It all depends on how it's wired
U wrote: "Even with the old fashioned way, if you change the switch to both after the engine is started, won't the charging source voltage overcome the flow of volts from the strong battery to the depleted battery? Also for you electrical experts, do I have the correct understanding of how a combiner works?" The old fashioned way? That's generally where the factory wired it this way: ONE measly wire from the alternator output to the C post (BOTH setting) on the 1-2-B switch, with a small wire between the alternator output and the starter. Why? Because it was the cheapest way to do it. One wire that served two purposes: to start it took the current through the wire FROM the batteries to the switch to the alternator through the small wire to the starter and then engine started. Once started, the flow of current reversed in the wire and went from the alternator output to the switch C post to the batteries, depending on which position the switch was in. [Some builders used two wires, one to the starter and one from the alternator, without the small wire between the alternator and starter. The downside was that these two wires still went to the C post, when the alternator output should have gone to the house bank, and the starter wires is really the only thing that needed to be on the C post. See more below.] That's why people had (HAD) to start their engines on BOTH, or risked shorting out their alternators if they started on the start bank and then switched the switch - if it's contacts were bad, the alternator output stopped and fried the diodes. Some folks started on their start bank, then only switch to their house banks when the engine was turned off. The downside here was that the house bank didn't get charged when the engine was running. OK if you've been plugged in all week, not so good if you've been on the hook and wanted to go to another anchorage. The 1-2-B switch served two purposes: it was the gatekeeper for where the alternator output went: 1, 2 or BOTH batteries; AND once the engine was off, it decided which battery bank was the source of power. Actually, when the engine was running it was doing both functions! A combiner works this way: it's a simple relay, whenever it sees a voltage higher than its set point, it closes allowing current to flow. It has nothing to do with the "resting voltage" of anything. It's on or off, closed or open, simply based on a setting. The easy way to think about it is, if there is a charge source available, that charge source is always higher than the even-full battery voltage of 12.8 V, so most combiners are set around 13.0 V or so. You plug your shorepower charger on, or run your engine and alternator and 14.2 V or 13.8 V or somewhere in between, shows up and the combiner closes, allowing juice to flow to the second bank. The reason smart electricians now suggest wiring ALL of your charging sources to the house bank is because that's where the input is required, because most of the time start banks (or emergency banks as discussed in another recent thread) are full anyway. Think about it: a 60 AH start bank, 30 AH usable -- the starting takes about 2 to 5 AH out of the start bank -- that's all -- glow plugs (for some) and start, is high amperage for an extremely short period of time. So, with all of the charging outputs going to the house bank, the combiner will send some juice to the start bank. One of the keys to understanding this is that it is much better to avoid the danged 1-2-B switch with charging sources COMPLETELY, and only use the C post of the 1-2-B switch for output from the batteries: i.e., power to the starter and power to the distribution panel FROM the batteries. Avoiding anything on the 1-2-B switch for wiring TO the batteries simplifies matters greatly and also avoids any problems with the switch messing up the alternator - you can move the switch all you want, even turn it off with the alternator running (!) because its output is going directly to the house bank. What a concept! The "charging source voltage" will overcome the drain of one battery to the other. However, there are two different scenarios here which most people tend to completely ignore or misunderstand. One scenario is this: been away from the boat all week, boat's been plugged in charging both banks. So, going to BOTH will have NO consequences. NONE. The batteries are charged, they're both just fine. Second scenario: been on the hook, start bank is almost fully charged, house bank isn't. Going to both will potentially drain the start into the house with dire consequences if left in BOTH too long without starting the engine right away. As I mentioned in another recent thread, you have to think about it based on the condition of the batteries as well as how you have the boat wired. A combiner is good for two equal sized house banks where they are switched daily (again when cruising, NOT when plugged in), or when one is retained for starting (a useless and wasteful approach in my mind - why have all those batteries sitting around doing nothing, besides, it reduces the life of the batteries). An echo charger is superior to a combiner when you have a larger house bank and a small start bank because the echo charger limits the amperage flow and avoids overcharging the start bank if you motor a lot. Alternatively, you could use a combiner and put a switch in the ground wire to the start bank and shut it off when motoring for long periods. The ACR is a combiner with improved technology to avoid spikes when the house bank is fully depleted. Pretty much the same thing, same purpose for all three. Theoretically, putting the switch on BOTH will do the same thing as a combiner to parallel the two banks if you have the alternator output wired to the house bank. BUT, and this is a BIG, HUGE "but", you again have to think about the WIRING. Most boat builders run #4 wires FROM batteries TO the 1-2-B switch. This is fine sizing for a meager OEM 55 amp alternator output which rarely gets anywhere near 20 amp output, and the total battery output rarely gets near that (when was the last time you ever had EVERY SINGLE ONE of your DC loads ON -- lights, fridge, electronics, stereo) add the DC loads all up they just don't get that high. So, with the alternator output gong to the C post, the #4 wires were being used for two purposes, in two different directions: alternator output to C post to batteries, and battery output to the switch and the DC panel for loads. Now, when a bigger alternator with external or internal regulation is installed, the alternator output gets tripled to 60 amps and the #4 wires are WOEFUL INADEQUATE (if the alternator output remains tied to the C post of the switch - you're just asking for trouble or undercharged batteries). That's yet another reason to rewire your alternator output to the house bank directly, and use either a combiner, echo charger, duo charge, ACR or a simple manual switch, to send power to recharge your start (or second) bank. The use of B on the 1-2-B switch, if the wiring and components are designed and installed correctly, should be a thing of the past, regardless of what kind of switches you have installed. We've redesigned our system, but did it retaining the 1-2-B switch very deliberately in lieu of buying new switches of any sort. The alternator output goes directly to the house bank, we use a combiner to the start bank with a switch in the ground leg to turn it off if we motor a lot, and the OEM #4 wires FROM the batteries TO the switch are just fine because they are ONLY used for battery OUTPUT to the C post: i.e., the DC panel and the starter wire to the starter. We never use the B switch position, can start the engine on the start bank position OR the house bank position, and can switch the switch when the engine is running with no fear of damaging anything. PS Ed, I also recommend changing your alternator output FROM the start bank through the ACR to from the alternator TO the house bank through the ACR to the start bank. Reason is described above, the start bank is almost always full, so why run that current through the ACR all the time? Same instructions came with the combiners (and Pathfinders) -- ALWAYS run the alternator output to the house bank.
