't being run to charge the batteries, the batteries are run down too far, and damaged.
Now we move to today, and the technology has gotten even more convenient. You have smart inverters and smart relays, and if you install enough of them, you don't have to think about anything. Things just magically work all the time, especially in a salt water environment.
The smart inverters actually check for shore power quite often, and when they find it, switch you over to shore power. There are some spiffy models from companies like Xantrax, and others, that include battery chargers as well, it will automatically start charging your batteries when shore power is found. Smart relays can be connected in various ways so that they disconnect a battery if they don't detect charging voltage from the alternator, or disconnect some or all batteries when the voltage goes below some point. There are a ton of possibilities. I spent many, many years as a controls engineer and we have a saying: if you can say it, we can do it. The idea being, that if someone can give a concise description of the behavior they want from a system, the engineer can convert those words into a logical system constructed of various devices that connect and disconnect different bits to perfectly model that behavior.
Even if you install a lot of great gear, you should still have a general understanding and plan for how you will maintain necessary systems when that equipment fails. That usually comes down to one or more big battery switches configured to suit the particular system on board. I've no doubt that is what LilRichard has recommended. I can't get the more details page to load so, I can't look at that right now but, chances are, if you have two engines and three batteries and a member of this forum has recommended that you buy the switch cluster for two engines and three batteries, they are probably pointing you in a good direction. If it is the type that includes voltage sensing relays, then take a close look and ask a lot of questions. Those things are great but, you should go into owning them understanding their limitations, and, how you will manage your system should one fail.
Two engines creates new possibilities of one fault taking out two alternators, eliminating part of the benefit of having two engines in the first place. There are some different strategies that can be employed to best enjoy the benefits of redundancy.
Please keep asking questions about this. Few understand it and all could benefit from knowing it better. I enjoy the opportunity to explain it in written form as it gives me time to think out how to explain it, and as a result, I do a better job of handling it verbally with my customers.
Also, I make errors. If anyone sees something I said that is wrong, or might be, please say so.
edit: I wouldn't think I'd need to add more to such a long post but, it occurs to me, that I didn't give the full definition of house power. It has a second meaning, which is "non-dedicated power." A circuit that is dedicated, say the starting battery on a boat, or the circuit that feeds the elevator in a building, is just that: dedicated to a single purpose. House circuits are not dedicated. They are shared, and when connecting large loads, one must make a careful analysis that they aren't overloading the circuit. On a boat, if the starting battery circuit had a problem, and the captain is able to configure things to start from the house batteries, the fact that they are called "house batteries" should remind him/her to turn off all other loads before attempting to start.
