One final note on the Sterling and Promariner units, though. They will handle multiple inputs and outputs, and favor the start battery (at least for the Sterling, not sure about the Promariner). So if you're dealing with either two alternators or three or more banks, they make a lot of sense. On the other hand, they haven't been on the market that long, while the Newmars and others have quite a track record.
The ProMariner is a Sterling made unit. As to "favoring the start" battery this is nothing but sales & marketing 101. It is a gimmick. In the time it takes a "start bank" to come up to "combine voltage", after starting an engine, you've really done little to no "recharging" because little to no capacity was used and the bank is still at its lowest levels of "acceptance". A starting bank can come up to combine voltage within seconds of the alternator exciting. Charging first? I challenged Charlie about this on the phone one day and he simply chuckled....
The average marine engine will use about .5Ah or less of capacity to start. Our 44HP engine starts in about 775mS or .75 seconds at an average current of about 280A (averaging the entire .75 seconds). If we call this a 1 seconds start duration it means we use 0.07Ah to start our motor. 280A/60 minutes = 4.66Ah per minute 4.66Ah/60 seconds per minute = 0.07Ah per second of cranking.. I recognize that not all motors fire up like ours does, so even if the start duration was 10 seconds, that's a LOT of cranking, we are still at just 0.7Ah removed from the battery to start a motor with an average starter current of 280A... So why on Earth do we want to "prioritize" replenishing less than 1Ah of use from a 70 - 100Ah battery... Marketing!!!!
BTW that data on our motor comes from a 27F day with an actual battery case temperature of 32F. Most boaters are not starting at those temps.
Charlie Sterling convinced people, who generally don't understand electrics, it was "necessary" to charge the start bank "first".. He did this to sell them "differentiated" products. It is marketing 101 and has virtually zero useful benefit IMHO.
The Sterling products have been in Europe for years and have a good track record and seem to be reliable. Charlie's marketing tactics however, are often called into question.
The best description for the "Zero Volt Drop Isolators" is that they are a "smart combiner"... Nothing wrong with them just one way to market to people. I much prefer to charge the house first and feed all charges sources directly there then let a combiner, Echo, Duo or other B2B charger do the rest.
Anything that uncombines/shuts off or disconnects the batteries, based on voltage, such as an Echo Charger, Duo Charger, ACR's, VSR's Zero Volt Drop Isolators, Sterling Battery to Battery chargers etc. are all technically "battery isolators" in that they isolate the batteries when no charge sources are present and they charge them when charge sources are present. The term isolator got a bad wrap from diodes but isolating the batteries automatically is still a great and very useful feature.
The Echo Charger, Blue Sea ACR's and Yandina Combiners are some of the most reliable pieces of equipment I've come across in the marine environment. I have Echo's and Combiners out there with 15,000+ operational hours of charging or combined charge states.. I even have a number of customers who have Heart Interface Echo Chargers going back well into the 90's that have been charging for 15+/- years.. Yandina actually backs their combiners with a lifetime unconditional warranty.