I received a reply from the
engineer who designed my combiner. Yandina who made it for West
Marine for many years. Her reply was very informative and I repost it here for the benefit of anyone else in future. It also speaks volumes that I should be able to get such great support all these years later:
1. What is the point of the external ON/OFF switch between the two poles of the VSR?
The on/off override switch on Yandina and West
Marine brand Combiners can be used to force the Combiner ON which connects the
batteries together for
emergency starting if the starting
battery is low. It will act like jumper cable and after a few minutes allow the
engine to be started.
The on/off override switch can also be used to force the Combiner to stay OFF. This may be used if a trickle
charger or small
solar charger is not putting out enough
current to support the 1/8 amp that the Combiner uses when ON.
Yandina Combiners draw no
current when OFF.
2. Why might the VSR be cycling through a process of switch on (13.3v) and off again at (14.1v) after an engine start
The Combiner makes sure that the alternator does not get overloaded. When the increased
charging load gets connected it can exceed the capacity for which an alternator was designed and cause excessive
heating. Under these conditions the Combiner will cycle off for a percentage of the time to allow the alternator to cool. As the level of charge improves the current gets less so the
heating is less until eventually the Combiner stops providing
cooling cycles.
3. Can multiple alternators with different profiles charge the same batteries?
The primary purpose of
battery charging profiles is to protect the battery from damaging voltages and to force the charge in as fast as possible. A generic alternator such as found in automobiles has a "safe" profile that will not harm any batteries. But by limiting voltage it is also limiting the amount of current it can get into the battery so charging will take longer. The voltage at which lead acid batteries start to form bubbles in the electrolyte is about 14.2 volts. Although bubbles are advantageous in liquid electrolytes, they can displace electrolyte in a gel battery which may not get re-absorbed and reduce battery capacity. During the bulk stage of battery charging, the alternator can safely put out a higher voltage than 14.2 and force more current into the battery because there is voltage drop in the battery circuits and the voltage across the electrolytes will be much less than the alternator voltage. As that charge voltage increases the extra "forcing" voltage has to be reduced to protect the battery from over-voltage and this is what is happening in a multi stage
regulator. The profile can be tailored to match the individual characteristics for maximum charging rate.
Using both your alternators is quite OK. It will mean that the total output of both alternators will less than the total of their individual capacities because subsequent to the bulk stage the regulators will be reducing output due to the higher voltage being introduced by the other alternator. Some
references characterize this as the alternators "fighting" each other which is a misleading term. In fact they are both charging in later stages but charge rates are reduced by the effect of the other alternator. Depending on alternator sizes and batteries, adding a "generic" alternator to a "smart" alternator of equal size may not decrease charging time by 50% because the smart alternator will be able to put out a higher voltage which will make the dumb alternator think the battery is already charged and put out less current. Only during the early "bulk" stage will both alternators be putting out maximum.
4. Would there be any harm in wiring the Delco directly to the starter battery, and therefore if the combiner is off I know exactly what the Balmar is doing to just the house bank.
Sounds like an excellent idea
It is unlikely that the Delco is going too high on voltage. Old alternators dating back to the era of flooded cells would often have top voltages well above 15 volts that can damage gel style batteries. The "High Voltage" input on Yandina Combiners can be connected to the gel battery to limit voltage to 14.2 on the gel battery. If you find that the Delco is spending any time above 14.2 you could use that feature. However there is a downside - the smart charger will get above 14.2 volts when it is safe to force current into the battery and since it is on the house battery the Combiner would be shut off until the later stage when the voltage was back below 14.2 so charge current from the Delco would not reach the house bank while it was shut off. Everything would still get charged, it would just take longer.
It is common to hear a Combiner cycling for quite some time after charging has finished since the artificially high voltage left over from charging will remain on the batteries especially if there is any other charging source. Don't forget that the Combiners are bi-directional. With no load at all on the batteries that residual high voltage can take many hours to dissipate. It represents no additional battery capacity since it can't support any current. If one of the batteries is low, the combiner will try to charge it since it still has more than 13 volts but the sudden load will immediately drop the voltage to 12.8 and the Combiner drops out in about 20 seconds. With the load removed the voltage on the fully charged battery will float back up above 13 volts and the cycle will repeat.