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Stephen Otto avatar image
Stephen Otto asked

SmartSolar 150/70 shifting to Float prematurely

I have a system that includes the SmartSolar 150/70 networked with a Venus GX and BMV 702. The battery is 24 volts and 258 Ah. The charge controller consistently switches from Bulk to Float seemingly prematurely -- when the BMV indicates that the battery is at around 81-82%.

The battery has a charge limit voltage of 29.2, but I have tried to be conservative with my max voltage setting for the charge controller -- I have it set to 29.0 volts currently. I understand that it shouldn't charge to 100% with that setting (nor do I want it to charge fully out of battery care considerations), but I should think that it ought to charge higher than 80%. Based on the history, I don't see it touching 29 volts even momentarily which might cause the battery to trigger the mppt to change to Float. It seems to get no higher than around 28.8 or so.

Any suggestions? TIA!


MPPT Controllersbattery chargingBMV Battery MonitorVenus GX - VGX
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klim8skeptic avatar image klim8skeptic ♦ commented ·

Hi Stephen,

"It seems to get no higher than around 28.8 or so."

You don't mention where this voltage is measured (battery, mppt terminals, Victron connect app), or what device this voltage was reporting (702, Cgx, dvm, Victron connect app/mppt).

What firmware is the mppt running?

Have you measured the voltage drop between the mppt and battery?

Is the mppt running voltage correction (via 702/smart battery sense/cgx)?

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3 Answers
Mark avatar image
Mark answered ·

To assist further please provide some screen images from Victron Connect showing your current settings for both the BMV & the MPPT.

It's also important to note that both act independently of each other so it possible for only one device needing optimization or both.

First thing to check is what the MPPT charge current is just BEFORE the MPPT switches from abdorption to float. It should be ~1 to 2% of your battery C20 capacity.

If it's more than you need to increase the absorption time on your MPPT to get the battery fully charged.

If it's is going DIRECT from bulk to float phase, the only way that this is possible is if the charge current is under the default 2A tail current threshold at the time that the absorption voltage is reached.

Once you know that the MPPT charge settings are correct then you can check/work on your BMV to get it to synchronize under the correct charge conditions and also to track the true battery SOC% as accuracy as possible in between synchronizations.

Regarding voltage differences - this is most probably due to temperature compensation. Do you have a temperature sensor connected to your BMV-702 and temperature compensation active?

As you have a Venus device you can also use DVCC to share the BMV voltage and temperature with the other devices.

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thomassolar avatar image
thomassolar answered ·

Have you installed the latest firmware, the v1.42?

What you described looks a bit like what I used to experience. And yes, Victron has stated that v1.42 "fixed a bug that made the MPPT regulators switch from bulk to absorption 200mV to early"

That bug was especially making trouble for us having a Li-ion battery pack, because the charge voltage and the actual battery voltage is almost the same in these systems. So the regulator switched to absorption 200mV before it should, causing the "absorption time Count Down" to start. And finally, the absorption time went out and the regulator switched to float before the batteries even reached the set absorption voltage.

After I installed the 1.42, the problem seems to be gone!

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Stephen Otto avatar image
Stephen Otto answered ·

Thanks everyone. I'm going to update to 1.42 and then I'll report back again.

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