I'm using a Victron Cyrix (12V 120A) Smart battery combiner to connect the starter and the leisure battery in my RV. I wanted to know when the two batteries are connected to each other, but unfortunately the device does not have any output signal with this information. What kind of circuit can I use to know this? Some kind of continuity test between the two (+) poles of the batteries?

Below there's a diagram with the connections. Thanks in advance for any feedback.

Connections diagram

Edit: Based on the suggestion from @joribama I think that measuring the voltage of the two Cyrix poles, can be a good indicator if the two batteries are connected or not. I'm thinking about using the ADC pins of a Attiny85 to measure the voltage using a voltage divider as shown below. Since the Attiny can read up to 5.5V we can calculate that for that limit to be reached the batteries needed to have 22V, so there's a good safe margin. The 100k resistance between the positive batteries' poles and the GND result in a max current of 0.14 mA (~2mW using 14V for the calculations), which I can live with...


simulate this circuit – Schematic created using CircuitLab

With this circuit, using the 10-bit converter, I have a resolution of around 21.5 mV (22V/1024), which seems enough to detect when the cyrix is closed. The Attiny would then turn on a LED inside the vehicle whenever the voltage difference between the two poles fall bellow a certain threshold (to be defined).

What do you think? Will this work?

  • 1
    \$\begingroup\$ I forgot to add the solar power charge controller to the circuit, I will edit that. But the Cyrix is there to avoid draining the starter battery. Basically the two batteries will connect only if the Cyrix detects that the Leisure battery is fully charged and so the solar panel can start charging the starter battery, or when the car is on, thus charging the leisure battery with the alternator. \$\endgroup\$
    – mwind
    Dec 3, 2019 at 0:51
  • \$\begingroup\$ not a trivial task to define cut-in, cut-out , with load, without load , full charged , CC mode, CV mode, engine On idle low V, Engine on & Charger On , Engine On & Charger Off, Battery OK, Battery Fault etc with all the thresholds and tolerances to ensure these simple requirements you require are guaranteed. \$\endgroup\$ Dec 3, 2019 at 4:46
  • \$\begingroup\$ It’s hard to answer without knowing more about how the battery combiner works. If I had to guess, I’d monitor the voltage between the two batteries. If it’s below a certain threshold (in the mV range) I’d assume the batteries are connected. I also notice the combiner has a status LED that stays ON continuously when the batteries are connected. It’s not very elegant, but you could have a light detector there. \$\endgroup\$
    – joribama
    Dec 3, 2019 at 6:14
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 I agree that the task is not trivial, that's why I decided to ask for help. \$\endgroup\$
    – mwind
    Dec 3, 2019 at 10:36
  • \$\begingroup\$ @joribama I would definitely go for the light detector solution, but the LED is not on my model (120A). It is only present on upper models. I like the voltage monitor idea. I will edit the post with a possible circuit. Please let me know of what you think after. \$\endgroup\$
    – mwind
    Dec 3, 2019 at 10:38

2 Answers 2


Current detection

I'm sorry to throwback to 1925 here, but it seems as simple as taking one of the power wires leading to the Cyrix and having it make several turns around a NC reed relay. If current is flowing through the Cyrix, the reed relay picks up. You could do the same thing with a Hall Effect.

Voltage detection

You are looking for hundreds of millivolts of voltage difference between the two batteries. If the Cyrix has pulled in its contactor, that difference won't be there.

Note that it should take the samples right at the Cyrix's lugs. Anywhere else and you're also measuring the voltage drop to the Cyrix.

My first thought is to connect the base of a transistor to one side (through a resistor), the emitter to the other side, and the collector to common B-. When there is a voltage difference B to E, current flows on the base, and you get current flow C to E, and that lights the LED. You would need one transistor per direction, which would be fine, because with two LEDs that would tell you which way charging was going.


Doing this negative-ground is obvious; the microcontroller measures 2 voltages of Bat1+ and Bat2+, and compares them. I don't like it, though -- it requires 2 voltage measurements, and if the ADC's are off, they won't compare properly.

I would prefer to do this positive ground. The device takes its power from common Battery -. It takes one side's + as its Vss. It makes one voltage measurement, the other side's +. This will typically be no more than 2 volts, but may be negative, so it would need to be able to cope with that.

This way nicely removes the "voltage drop" problem from wiring between batteries and Cyrix.

  • \$\begingroup\$ Thank you Harper! I really liked your solutions but unfortunately my upvotes are not counting yet.. The problem with the first solution is that the cable that is connecting the battery to the cyrix (and then also from the cyrix to the other battery) is a 35mm2 (2AWG) cable, so I cannot twist it around the reed relay. Do you think it would work if I add another cable parallel to that connection but thin enough to twist is around the reed relay? \$\endgroup\$
    – mwind
    Dec 4, 2019 at 0:25
  • \$\begingroup\$ Maybe I should add a resistor to that cable to make sure that the current is not too high, but small enough so that some current still goes through it when the cyrix is closed. I really like the second solution as well. I can also add an AND gate to the output of both transistors to have just one LED. Do you have any idea of the best transistor for that? \$\endgroup\$
    – mwind
    Dec 4, 2019 at 0:27
  • \$\begingroup\$ No, since you are trying to measure current, using parallel wires will not work at all. That is the problem with measuring current; all the current has to go through the sensor! On the transistor, these guys here would know exactly what to use. When I said "which direction charging was going" I meant which direction it was not going, i.e. which battery was stronger. \$\endgroup\$ Dec 4, 2019 at 4:27

1) Micro switch for remote monitoring = Yes ( connect to this)
2) LED status of switch ( add shielded phototransistor or remove cover to access driver switch to gnd. )
3) add gate driver detector logic AND/NOR gate to inside signals. If both FET drivers are ON then activate a switch

Since options 1,2 are not avail, consider as easiest solution. Measure Vgs on both FETs. Choose best option and maybe add a phototransistor.

Example of low current switch.

enter image description here

  • \$\begingroup\$ Yeah, unfortunately the 1st and 2nd options are not available. Does the third option require to open the Cyrix? I really wanted to avoid that. If not, can you please provide a schematic with your suggested circuit, as you did for the low current switch? \$\endgroup\$
    – mwind
    Dec 4, 2019 at 1:08
  • \$\begingroup\$ Yes. If you can build and test anything you can open it , look for these connections from power gates to collector and measure it. \$\endgroup\$ Dec 4, 2019 at 1:29

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