It won't do this. See the combine / un-combine parameters below.
Not this either. The batteries are simply charged in parallel providing the parameters shown below are met.
Charging a start battery first does not ensure it is fully charged before paralleling occurs it only means it is charged for 30 seconds or 90 seconds before the parallel conditions are met.. One common misconception of the ACR is that a start battery is first charged until full, then separated, and then the house bank is charged. This is not the case, they are simply charged in parallel together, so long as the bus voltage, measured at the "A" or "B" terminal (bi-sensing) remains above 12.75V for 30 seconds or 12.35V for 10 seconds.
To address the question of the house bank taking a long time to combine with the start battery, we first need to consider a few things:
Start battery energy use?
The start battery is using very little stored energy to start the engine. Usually considerably less than .5Ah. This is due to the cranking duration, loaded to unloaded, averaging 0.75 seconds to about 1.5 seconds. This means your previously full start battery will still be at about 99%+ SOC. A 99% SOC battery does not really require immediate charging and has many, many, many more starts in it before any charging would even become necessary. In this image we have a 44HP diesel cranking diagnostics. Average cranking voltage = 12.04V, Average cranking Amps = 286A and the loaded to unloaded starter duration is 0.765 seconds.
Even if we round up the cranking duration to 2 full seconds we are using just 0.17Ah
If we correct for Peukert we are looking at a max of about 0.29Ah's.
How long to attain combine/parallel?
From 50% DOD/SOC, the max depth of discharge recommended by most lead acid battery makers, it takes about 2 minutes at a .2C charge rate for even a high acceptance AGM battery to attain 13.0V. Blue Sea knows this and this is why they have two differing combine points, one at 13.0V & 90 seconds and one at 13.6V & 30 seconds.
This battery began charging at 50% SOC when the clock read 12:00. The charge rate was .2C or the bare minimum recommended charge current for this Lifeline AGM battery. As can be seen it is already at 13.1V.
Think about this snap shot if you are concerned about an ACR for charge management.
The rumor goes something like this: By using a battery combiner, on "
high acceptance" AGM batteries, and feeding the alternator or battery chargers charging current directly to the house battery bank first, “
it will leave your start battery under charged“ because it will never get to the combine voltage or will take too long to get there.
If you are practicing good battery management, and have even the minimum suggested charge current for an AGM or flooded battery, this is really a
non-issue. In 2 minutes of charging, at .2C or 20% of Ah capacity from 50% SOC, the AGM battery voltage is already at the parallel/combine level for the Blue Sea ACR. Even at .1C or 10% of Ah capacity the time to attain 13.0V is not very long, just a few minutes more.. To get from 13.0V to 14.4V+ does take more time but the relay has already combined at 13.0V and both banks are now being charged.
Battery voltage will rise pretty slowly from the low 13's on but, to get to an ACR's
combine level, is relatively quick and easy, especially if you have your system set up properly. Echo Chargers, Duo Chargers and a number of other DC to DC chargers also turn on at similar voltages and those devices
require all charge sources to be fed to the house bank. On cruising boats with disparate sized banks Blue Sea recommends feeding charge current to house first, not start, to avoid relay cycling.
What about relay cycling?
In a system with similarly sized banks, for both house and start, the charging sources
can be fed to the start battery first. This is often easiest because it is how the factory wiring often exists from the builder. On cruising boats, with disparate size banks, it is recommended to feed the alternator to the house bank to minimize the risk of relay cycling and voltage drop.
The ACR relays are smart enough to detect a voltage trend upwards between 12.35V and 12.75V. If voltage is trending UP, and 12.35V is attained before 10 seconds, it will remain closed unless 30 seconds expires before it attains 12.75V. If wired incorrectly, for the application, the relay can still suffer from relay cycling, especially with low current charge sources, despite the in-built logic to help minimize it.
I would urge anyone reading this to take a few minutes to read the Blue Sea technical document on relay cycling. Most installers and DIY's miss this:
Blue Sea - Preventing Relay Cycling (LINK)
Think of an ACR/COMBINER/VSR as nothing more than an electronically
voltage-change triggered BOTH/COMBINE switch and it will become much easier to understand.
ACR Closed:
ACR Open:
This part of the ACR installation instructions is also very often missed on cruising boats.