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I have two 12v domestic batteries connected in parallel and one 12v starter battery. Starter and domestic batteries are connected via negative pin to main switch which then connects to engine and appliances. The alternator is connected to LUCAS 33RA SRB600 relay and charges both starter and domestic batteries. There is also solar panel which is connected into regulator with two outputs, each connects to one domestic battery.

Now my question is if there's any point having the regulator connected to each battery? shouldn't be enough to connect it to one and second would be charged because of the parallel connection? If so, I would save one output on the regulator which I'd like to use to charge starting battery. In that case would I need some over-voltage protection to save regulator from high currents on engine startup?

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In practice the paralleling of two identical or even relatively equivalent batteries will work "well enough" but if you want maximum ;ifetime and capacity then extra care is needed.

IF the two domestic batteries are

  • the same brand & model (& so capacity)

  • and the same age

  • and are discharged in parallel

  • and have been treated identically since new,

then connecting them in parallel for charging "may be bearable".
If they are not identical twins in manufacture and use then you run the risk of them being treated less gently than otherwise.
This is because the batteries have 3 main charging modes and if they do not walk in approximate lockstep through these modes then the charger may not do what it "thinks" it is doing.

Lead Acid batteries are usually charged in a constant current mode until they reach a float voltage, they are then charged at ABOVE the float voltage in boost or equalise mode to cause all cells to reach the same full state of charge and are then dropped back to the float voltage. Two batteries in parallel at different states of charge are liable to not be properly cycled through this process by the controller. A capable controller will hold a battery in equalise mode for a period of time related to its prior depth of discharge. To do this the capacity of the battery needs to be known. In many cases a lower specification charger will not change what it does if battery capacity is doubled.

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References:

Battery University:

BU-403: Charging Lead Acid
Learn how to optimize charging conditions to extend service life.

BU-201: How does the Lead Acid Battery Work?
Learn about the differences within the lead acid family and find out what the cons and pros are.

BU-804: How to Prolong Lead-acid Batteries
Explore what causes corrosion, shedding, electrical short, sulfation, dry-out, acid stratification and surface charge

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