Hi Ray,
By "no practical use" I didn't mean that the house batteries should not be charged underway, only that an echo charger on an outboard powered boat is not practical from an installation and/or cost standpoint. I believe that it technically could be done but it would be an exercise to see if could be done and would not result in a meaningful improvement over what can be done with off-the-shelf parts and no alteration to the outboard.
It's really a matter of scale and use given that a C-Dory is using relatively inexpensive and small batteries and the outboard is outputting low amperage that cannot (easily) be regulated beyond what's built in by the manufacturer. This is a totally different scale of operations when contrasted with a cruising sailboat where much attention and (usually) dollars are lavished on the batteries and electrical system.
Typically the largest house battery on a C-Dory is a group 31 deep cycle of about 105 to 115 amp hours and the starting battery is usually a group 24 (though we use group U-1 batteries on small twins like the 40/50). It's difficult, without an external charging source or lots of motoring to keep more than 105 to 115 amp hour batteries charged up; there just isn't enough output from the engine alternator to do more. The rule-of-thumb is to keep the house battery (or bank) no more than 5 times (in amp hours) the capacity of the alternator (in amps) and 4 is really a bit better unless the boat is motored a lot.
I agree that using a VSR/ACR (just different names for the same thing) is not, in theory, as efficient as using the echo charge but in an everyday practical application on a boat like the C-Dory there isn't enough to be gained (for most folks and/or applications) to warrant the effort to install an echo charger system.
Most VSR/ACRs are dual sensing units and "see" both the starting battery and house battery voltages. When either meets the cut-in requirements the VSR/ACR closes and parallels the batteries. In most cases this means you're paralleling a group 31 deep cycle and a group 24 cranking battery. Assuming that they're the same "type" (flooded, AGM, or Gel) that isn't an issue given their relatively close sizing. On a boat with a very large house bank (say several hundred amp hours) this would/could be a problem but since that's not typically the case on a C-Dory it isn't an issue in everyday use.
Much depends on the capacity of the house bank and how one uses and maintains the batteries. If there is a large(ish) house bank involved that is routinely pulled down to deep depth of discharge levels using a VSR/ACR is more challenging if a good alternator output is not present. A worst case scenario on a C-Dory might be a boat with twin 40/50s (I'll use the older Honda BF40/50 in this example with an alternator output of 10 amps) that's equipped with a group 31 115 amp-hour house battery and a group 24 cranking battery and the house battery is discharged to 10.5 volts (as an example). When the VSR/ACR is closed (one the starting battery is up to cut-in voltage and the VSR/ACR parallels the batteries) the much depleted house battery will take what the starting battery will give up and everything the engine alternator can deliver. There's a good chance that the system voltage (the combination of the starting and house battery) will fall below the cut-out voltage on the VSR/ACR and that it will disconnect the house and starting batteries. As soon as the starting battery comes back up to cut-in voltage the VSR/ACR will cut in again. This scenario causes a "chattering" VSR/ACR that continuously cuts in and out until the system voltage is finally raised enough for the VSR/ACR to stay engaged.
This is not a "equipment" or technology problem but rather a design situation; the "balance" of the system was either not taken into consideration or a decision may have been made that the house bank wouldn't (typically) be allowed to drop to a deep depth of discharge. That is, the owner/operator understands enough about the system to regulate the depth of discharge and keep it above a particular lower threshold.
With a larger outboard the same scenario rarely occurs if the house bank is "balanced" with the output of the alternator(s). Even with a deep depth of discharge of the house bank the larger alternator can put out enough to keep the system voltage from dropping below the VSR/ACR cut-out point.
So...given that the batteries on a C-Dory are relatively inexpensive (nothing like the cost of a good house bank on a cruising sailboat), that the relationship of the house and starting battery are relatively close, that a VSR/ACR is an affordable product and easy to install, that no alteration to the outboard is necessary, and that the system works in most typical scenarios, the VSR/ACR is the "go to" product when trying to optimize the charging system.
That system allows the house bank to reach about an 85% charge level, which is as good as it's going to get without a multistage regulator, which is something that can be added to an automotive-style regulator on an inboard but not to an outboard.
If the starting battery happens to suffer in this scenario and "dies" as a result of being rather continuously paralleled to the house battery then at least it's rather easy and inexpensive to replace. Real world experience does not indicate that starting batteries in a VSR/ACR equipped system are any more likely to die than in other system.
I would opine that in the case of a typical cruising C-Dory with a properly balanced charging system that not only does a VSR/ACR system work but that it keeps the batteries as charged as any other system that might be far more complex, expensive, and require technical alteration to the outboard's charging system.
If one has a use scenario for a C-Dory outside the "norm" and is trying to develop a system to support that use scenario then likely specialized systems may be needed to meets those needs. For instance, if one were to wish to spend as much time away from marinas (and therefore shore power) as possible, and every day motoring is also not the intended use, then outfitting the C-Dory is really no different than outfitting a cruising sailboat for the same use. It will take a large battery bank to support the needed loads over many days, and a way to recharge the battery bank, which would include things like solar panels, wind generator, and such. On the C-Dory a substitute for running the diesel inboard on a sailboat with a large capacity alternator for some time every day or two is to run a suitcase-style generator (like a Honda EU2000i) powering an onboard 115v battery charger.
Basically, if the goal is a balanced system, the defining item for the typical battery system on a C-Dory is the output of the alternator on the main engine(s). If that system does not have enough capacity to support the intended use than a custom solution must be sought and ways found to balance it with some external charging source (or sources). Using an echo charge system rather than a VSR/ACR will not fundamentally change the characteristics or usability of a basic battery system designed for support from (only) the engine alternator(s).
I know this is awfully long-winded and I also assume that you (Ray) already know and understand much of the information. I've written as much as I have in the spirit of education so that others might gain from the conversation.
With that in mind I don't believe your proposal for an echo charge system is "wrong" in any sense of the word. I do believe that you won't see a real world return on your investment when compared to a basic VSR/ACR system unless you incorporate much more than typical battery capacity and external charging sources.
Les
