Some of the problems with ACRs
1) If you have banks of mixed battery types, you must program the charger to comply with the lowest common denominator of charging profiles so AGMs may not be fed as much current as they can take because of a different battery type in another, now connected bank.
Only if the charge voltages are different. The same can be said of any battery charger with two or three outputs or an alternator or inverter charger or solar controller.. With mixed banks that have varying voltages eg: GEL (14.1V) and Trojan (14.8V) or GEL and Odyssey (14.7V) or GEL and Lifeline (14.4V) then you need a way to either boost or drop the voltage to the other bank..
2) With different states of charge between banks, there can be some currents flowing between the banks as opposed to the charger feeding all banks.
Once you have voltage parity current simply flows where it is needed. Both banks will be in absorption for the same amount of time. I have solar charged banks that are world cruised and charged via ACR's or Combiners. Some of these starting banks are 10+ years old with over 30,000+ hours of solar charging. In not one case have I seen a start battery "over charged" or fail before the house bank but internet lore will have you belive that an ACR will "cook" a start battery. Never once seen it.....
3) If a battery is fully charged most of the time (starter battery, windlass battery when it hasn't been used in a while, etc.) but the house bank is continually used, the charger will put higher than float voltages to those batteries, as well, possibly causing higher gassing, boiling off the electrolyte...
This internet lore continues and all I can say is it does not happen in the real world
... However, seeing as you make that point a Duo or Echo will provide the same voltage to a second bank that the house battery is getting so in reality there is no realistic difference.
Duo's and Echo's DO NOT stop charging or independently float from the house bank. This is very often misunderstood. They simply follow the voltage of the house bank. The Echo limits max output voltage to 14.4V and the Duo's max voltage is user adjustable but neither will drop
voltage to float level unless the house bank goes their first. They get the same duration of absorption voltage that the house bank gets because they are voltage followers
..These two devices are often grossly misunderstood about how they actually operate. In full disclosure I am a dealer for Balmar, and really do like the Duo, for the right application.
When I use the term "smart" controller, I recognize there are levels of intelligence but, at least the duo-chargers on my boat, can be programmed for the battery type and have some knowledge of charge states to stop charging a fully charged battery or at least reduce the currents to a low enough level to be relatively harmless.
Neither the Duo Charger or Echo Charger stop charging when a bank is full. Many misunderstand this because the manuals and marketing can be a bit misleading. The Duo can be programmed for max voltage, battery type voltage and different turn on voltage but otherwise it simply follows the voltage & charging stage of the house bank. The drop in current is dictated by the battery at % SOC & XX.XX voltage and not the Duo Charger. The same occurs with an ACR. At X volts and X% of charge a battery can only take what it can take and you either need to raise or drop voltage to change accepted current.
In a battery over temp situation the Duo simply ceases operation because it can't independently drop voltage to a lower level like an alternator regulator or battery charger or solar controller can.. Once the battery cools it simply resumes.
Things like this were added to the Duo Charge manual at the request of marine electricians, such as myself, so that folks can make good decisions on how and where to use them:
"The maximum output amperage of the Duo Charge is 30 amps. When used as a stand alone charge source, the Duo Charge is designed to discontinue charging when demands exceed its capabilities. At that point, the Duo Charge will wait for a short period and query if the demand has diminished to below its 30-amp capacity. If so, the Duo Charge will continue charging. If demand continues to exceed capacity, the Duo Charge will continue to shut down, while checking periodically to see if demand has diminished. So, if you decided to install the Duo Charge to charge a bow thruster battery, windlass battery or a battery that can be highly discharged the Duo Charge may go into the over current condition because the demand is greater than 30 amps."
Guys like myself have been bugging Balmar for years to fix this issue
but now that they have been sold I don't see this ever happening..
Many boat owners don't even know this may be occurring and get saved by solar or wind. If you don't have 30A+ of input current the Duo can't shut down on over-demand by exceeding 30A on the output.. It is situations where owners only charge via dock or alternator where a solenoid drive is necessary. If you have solar you may never notice the Duo cutting in and out because if input current from solar is not in excess of 30A this will not occur...
The Duo's and Echo's are great devices, I install tons of them, but now that Sterling has the B2B's out they are an arguably better choice for a bow bank because they can both boost, drop and do float independently as well as not time out on over current.. My one complaint with these is no battery temp sensor, and the Duo wins here. There is no one perfect battery to battery charger but the Duo's Echo's and Sterling B2B's are all great if appropriately chosen for the application. ACR's / Combiners are also great devices and perhaps one of the most reliable products in the marine environments.
Again, many ways to skin a cat, I just want people to have as much info as they can to make the right decision for their application..