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damien-c-tanner avatar image
damien-c-tanner asked

Do I need a BMS when using a single MPPT 75/15 and single Victron lifepo4

Hi,

I have a simple 12V solar setup: 200W panels, MPPT 75/15 and a Victron lifepo4 160Ah (manufactured in 2015).

I have set this up with a Victron minibms and connected it to the MPPT with the non inverting remote on/off cable.

But now I am wondering:

1) Do I actually need the minibms? It looks like the MPPT supports lifepo4 and will shut off the solar charging when the battery reaches max voltage.

2) If I don’t use the minibms, will my Victron lifepo4 battery cells balance automatically? The minibms connects to the two Victron battery data cable, but it’s not clear to me if the battery has internal cell balancing which works even if a BMS is not connected to these data cables.

MPPT ControllersBMS
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2 Answers
nebulight avatar image
nebulight answered ·

Yes you do. Cell balancing is limited in the current in which it can balance, espeically the older victron lifepo4 which use passive balancing. You can charge faster with your panel than the battery can balance which could lead to an over voltage condition even with balancing.

What are you using to discharge the battery? You should have protection on there as well. If it's small DC loads only, you should add a battery protect in li ion mode to protect from over discharge.


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damien-c-tanner avatar image damien-c-tanner commented ·

Hey I just replied, but accidentally to my original post. See above :)

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damien-c-tanner avatar image
damien-c-tanner answered ·

Thanks for the quick reply!

Okay that makes sense. So does the minibms use the data cables to do individual sell balancing? What is confusing to me is that the minibms doesn’t mention anything about cell balancing - just load an charging on-off.

My dc loads are max 15A, so I have connected them directly to the load output of the MPPT 75/15. Am I right in thinking the MPPT will disconnect the loads at low battery voltage? If this is the case, I don’t need a battery protect do I?

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nebulight avatar image nebulight commented ·

The balancing and monitoring takes place inside the battery. The external BMS is just for external control to disconnect chargers/loads based off the monitoring of the battery.

As for using the load output of the MPPT, that will not work as the BMS will not disconnect the loads of the MPPT because the output of the BMS will only disconnect the chargers how you have it wired. Another undesirable outcome of wiring it this way is if you have an over voltage, the MPPT will shut off and your loads will also shut off as well (I believe, I've never tried this, but I think the non-inverting cable just turns off the entire unit).

I would add a small BP65 to your system and wire the load terminal from the BMS to the BP. This way the BMS can control the loads and charger separately. Just don't connect an inverter to the BP.

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damien-c-tanner avatar image damien-c-tanner nebulight commented ·

Ok sounds sensible. Since I have a BP anyway, I’ll do it like that :)

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