Michael Moorman avatar image
Michael Moorman asked

How to restart MPPT charge cycle

I use MPPT 75/15 charge controllers on my sailboat. They work great, however, when I run the motor and the alternator is also charging the batteries, the MPPTs get fooled into thinking that the batteries are charged and go into float mode way too early. They stay in float mode even after the engine is off.

Is there a way to avoid this or to restart the MPPT charge cycle?

I can make this happen by putting a blanked over the solar panels for a few minutes - but that is a difficult way to do it.


ps - what I'd really like to see is the MPPTs use the information/intelligence from the BMV-702 to manage the charge cycle :)

MPPT Controllers
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2 Answers
Paul B avatar image
Paul B answered ·


If the batteries get to 14.4 (depending on type) then they are at around 80% charge then at the end of a absorption phase held at 14.4 they are full .

Now the reg does not care if another charger is also used it just sences the battery voltage and will switch to absorption then to float and then it will just supply all the loads (if it can that is) and hold the float voltage.

You really should not keep putting the batteries back into bulk all the time when they are full.

If you really want to reset the reg then put a switch in the PV and Battery lines and turn them both off, then wait say 2 min then turn them back on

When turning OFF ITS ALWAYS BEST TO turn the solar off first then the battery

When turning ON ITS ALWAYS BEST TO turn the battery on first then the Solar

Also another possible way is if you have a smart solar reg or a victron bnluetooth dongle attached you can go to settings and turn the charger OFF then back on this may work.

I have not done this myself so cant confirm that.

I would also HIGHLY recommend that if you don't have the bluetooth option, then buy the add on and get it as you also get 30 days of battery charge history ie max charge voltage and MIN battery voltage, very handy for reference plus many other setting options, Its a must have in my opinion.

in regards to getting the info from the BMV, well this is not so wise ie I presume you are referring to SOC here, well that maybe fine when the batteries are NEW and the real battery capacity is there and available,

However these battery's quickly fall off in capacity and thus the SOC is a not a really the correct capacity say after a year.

Voltage is the way to only measure a sealed battery, (or a special capacity test meter)

a good way to describe this is

lets say you have a 200 amp hour bank and yes we can use 50% so we have 100 A/H usable

now in Theory we can do this 1000 times and they are finished. well .

lets convert to a 200 liter water tank for (example purposes only)

and every time you go down to 50% you then add 500ml of sand (simulated aging of the battery) so as you can see the bottom half of the tank slowly becomes sand and you no longer have your original capacity of 200 ltr, but you are still pulling 100 liters out SO you dont now know you are pulling 100% every time. (See the point)

Its no longer a 200 amp hr battery its a 100 amp hr battery and you are pulling 100%

this is why the batteries fall of a cliff as they get old as people forget to adjust for the real avalable size based on AGING and start to cycle very deeply.

Its also why people who only use 20% of there bank get extra time (yes there number of 20% cycles goes up) but they are not also going below the 50% level even after 2 or 3 years

other factors are involved also (especially heat and charging to quickly ie to many amps) but lets leave this for another time.

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Michael Moorman avatar image Michael Moorman commented ·

Hi Paul,

Thank you for your thoughtful answer.

I believe what is happening is explained in Sec 1.7.2 of the MPPT 75/manual:

"Additionally, the absorption period is also ended when the charge current decreases to less than 1A."

I suspect that the absorption voltage of the alternator regulator may be slightly higher than the solar, so MPPTs see the battery accepting less than an Amp. I'm leaving the dock today and will try to monitor closely.

On a related topic, I do have one dongle, however I have four MPPT 75/15's (one for each panel to theoretically minimize the effect of partial shading.) I switch the dongle between MPPTs to change the settings or to update the firmware. Will the Victron Connect app work if I have multiple dongles set up to the MPPTs?


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Michael Moorman avatar image Michael Moorman Michael Moorman commented ·

FYI - I monitored my BMV-702 and MPPT on a pair of SmartPhones today as I started the engine and confirmed that the MPPT went quickly from Bulk to Absorption to Float once the alternator kicked-in. Potentially I could raise the Absorption Voltage setting, but that is risky.

I also confirmed that if I momentarily toggle the "Charger Enabled" slider off and on in the battery settings for the MPPT that it will revert to Bulk charging mode.

Finally, I also confirmed that if I momentarily through a tarp or a blanket over the solar panels that the MPPT will go back to Bulk mode.

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jeremie-simonet avatar image
jeremie-simonet answered ·

Hmm I have the same issue, my amp is <0.6 from solar panels. I believe that is whys its switching to float.

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