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billknny avatar image
billknny asked

Combined system Solar/generator charging strategy.

System is on a sailboat, off grid. Solar supplies about 50% of daily power needs with the rest supplied by a DC generator.

System Description:

  • Eight g31 Firefly batteries parallel series to 24V, 464 A-hr,
  • Cerbo GX,
  • BMV-702,
  • MPPT Bluesolar 100/50,
  • 125 Amp DC Generator.
  • Total daily consumption ~6kW-hrs

The plan was to use the generator first thing in the AM to bring batteries to ~90% SOC, get the heavy lifting of bulk charging out of the way, then let the solar system top them off.

My understanding is that the battery voltage on "wake up" is how the MPPT estimates battery SOC and uses that to decide how long an absorbtion cycle to run. The issue I am having is that the generator is typically running when the MMPT turns "on" with the sunrise in the morning. Because it is seeing a voltage significantly higher than battery resting voltage, it cuts the absorption cycle short leaving the batteries less full than they should be.

Is there a way to force the absorption cycle longer? Or an alternative generator running strategy that doesn't involve running it at midnight, or during the solar charging day?

Right now my best idea is to reset the solar float voltage to be equal to the absorption voltage. This will work fine when off grid, because the chances of overcharging are very small. It would need to be re-set every time I plugged into shore power. An annoyance, but not impossible.

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2 Answers
JohnC avatar image
JohnC answered ·

Hi @billknny

You have all the kit there to set up DVCC with SCS (Shared Current Sense), so the mppt can forget it's defaults and follow a real-time charge algorithm. https://www.victronenergy.com/media/pg/Cerbo_GX/en/dvcc---distributed-voltage-and-current-control.html

The manual may seem complex, but it's actually quite simple to set up, but you may need to do a little fine tuning. I love it. Akin to setting up a BMV 'Tail' to sync, but now you have an mppt Tail to drop from Absorb to Float.

I use a generator too, but I like to leave the daytime for the solar alone. I start it manually at sundown when necessary, but could have it auto-start. I use a Tail there too to determine when to stop it. The same level as I use for the mppt Tail when Abs is done.


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billknny avatar image
billknny answered ·

@JohnC Thanks for the reply. I have DVCC enabled, but it seems not to be doing as I hoped with the MPPT and tail current. Why I am not sure, and I'll have to dig a bit to see if there is a system problem or (more likely!) a user issue.

But... I am very interested in a clue how you use the tail current to stop your generator. That would be WAY better for me than my current practice of using the SOC as the stop trigger. Is this something that I can do with the standard install? Or do I need to use the Node-red programming system? I have a couple things running on that, so it's not out of the question by any means.

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JohnC avatar image JohnC ♦ commented ·

@billknny

Good you're well advanced. Re the DVCC, in the mppt I just use the 'Adaptive' Absorption Duration, 4hr Max Abs (irrelevant really), and 7A Tail Current (over 225Ah of 48V batts). The 7A determined by trial'n'error, but gives me less Abs time on average. Usually 1 to 1.5 hrs. So I'm happy.

I don't think I can help much with the genny auto-stop, as I do both manually. You don't mention having a Multi/Quattro, so the Assistants there are out. I wouldn't have much issue using SOC myself, but my floodeds are quite reactive. So much so, that my mppt's 7A flip is invariably reached at 99.0 +/- 0.2% SOC daily. No point forcing more Abs V by then. The same for the genny - no point wasting fuel for little input. The 7A is my lower limit there, and I'll often stop it earlier.

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