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Lars avatar image
Lars asked

MPPT 75/15 never reaches float mode

H all, first question on this forum.

We got a 120W solar panel on deck, connected to a MPPT 75/15 and two 100Ah lead-acid batteries when we bought our latest sailboat some four years ago. This seems to have worked fine, but last summer we found it difficult to get enough power to operate the fridge and have the anchor light on throughout the night. It may be the batteries are going tiered, but when I started to investigate I noted that the installation was not the best. The regulator was installed behind the panel at the chart table with BATT connected to the 12V after the main switch and PV connected to the panel, in such a way that it lost contact with the batteries when the master switch was turned off. This left the system with some 15V on the 12V bus bar with everything switched off including the main switch. Moreover, I noted that the regulator never left Bulk mode, no matter how well charged the batteries where, but the charging current was very small (about 1A) even though the sun was shining directly at the panel.

I have now modified the installation, moving the regulator down to the battery bay and running separate cables from the panel so that it it always connected to the battery, but the problem with low charging current and no Absorption or Float remains. Is there a risk that the regulator was damaged and if so, is there a simple way to test it?

Cheers!

/Lars

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klim8skeptic avatar image klim8skeptic ♦ commented ·

Sounds like you have a 32 cell solar panel, with a Vmp of around 16 volts.

Pl confirm solar panel specs.

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4 Answers
seb71 avatar image
seb71 answered ·

Start by checking the MPPT settings.

That single 120W panel is the only charging source for your 200Ah 12V battery bank?

Does the charging current increases when the batteries are discharged and you have full sun? Or it is always 1A?

Having a battery disconnect for the MPPT is not an issue as long as you also have a PV disconnect (and you disconnect the PV first, then the battery; when connecting, connect the battery first to the MPPT, then the PV).

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Lars avatar image Lars commented ·

Hi and sorry for the long silence. As said, the boat is on land for the winter and no experiments can be made, so I don't really know the exact behavior. Also, as stated above, due to the previously faulty installation the MPPT has been subject to repeated BATT disconnects with the PV still connected for long periods of time, so my main concern is that it might have been damaged. I guess all I can do is to wait for the spring and make better measurements.

For what it's worth, the solar panel make and model is Sunbeam PS100img-20210203-164730237.jpg

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Lars avatar image
Lars answered ·

Thanks for the input. As said, the system came with the boat and I am not 100% sure about the make and model of the solar panel, but it looks very much like the Solarme Flex 110W EFTE, so I may have been a bit off regarding the power rating. The solat panel is not the only source of charge, there is the 50A generator on the engine and a small mains charger for the rare occasions when we have a land line connected. But despite these other sources, I have never seen anything but the fast blinking yellow LED indicating Bulk charging.

The main issue with the original installation was that there was no PE switch and the BATT was connected on the 12V bus after the main switch, so the regulator would have been repeatedly subjected to battery disconnect with the panel still connected. This has now been rectified, but my remaining concern is if the regulator is still in good health. The batteries have a "green eye" indicating good charged condition and I have put them through a reconditioning process with a smart 15A charger, but that of course is no guarantee for their actual condition.

The boat is now on land sleeping under a snow covered tarp, so any further experiments will have to wait until spring. I have the regulator on my desk, so any means to verify its health would be appreciated.

Cheers!

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michelg avatar image
michelg answered ·

Hi Lars ! A few things, first being to recharge manually your batteries with a charger, in order to be sure they are fine. The Victron Blue Smart Chargers are pretty good, it saved my "dead" battery.

Then, add on your boat a 2nd PV in serie or a bigger one to get more power for the battery bank. 4 years on a boat for "service batteries" not well charged can also mean they are sulfated / depleted and it will be better to change them in the future AFTER being sure that your setup is providing enough energy off-shore, and on-shore.

Michel

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Lars avatar image Lars commented ·

Hi Michel and thanks for the input. The batteries are obviously the prime suspect in this, and I have put them through a couple of reconditioning cycles with a smart charger (albeit not the Victron). Fitting more or larger panels is not feasible on a boat the size of ours, but as said, the solar panel is not the only (or even prime) energy source on board.

No one seems to address my prime question though. What will repeated and prolonged battery disconnects with PV still connected do to the MPPT? I guess I will have to wait till spring and see how it works with fully charged batteries and correct connection sequencing.

/Lars

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alan avatar image
alan answered ·

That panel is way undersize for 2x 100A batteries imho, when new it might have just cut it, once aged it will struggle.

Your trying to charge 2400w with 100w, in optimal conditions and performance that’s 24hrs.


If you can, try to fit a 36v 300w “grid” type panel ~1650x1050.

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Lars avatar image Lars commented ·

Thanks for the input! The solar panel was never intended to be the sole energy source, but to keep the batteries fully charged during times when the boat was not in use. The main source would be the 50A generator on the engine. With the boat idle for a couple of weeks during the summer I would expect full batteries.

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