svsimple avatar image
svsimple asked

MPPT Controllers Going to Float Too Early

I have a Cerbo GX, BMV-712, and 4 x SmartSolar 75/15 MPPT controllers.

Controller Settings:

Bulk/Absorption Voltage: 14.4
Float Voltage: 13.4

Max Absorption time: 6 hours

Tail Current: 6.0A

Some of the controllers (sometimes one, sometimes two or three) often go into float before the 6.0A tail current is reached (as recommended for my battery bank), while the others stay in absorption mode. The controllers in absorption mode cannot finish the charge cycle (because they cannot maintain 14.4V until tail current is < 6.0A) and the controllers in "Float" mode are not producing (0W) because they are waiting for the voltage to drop to 13.4.

I was under the impression the the Cerbo would coordinate the controllers so they would act in unison. Perhaps I missed a setting?

I am running firmware 2.54 on the Cerbo GX and cannot upgrade yet because I am waiting to get a question answered about using a powered hub since I will lose one USB port currently in use with firmware greater than 2.57. I have read the release notes for later firmware and it does not seem to mention this issue.

Any thoughts on what to look for would be greatly appreciated.

MPPT Controllerscerbo gxfloat
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4 Answers
JohnC avatar image
JohnC answered ·

Hi @svsimple

Try setting your mppt's to a Fixed absorb time rather than Adaptive, and ensure that all their settings are the same. This is just to get Adaptive out of it so you can test, free of interference.

To get all the choir singing from the same hymnbook, turn on DVCC in the Cerbo.

Check SVS so they all see the same V (the BMV).

Check STS for the same Temp. (maybe the BMV, or a Multi/Quattro). This is just for continuous temp-compensation and you may be able to get by without it. A good feature anyway..

Check SCS for the same Current (the BMV). They should all then see the same total Tail instead of their own current measurement for the purpose of switching to float.

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svsimple avatar image
svsimple answered ·

@JohnC, thanks for the reply.

I do have DVCC on and all the settings you recommended. I also changed the config on all MPPT controllers to Fixed Absorb time and will monitor what happens tomorrow morning.

I went ahead and set up the USB Hub and upgraded to 2.60 this evening, so maybe that will make a difference as well.

I was looking at the logs and have found examples of how the MPPT controllers are seeing different voltages (although the difference is slight, it is still different). Seems DVCC settings are not really working and the MPPT controllers are not really using the voltage data from the BMV?

See these screenshots for 4 days ago:

You can also see on the screenshots the difference in Absorption time between all the MPPT controllers (1 & 2 stayed in absorption while 3 & 4 went into float hours earlier).

Let's see what happens tomorrow, but is there any other data I can gather to help figure this out?

Thanks again for the suggestions!

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JohnC avatar image JohnC ♦ commented ·


Good you already have DVCC up. Don't get too excited about very minor differences in readings, as the sample times don't happen all together.

But there's something wrong there. To start the day at 12.5V and have 6 hours in Absorb ain't right. Mppt's #3 & #4 look realistic, but the first two show excessive times.

Maybe firmware?

Your 6A Tail doesn't mean much without knowing your batteries. I use 0.02C on my floodeds, and gives me usually less than 2 hrs Abs. By then SOC is close to 99%, and Float over the next 2-3 hrs will pick up the last 1%, about the time it's due to sync at 0.006C (0.6%).

Everyone's batts are different, but a 6hr Abs is way out there.. especially when it hasn't even finished..

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svsimple avatar image svsimple JohnC ♦ commented ·

@JohnC, I believe the reason for the 6 hour Absorb time is because when MPPTs 3 & 4 got to float, MPPTs 1 & 2 cannot reach the absorb voltage of 14.4... and since the tail current has not been reached yet, they stay in Absorption mode but generally cannot keep voltage beyond 13.9 or so.

I believe the adaptive "timer" does not account for the time when the voltage is not at the absorption Voltage setting, right?

The 6A tail current is per the battery manufacturer specs.

I have a bank of 12 x G31 Firefly batteries and their specs call to stay in Absorption until the bank reaches a tail current of 0.5A per battery at 14.4V (0.5A x 12 = 6A).

I'm watching today after I made the config change last night to Fixed Absorption time (set at 6 hours). From all the information I have from the manufacturer, the important value is the tail current to ensure a full charge.

Will see what happens and report back in a few hours.

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JohnC avatar image JohnC ♦ svsimple commented ·


Have any luck?

