question

the-salmon avatar image

Off-grid installation used only during the weekend: best MPPT weekdays configuration

Hello,

I recently installed the following off-grid system in a country house that will be used only during the weekends:

  • 4 300W solar panels
  • MULTIPLUS-II GX 48V
  • SMARTSOLAR MPPT 150/35 48V
  • 8 Acid Lead batteries of 6V each

Normally I switch on the MP II only when I'm onsite and I need power (once a week so far), during the other days I leave the MP II off. The solar panels and the MPPT are always active (see the screenshot below taken from VEConnect).

I see from the App that every day of the week, even if the MP II is OFF, the batteries are exposed to a charging cycle (bulk, absorption, float). The voltage is raised from 51V to 59V everyday.

That is the behavior suggested from my Victron and battery re-seller.

I'd like to have the confirmation from you that my settings are the best ones in order extend the life of my batteries as much as possible.

Is the daily charging cycle really needed? Wouldn't be better to disconnect the 4 panels during the weekdays and to connect them to the MPPT only during the weekend?

Thanks for your suggestions!

MPPT - Solar Charge ControllerMultiplus-IIoffgridlead-acid
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2 Answers
JohnC avatar image
JohnC answered ·

Hi Salmon.

You have modest needs, and I too don't think an aggressive profile is warranted. For batt longevity you could just use a flat 'Storage' profile of (say) 53V. And even when visiting once a week, probably no need to change that even then.

If you were seriously cycling them, then things may change, but that's usually associated with fitting the solar day into the charge needs. That doesn't apply here, so just *stroke* them.

The '50.7V' day when you boosted, is still around a standing V for fully charged pb's, and that's a healthy state even before you started.

They'll inherently self-discharge a little every day, so you get to put that little bit back. But no need rush it. Akin to using a garden hose to give a bottle-fed baby a drink. Easy does it, hey..


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Thanks @JohnC! What do you think about disconnecting solar panels during the weekdays? Wouldn't be better for batteries longevity?

Yeh, possibly the best advice re that might be from your batt manufacturer. I suspect a 'storage' charge would be preferred, and after all it'll only happen while the sun is up.

Nor does it have to be an on-or-off decision. You could even ease back further on the storage V if you choose.

Mark avatar image
Mark answered ·

I have a similar setup and usage profile to yours.

Working with the existing logic available, my advice is to utilise the MPPT 'tail current' feature and set it to 1 to 2% of your battery capacity rating. This option can be found after enabling 'expert mode' in the MPPT battery settings page.

That way, on days when the battery is already full or very close to full in the morning the absorption phase will end as soon as the tail current threshold is satisfied (whilst also at the absorption voltage) and the MPPT will move into float phase. This could be as fast as 1 to 5 minutes...

This will prevent the MPPT unnecessarily charging in absorption phase for an extended time period every day.

If you already have or add a BMV to your system then you can also enable DVCC-SCS (shared current sense) and the MPPT tail current feature will work perfectly even when you are using the system with some MPPT charge current going directly to power loads (rather than all current going towards battery charging).

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