Does this need to be sized to match the maximum amperage of the alternator?

It needs to be sized to the potential constant current flow, so that usually means the battery charger or alternator. If the destination battery is small, that would limit the current. For example, a 100AH lead/acid battery would only accept 15-45 amps, depending on type, no matter how large the alternator. It can handle up to 180A for 5 seconds when needed for starting an engine.

Can this be wired with an LED light to show when relay is on?

No, not easily anyway. Another answer mentions Post #85, but that just manually combines the battery systems when a battery(+) is applied to it. I don’t think this could be used for an led ON signal. Post #86 is the ground reference for the combiner. I don’t think you want to add resistance (resistor, diode, or led) to that path to battery(-) because it will alter the voltage read by the combiner. The combiner needs the actual voltage for delayed relay intervals. One solution is to compare the two battery system (+) posts (87, 30). When they are exactly the same, the combiner is ON (or very coincidently equal). You might be able to use that as a signal for an led ON relay but ??? Here is the wiring diagram for this combiner (I think the other answer shows the lithium version) https://www.victronenergy.com/upload/documents/Manual-Cyrix-ct-120-EN-NL-DE-FR-ES.pdf

Can you use this for a secondary battery on a Honda Accord hybrid one that doesn't have an alternator?

Your question is in regards to whether your hybrid power system will trigger the relay when the car is engaged. It is likely possible that it will, but it honestly depends on the voltage curve of the cars high voltage dc-dc converter (144v to 12v)

could this be used to combine two 12v power sources to one output without feeding back from one to the other? solar bank + alternator for reduced load

The main function of this unit is to protect two battery sources from discharge when no charging sources are present. Protects starting battery from discharge when there is no charger or alternator (engine) running and there are loads on the 'house' battery. These are controlled by the voltage present in either bank. Once voltage is high enough it turns on and allows the charging source to charge both banks.

Connector Type	Kräuselung
Contact Type	Normally Open
Current Rating	120 Amps
Mounting Type	Schraubmontage
Brand	Victron Energy
Coil Voltage	24 Volts
Maximum Switching Voltage	24 Volts
UPC	616913923864 619639706902
Global Trade Identification Number	08719076022145
Manufacturer	Victron Energy
Model	Victron Cyrix-ct 12/24V-120A Intelligent Battery Combiner Retail
Item Weight	0.247 ounces
Product Dimensions	1.81 x 3.15 x 1.81 inches
Item model number	CYR010120011R
Batteries	120 A batteries required.
Is Discontinued By Manufacturer	No
Manufacturer Part Number	CYR010120011
Amperage	120 Amps
Voltage	12 Volts

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