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zabbaskeema avatar image
zabbaskeema asked

MPPT 150/70 as a LiFePO4 charger in a DC-DC context.

Hi Gurus,

I have a 48v LiFePO4 bank that is charged with a big genny.

I am soon changing the 12v house bank to lithium as well.

Here: https://www.victronenergy.com/live/drafts:mppt_faq Q7 it is mentioned that the MPPT can be used as a DC-DC charger.

The rationale is that a buck converter has no lithium specific/programmable charging algo but the MPPT has this csapability hence when not on shore power we would have this scenario: Genny -> 48v bank -> mppt -> 12v bank.

The problem is that all grounds are common on a boat as per code. The MPPT documentation mentions this: "WARNING: DC (PV) INPUT NOT ISOLATED FROM BATTERY CIRCUIT." -> no problem, we have a common ground anyway,

Then more problematic apparently: -> "The plus and minus of the PV array should not be grounded. Ground the frame of the PV panels to reduce the impact of lightning."

In our case the PV is the 48v bank which shares the same common ground as per code for the whole boat, which essentially is the... sea.

Is the FAQ right? Can someone from staff tells us the lowdown please?

Merciiii!

Patrick

MPPT Controllersbattery chargingLithium BatteryOrion DC-DC Converters not smart
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benm avatar image benm commented ·

Victron has updated the pag mention above:

Here: https://www.victronenergy.com/live/drafts:mppt_faq

Q7 it is mentioned that the MPPT can be used as a DC-DC charger.


The new page says:

Q7: Can I use the MPPT Solar Charger as DC-DC charger?

For example to charge a 12 Volt battery from a 24V battery bank?

No.


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zabbaskeema avatar image zabbaskeema benm commented ·

You should write this as an answer as this is the answer :D

What a shame that my question triggered them to change this FAQ.

I am glad I did not bought the unit based on this FAQ. Thanks again Ben!

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4 Answers
wkirby avatar image
wkirby answered ·

It will work since your 48V bank and your 12V share the same negative ground point. So, since your source and destination are not isolated a MPPT controller will work as a buck converter.

The meaning of "Don't ground the array" is to only run + wire to the controller and then use the chassis / hull as a return path to the PV. You should have a separate + and - from the PV to the controller. The PV - will then be grounded close to the single central ground point (near the batteries I expect) thus reducing the chance of ground loops.
You can simply bond the PV frame to the ground at the PV array location since no current flows through the PV frame under normal circumstances.

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markus avatar image markus ♦♦ commented ·

Hi, how does Q7 deal with MPPT input short circuit current limit? A fuse should be mentioned here in my opinion.

Regards,

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zabbaskeema avatar image zabbaskeema commented ·

Thanks, when you say it will work as a buck converter do you mean the mppt would lose its capabilities to intelligently charge the lithium bank?

That would render the idea/strategy moot ;)

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Paul B avatar image
Paul B answered ·

Patrick, there is no cut and dried answers in regards to the ground loops and lighting, but I think the best way is to keep all grounds or Neg common and if this is a sail boat then tie the mast in as well.


plus also lighting and boats, everyone will have a different answer, and no one is game to put there name to a 100% correct answer. so in this area its your call I think.

however there are some quite good lighting and EMP damage suppression systems out there. the one that we use is the one from Tasmania Australia


they seem to have it rapped up for 12 to 150v DC and 120 to 230 AC as well there web site link is below

we use the surge protectors

http://www.novaris.com.au/

http://www.novaris.com.au/product-category/power-protection/dc-surge-diverters/

http://www.novaris.com.au/products/

http://www.novaris.com.au/lightning-facts/



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boekel avatar image boekel ♦ commented ·

There's a big difference between 'ground' and negative, make sure there is only one connection between ground and the negative side of the electrical system (usually the engine: starter motor and often the alternator also)

yes the mast should be grounded, but a lot depends on the material the ship is made of. There is enough information on this topic online.

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zabbaskeema avatar image zabbaskeema boekel ♦ commented ·

In the question I mention lithium. I already have three damn buck converters - they are dumb. The mppt has built in logic to charge lithium and this is what we want. Last resort would be to use the buck converters but this is not the right way to rely on the BMS to shutdown a charging source through an output relay.

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boekel avatar image boekel ♦ zabbaskeema commented ·

It depends on the type of battery what charging profile is necessary, does the BMS always balance, or only above x volts.

Lithium is actually very simple to charge: just give it a voltage (CC-CV is the fancy name). just don't trickle charge at 100% SOC.

For LiFePO4 batteries: 3,45 V per cell (13,8 V for 12 systems) is the max I'd charge continuously, if you don't need all the capacity go down to 13,6 or 13,4 V

Only question is: does the battery need a higher voltage from time to time to balance, or does it also stay in balance when kept at 13 point something volt.

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zabbaskeema avatar image zabbaskeema boekel ♦ commented ·

Thanks for the feedback!

You cannot do CC-CV with a buck converter. It’s pouring all it can at all time. So depending on various factors it will do its best to fulfill the rated voltage up to the maximum Amps it can deliver. It’s brute force! No concept of constant current here even. One could say that being lithium, the bank will load the converter to its max until the « knee » where the Amps it accepts drops significantly. But we don’t want to get there do we ;)


As I said, it’s possible to use the dc-dc but Theo is relying on the bms to actually disconnect it when a given voltage or capacity has been put in the bank. Not the nicest design... with the mppt in charge (pun!!) to apply the CC-CV then the bms does what it should do, eg. monitor and act if parts of the system misbehave. Is that clearer?

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boekel avatar image boekel ♦ zabbaskeema commented ·

What you describe (pouring in all it can) is exactly what CC-CV is, so the DC-DC converter gives it's maximum amps 'Constant Current' up to the set voltage, that's when the 'Constant Voltage' part starts.

End no I wouldn't charge (far) up to 'the knee', but that's nothing to do with CC-CV charging.

relying on the bms to disconnect is indeed not the way to go, that would reduce the lifespan of your batteries.

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boekel avatar image
boekel answered ·

Why not use a DC-DC converter for this? Orion-Tr 48/12-30A (360W) is cheaper than a 30A MPPT and is even isolated, this might safe you the trouble of ground-loops.

I do not know the exact configuration inside the MPPT's but if they measure current on the negative side this will -not- work with the situation you describe (common ground) and IIRC I've seen a problem with this before where someone had a connection between negative of battery and solar posts.

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esteban avatar image
esteban answered ·

Since you already have "dumb" buck converters, I suggest that you add your own custom intelligence. Add a relay interrupt (Cyrix-Li charge or Li -CT) and manage it using a BMV (battery monitor) relay control output.

Then set yr custom relay close and open thresholds and the buck converter will connect and disconnect at the charge levels or voltages you wish.

Yes it will cost, but you should have one anyway, lithiums are too valuable to operate blindly. There will be a host of other advantages to having the BMV in the system as well.

Suggest the BMV 712

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