zuzullo avatar image
zuzullo asked

Cyrix -ct 120, draining both batteries?

When I leave the boat and the batteries are:

STARTER Batt.=13,0v

HOUSE Batt.=12,5v

after 24h with everything OFF the batteries have both the same voltage (~12.7v).

I thought right away that the Cyrix is connecting both batteries and when I tested with a voltmeter, there is indeed connection from both poles of the Cyrix. Is this a normal behavior?

See video HERE.

My current setup is:

BMV Battery MonitorCyrix Battery Combiner
bmv-cyrix.jpg (186.9 KiB)
1 comment
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esteban avatar image esteban commented ·

I think that I've seen the same thing in my similar RV system. I did not have time to verify anything as it was only once while testing an alternate configuration.

In my case, I control the Cyrix with the BMV702 to shut it off when starter side voltage drops at engine shutdown.. BUT I wanted to use the BMV for a different function and tested to see if the built-in isolation thresholds would still open the relay when engine is off. But that one time I tested that, the relay did not open when dropping to the "engine off" voltage and the batteries remained connected.

I would hope that Victron answers you. We need a better understanding of the various Cyrix programmed behavior than just that data sheet. I think the issue is that they set it to watch BOTH sides of the relay for "charging voltage" to open bidirectionally and then also the thresholds are not quite correct maybe. But I probably just don't understand. I also can't tell if the Cyrix charge would be better if it is setup for a one side only "high voltage" sensing.

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7 Answers
zuzullo avatar image
zuzullo answered ·

Hi there I got more evidence that the Cyrix is NOT isolating the Starter Battery (B1):

  1. Batteries were at 12.8v
  2. Then I fully charged them with the alternator for over an hour.
  3. Left the batteries overnight.
  4. Next day the Starter battery (B1)=13.1v , House battery (B2)=13.0v
  5. Turned on the diesel heater (7amp) for 30min and batteries are now reading: B1=13.1v , B2=12.4v (Great, the Cyrix is still protecting the Starter battery (B1)
  6. Turned on the solar pannel, which is giving +0,3amp
  7. The BMV is now showing around (-6,5amp) being consumed
  8. After a few minutes I hear a "click" the Cyrix connects!
    • (NOT GOOD, the Cyrix is now DRAINING the Starter battery (B1)
  9. I continue to use the diesel heater for another 30min and then turn it off.
  10. The batteries are reading now: Starter battery (B1)=12.6v , House battery (B2)=12.5v
    • (The cyrix did indeed use the Starter battery to power the house appliances)

Is this a normal behavior of the cyrix? How can I protect my Starter Battery so that nothing can drain it apart from the engine? An I missing something in the configuration?

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Michael Connolly avatar image
Michael Connolly answered ·

Has anyone come back on this? Looking at a 24v Cyrix connected to an MPPT which willcharge a 12v lifepo4 house battery. I’m concerned with these comments!


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zuzullo avatar image
zuzullo answered ·

I contacted Victron and was told to ask the shop where I bough it. The shop told me that this is a normal behavior so I had no choice but to get the Argo FET isolator. According to them, it is the only way to prevent 100% that the engine battery does not depletes.
At the same time, they told me that now I cannot connect the MPPT to the input plug of the Argo FET, because it wont properly charge both batteries. I cannot understand why does it work with the alternator and not with the MPPT. I will make the test anyway and let you know.

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cris avatar image
cris answered ·

Same problem,

starter battery (B1) lead acid 13.5 V

service battery (B2) LiFePo4 14,6 V

after some hours both battery at the same level 13,8 V because also with motor OFF the Cyrix

doesn't open the contact and drain the service battery in to the start battery.
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esteban avatar image
esteban answered ·

It's too bad Victron still won't weigh in on this. As far as I can tell, the default voltage settings for opening up the relay are not set at a very good point in the firmware. They must have had some reasons for choosing the open/close voltages and logic, but they don't work for me either, though I'm using the LI version.

