question

Michael Schmidt avatar image
Michael Schmidt asked

Multiplus II 48/5000 use with 18s LiFePo4

In Multiplus 48/5000 i can set up chagring end voltage up to 64V.

is there any reason why not to use with 18s (3,56V per cell charge termination voltage)

or at least 17s (3,65V per cell charge termination voltage is possible)


best regards

michael

Multiplus-II
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2 Answers
ben avatar image
ben answered ·

(a) It would be a little harder to find a 17- or 18-channel BMS.

(b) 50VDC is considered the "touch safe" threshold. 64VDC is a good bit higher. But even 16s will be at just over 50VDC.

(c) Nominal ratings on Victron inverters are often hard to achieve except in perfect conditions, in my experience. You might encounter a hidden derating factor that makes it hard to achieve the charge voltage you desire.

Those are some things that might give you a reason to skip 18s. I'm not sure any of these are enough to convince me not to do it, if 18s or 17s is somehow better for your install.

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petajoule avatar image petajoule commented ·

60V is considered "touch safe". For BMS you need either decentral ones with "1 PCB per cell", or some 24cell BMS where you leave some connections alone.

As there are BMS systems also for LTO cells - which have lower cell voltage and thus are encountered in 20s or 22s configurations, these BMS do exist plenty.

From quite a lot of experiments of tinker-diehards, it seems the derating is more depending on current than on voltage. In that respect, driving inverters/chargers with higher nominal DC voltage actually seems to be beneficial.

Not only have been no adverse effects observed, temperatures of the devices indicate there is less strain. Of course someone from Victron may start cursing right now, in which case I would appreciate any enlightement, but in absence of a "red paper" or somesuch experiment-experience is on.


Good old quora:

If you look at electrical safety standards eg EN62368 (for AV & IT Equipment) states that “For any voltage up to the voltage limit, there is no limit for the current. Likewise for any current up to the current limit, there is no limit for the voltage”

The limit at which under no safeguards are required is, for DC, 60V or 2mA. Which means you can have an exposed DC level of under 60V that can deliver as much current as it can, or you can have more or less whatever DC voltage you want, provided the current limit is under 2mA.

In different environments however this changes, for example, EN61010 (laboratory equipment) specifies 70Vdc -dry, or 35Vdc in wet locations. 2mA still applies.

So the answer of what voltage is safe, really depends on the environment and circumstances, but generally speaking 60Vdc is considered safe in normal use.

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Michael Schmidt avatar image
Michael Schmidt answered ·

Hey Ben,

so i tested it by my self now.

and you are right there is no more charging or discharging power!

The limits are Power based so now the max discharge is around 80amps! And charging i

os aourounf 57amps maximum.

The max voltage is working at 63.9V so it is no problem to use 18s

but there are also no big benefits expect the lower amp rating, maybe slightly higher efficiency (dind´t checked this) and little more kwh with 2 cells more.


best regards

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ben avatar image ben ♦ commented ·

That's interesting to hear, that it seems power-limited.

I suspect it is actually heat limited, but the power output correlates closely with heat and so it appears to be power.

You might be able to super-cool it and see it reach the rated current, but then, who actually has that much cold in real life?

I'm glad it worked nevertheless. You might be the only person with an 18s battery on this community. :)

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Michael Schmidt avatar image Michael Schmidt ben ♦ commented ·

Hello Ben My MP II is housed in anti vibration anti noise housing.

The Housing is cooled with 2x 120mm Fans (MP2 fits exactly in the foam) so all air from this fans must run through the inside of the MPII housing.

The MPII is running much cooler than with the original crappy 24V Fan.(wich is still also operating inside)

But unfortunately there is not in single watt difference in Power.

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petajoule avatar image petajoule commented ·

This has been tested and discussed in length on mypower.cz by some tinker-enthusiasts. Especially for the lower voltage Victrons (12V and 24V), there seem to be some efficiency gains, but first and foremost power gains when operating on the upper end of the voltage specs.

As in: 1.2kVA phoenix being able to deliver 1.6kVA etc. And so far no adverse effects have been observed.

Unfortunately I have not been able to extract information from Victron staff regarding other DC components (such as SmartSolars) and their DC range specs.

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