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.

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Reviews

N**K

Good quality, easy to install

Good quality sturdy relay. Paid a few ££ more however victron make quality products. Easy to wire in, one connector to starter battery + other to + busbar on leisure battery. Negative to ground. It’s as simple as that. I now can be sure that any surplus charge to my leisure charges the starter.

8.02.2026

B**N

AGM battery combiner

Great piece of kit for trickle charging two agm batteries with different amp hour capacities

20.01.2026

W**N

2 Years and still going strong.

Been using this for 2 years now and it's brillant. Had no issues whatsoever. Easy to install and has been reliable where as my previous device i would have some issues and cost the same. All my devices are victron on my campervan solar system as victron is reliable, costs more but hey you get what you pay for.

10.05.2025

T**S

Quality

Very nice quality!

8.10.2018

D**D

Good spec.

Not fitted yet. Was bought because the specs seem to be better than a VSR & has the combiner function if start battery if flat.

29.07.2024

M**B

Great device (less prone to tripping out due to heat than the company’s smart B2B chargers)

Although specs seem to suggest this has a thermal cutout for ambient temperatures it seems to be happy in the engine bay of my van where a Smart 12-12-30 keeps tripping out.

31.10.2022

M**N

Price

Just what I needed recommended by marine electrician

30.09.2023

A**R

Well built and excellent value

This does exactly what it is supposed to do. Only just fitted it so longevity not yet tested but so far does exactly what it is supposed to do. Would definitely recommend

9.11.2021

C**N

Acheté pour remplacer un coupleur séparateur chinois qui a fonctionné une semaine. Celui ci fonctionne parfaitement. Matériel de qualité. Utilisé dans mon utilitaire atelier pour alimente une batterie auxiliaire de 100a.

11.05.2025

A**O

Prodotti assolutamente affidabili e intelligenti. Permettono di tenere le batterie in parallelo ma solo in ricarica

10.04.2025

O**E

Works as it should

17.09.2025

O**O

Ich finde diese Zeil genial, funktioniert in beiden Richtungen, je nachdem, ob über Generator oder Solarpanel geladen wird. Sehr einfache Installation

24.10.2025

L**I

Świetnie działa jako separator baterii

16.12.2022

ASIN	B071Y7C9MM
Best Sellers Rank	9,164 in Automotive ( See Top 100 in Automotive ) 226 in Motorhome Parts & Accessories
Guaranteed software updates until	unknown
Item model number	NT-1231
Manufacturer	VICTRON ENERGY
Product Dimensions	6.5 x 5 x 7.5 cm; 110 g

Cyrix-ct 12/24-Volt 120 amp Intelligent Battery Combiner

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