I haven't forgotten, just been chewing this over - slow chewer.. :)

What I think has happened (in your link) is that all mppts have reached Absorb at about the same time. Then the current has tapered off, as it will. #3 and #4 have got 'lazy' and tapered right off to near zero A, leaving #1 and #2 to carry the load. Doing that they've reached the set Tail and dropped to Float. The others continue, but they can't get as low as the Tail at Absorb V (or anything above Re-bulk V), so they don't reach it.

And frankly they may *never* reach it. I've read the sales blurb on those batts. Call me a cynic or skeptic, but that quoted 0.5A is unrealistic for any batt with Pb in it. Its actually lower than what I use to sync a SmartShunt at Float V, let alone function at Absorb V.

My guess is it's too low by a factor of at least 4-5 times.

But there's other things happening in your system I can't explain. Like when the sun sets, your 'busy' mppts still haven't seen a Re-bulk V until very late.

And oh, I'm not sure what the timer counts as Absorb. Seems to me it keeps counting until it reaches Rebulk V, but it won't honour the mppt tail until it's back at the 'real' Absorb level. Given that can sometimes wander a bit with loads, etc, maybe there's a tolerance involved there?

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svsimple avatar image svsimple JohnC ♦ commented ·

@JohnC, I replied to you then tried to convert my reply as an answer, but it is hidden for some reason. Do you see it? Can you accept it as the answer if you agree?

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JohnC avatar image JohnC ♦ svsimple commented ·


Dang, I found your answer stuck & spammed in the moderator queue. I've limited moderator powers, but I think I've revived it successfully. I'll check it a little later when I get some time to analyze the data.

Others can see it now too. And welcome to comment..

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svsimple avatar image
svsimple answered ·

@JohnC, I think I spoke too soon.

Today one of my MPPTs is stuck in Bulk, while the other 3 MPPTs went into Absorption already.

Looking at the console for MPPT 1, the "battery voltage" is showing under 14.4, while the BMV-712 reports 14.42. Looks like MPPT 1 is not using the Shared Voltage (SVS) data? Or is it displaying what it sees and still uses SVS data regardless?

I have double checked that that SVS setting is on. I even turned it off and back on just in case.

Here's the log screen:

The other MPPTs' log screens also show lower than 14.4V, but they went to Absorption.


mppt1.jpg (67.8 KiB)
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JohnC avatar image JohnC ♦ commented ·


Responding to both posts..

Yeh, getting your head around how it works (or supposed to) is key. The 'Panel 1 MPPT' screen above is showing the data it sees itself (after all, you clicked on it to get there), and not necessarily equivalent to the data it's being sent by DVCC and expected to follow with it's subsequent decisions. And it's not just V, the '6.8A' there might well be equivalent to something far lower from the BMV if there's a load coming off the batts.

But it's awfully hard to diagnose this from afar. You need a sixth sense, and mine's telling me the 'Abs > Float tail' is far too low, and may be the base cause of your issues.

VRM graphs are also a great help, not the individual mppt's but the overall. Begs the question, what is the overall end result?

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svsimple avatar image
svsimple answered ·

(Mod note: This post was delayed and the posted time is incorrect, so may be out-of-sequence)

@JohnC, yes, I believe it is all working as it should now.

Your replies gave me several tips that helped me understand better how the system works as well as the solution, which I will describe below to hopefully help others in the future.

First, I was operating under the incorrect assumption that the Cerbo GX was "telling" the MPPT chargers when to switch state (Bulk => Absorption => Float). From this "experiment" I understand (hopefully correctly) that the MPPT chargers get shared voltage, temp, and current data from the Cerbo GX, provided these are enabled in the DVCC settings. The MPPTs then make their own decision about when to switch state based on their individual configurations and the DVCC shared data.

Provided I am correct above (please correct me if I am wrong)... based on. your comment regarding the slight voltage differences in the logs, because the measurements are not taken at the exact same time, the symptoms I was experiencing are easily explained.


Given the slight time difference in measurements between MPPT chargers, the Adaptive Absorption timer expiration would be reached in one MPPT charger before the others (perhaps just a few seconds before).

When the timer was reached in one MPPT, it would go into float. When this happened the other MPPTs would not be able to keep the voltage at 14.4V, which would then "pause" the Adaptive timer for the other MPPTs, thus leaving them in Absorption mode for the rest of the day.

The solution for me, as you suggested I try, was to set the Absorption Timer to Fixed, instead of Adaptive. This allows the MPPT chargers to stay in Absorption mode as long as required to reach the Tail current I have configured based on the battery manufacturer recommendations.

Here are screenshots of the daily log for Sept 17th and 18th showing that the controllers spent roughly the same mount of time on Bulk, Absorption, and Float:

Thank you so much for your insights and helping me figure this out!

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