My workaround is to ignore the Cyrix's firmware programming and just use it as a switched relay controlled by a BMV-7xx. Set the BMV's programmable relay in the app to do what we need the Cyrix for. Relay closed if starter voltage is above 13.x. Open if it falls below 13.x. Done. Of course you must dedicate your second input on the BMV to starter voltage instead of temperature.

I wish that Victron would:

1) make a unidirectional Cyrix that prioritizes separating and protecting the two systems when alternator is not running instead of making bidirectional which seemingly optimizes for charging your starter battery while charging house battery (what percentage of user actually does/needs this?) at the cost of adequate separation. A version for lithium also.

It seems that the typical vehicle setup is a lead-acid starter battery (who needs lithium there?) with (increasingly) a lithium house system. This would be optimized with a unidirectional combiner that simply opens and closes around 13.3 on the starter/charger side. With some delay for controlling rapid oscillations around setpoint.

2) Improve the logic controls for the BMV series relay to allow multiple rules (as now) but with individual settings for AND/OR and ordering of priority. Rule1 - choose thresholds. Rule 2, (second priority) choose AND/OR, choose thresholds. Rule 3? same.

Would love to have even two rules with setting AND/OR control for second. Extending relay control list to three or more rules/parameters would be fantastic.

Currently the only way to do this is by fooling around with fixed AND/OR rules for charge mode in combination with INVERT. Needlessly complicated. Just add a logic switch to each parameter.

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zuzullo avatar image zuzullo commented ·

Hi Esteban ,I totally agree. That is the new configuration that everyone I speak with wants :)
And yet, no comments from Victron :/

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Justin Cook - Bay Marine Supply USA avatar image
Justin Cook - Bay Marine Supply USA answered ·

I'll add a couple of notes here, because I was just back on the test bench playing with a couple of Cyrix-CT-120A units and trying to nail down what they're actually doing.

1. As far as I can tell, a properly functioning CT always prioritizes the voltage of Bat 1; I've tested with chargers at both ends and while it definitely closes when there's a charge on Bat 2, its 3-minute opening timer starts when Bat 1 reaches 12.8v, not Bat 2. On the closing side, it seems to close when either battery reaches ~13.0 - 13.2v (what the CT defines as a "charging voltage").

2. Based on this observation (tested on 3 separate CT-120s to verify readings), I think that anyone experiencing a discharge of their batteries below 12.8v while the CT is closed, such as @zuzullo, has a defective unit. The datasheet for these says a 3-minute opening timer starts at 12.8v, and multiple tests confirm this, so I think behavior other than this indicates a faulty unit.

3. For those seeking more control over the connect/disconnect voltage settings, I might point you to the Cyrix-i 400, which offers user-selectable connect/disconnect profiles; for those seeking lithium-compatible Cyrix devices I would point you to the Cyrix-Li line, which includes the Li-Load, the Li-Charge, and the Li-CT. Although certainly the documentation on these things could use some work, Victron has thought ahead and provided solutions that apply to most potential installations.

4. As a final note, for anyone who already has a regular CT and wants to be absolutely positive that it's opening when there isn't a charge input present, I suggest a bit of a rigged solution but one that's easy enough to do and can look good if done right: use a momentary-off switch wired inline to the Cyrix's ground wire (pin 86 on the CT-120). A momentary disconnect of the ground will open the CT, at which point it will remain open until charging voltage is again sensed at either of the batteries.

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morissen avatar image
morissen answered ·


I am new to this but read above very carefuly. I think my friend zuzullo wants a cyrix work as a logical isolator. It is not designed for this purpose. If one wants to keep the starter always isolated that is, not drain unless engine starting needs it, then one shoul buy a mosfed isolator. Cyrix will let you drain the starter to the service bank (hopefuly it does not melt the cables doing it) but it will not let it go dead below a usable fixed voltage. This must be why they designed it. If you start with starter 13.4 and service 12.1 and after cyrix works you get 12.8 both it is not bad at all. You still have starter power and you have improved the service.

correct me if my logic is wrong pls. Thx.

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