question

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porcini asked

Offgrid PV Inverter Support Assistant and the fixed 13.5V / 27V / 54V limit

To prevent DC and AC voltage overshoots because of varying solar irradiance and/or load fluctuations, it will limit the charge current already before battery voltage has rised up to the absorption voltage. It will derate the maximum charge current from 100% at 13.5 / 27 / 54V to 10% at 14.4 / 28.8 / 57.6V. These thresholds are not related to the configured absorption voltage.


Hello, knowing this has already been increased a few years ago and is somehow necessary, I'm still wondering about the reasonableness of the fact the voltages are fixed.

In my opinion this should consider capacity:inverter ratio and especially battery type and voltages! Using a relatively small inverter in a big sized lead acid offgrid installation this makes the AC-coupled power pretty useless for charging, although lead acid doesn't care much about overvoltage!

Now let's look at the Pylontech or other 15s Lithium-Storages, that probably don't even have a GX. Their absorption voltage is below 54V so there is no derating at all, although Li can be damaged by overvoltage? Further this means 15s systems can use the full power for charging while 16s or lead acid systems are extremely disadvantaged. Due to the fact that this is for protection I don't understand why some systems can use the full power up to absorption voltage and others cannot.

Can someone tell me the technical background of the fact that the voltages and the derating are fixed and anything else isn't considered? Probably @Johannes Boonstra (Victron Energy Staff)



Thank you very much!



regards,


porcini

MultiPlus Quattro Inverter ChargerPylontechoffgridAC PV Coupling
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3 Answers
porcini avatar image
porcini answered ·

Hello @Johannes Boonstra (Victron Energy Staff) and all who are responsible for this update. I just recognized that finally something changed here in 08/21 (since 017D). Thank you for that! :)

In the changelog it says:

"When charging with PV, the IBat limitation as function of UBat now takes the configured Absorption and Float voltage into account."

Great, but at the moment detailed information is missing and also hasn't been updated on victron live, yet.
Is it 100% up to the configured float voltage now and 10% at absorption voltage or how does it work. What exactly has changed?
Does it mean the assistant automatically adapts to 15s/16s LFP now, even if a battery needs more than 57,6V (e.g. untypical 17s)? And with lead acid batteries does it throttle later than before, when the float voltage is set to 2,3V instead of 2,25V/cell?


There are some more things I want to talk about.

In my opinion it should be the battery capacity (and type) and not the inverter size the derating curve needs to be adjusted to. Depending on system design and location the nominal power of the inverter(s) can be C20 (even less) or C5 currents for a battery, which makes a huge difference in voltage rise. So in a system with a relatively large battery and a relatively small inverter the 10% limit is already below the absorption tail current. With a good system design this is not a real problem, since the MPPTs will take over, but I just wanted to make aware of this. E.g. someone can use 5kWp of AC-coupled solar power with an 5kVA or an 10kVA inverter. With the bigger inverter there is only half the throttling on the same battery size which is a bit illogical, isn't it?

There is another "problem" which only (mainly) affects lead-acid battery systems, but since these batteries are still popular in big and reliable off-grid systems this is something I would appreciate to be worked on.
When charging with the multi in AC-coupled systems it isn't possible to have adaptive absorption times at all. It is more like the max. configured absorption time every day and no real difference between DOD10 or DOD70 after some days of bad wheater. It would be great to get the tail current option from the MPPTs (and then possibly even synchronisation with the MPPTs) implemented on the multiplus. Also the charging states are only visible by LEDs but missing in the VRM which can be confusing.

I know I'm annyoing but it would make things so much better.

Thank you for listening.


kind regads,

porcini


(accepted my own answer to make the actual question clearly visible)

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Johannes Boonstra (Victron Energy Staff) avatar image
Johannes Boonstra (Victron Energy Staff) answered ·

Hello, the back ground is that overshoots are not desirable for either batterie, inverter and also loads. Realize that if the DC is overshoot inging al dc loads get this also. Specially DC diesel heaters and refrigerators will give alarms. Also the inverter itself can handle a limit amount of overshoot as there is a DC power supply active ro run the inverter itself.

As the freq control is quite slow (it needs to be to protect loads running on AC ) the overshoot needs to be taken care of by throttling down soon to avoid issues

most of the time the batteries are reasonably full by then already anyway, and the charge current isn't stopped completely. So the battery will get full eventually anyway, although fro safety reasons this might be taking a bit longer then theoraticly possible.


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Thank you @Johannes Boonstra (Victron Energy Staff) for answering this fast, but I'm not sure you understood the question correctly. I think my asking was bad, I will try again. What happens in a 15s LiFePO4 system where the battery is full way before (<54V) throttling starts? Why it is not needed to adjust the voltages there?

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Johannes Boonstra (Victron Energy Staff) avatar image
Johannes Boonstra (Victron Energy Staff) answered ·

there is a throttling back anyway of course, but then on the charge voltage setting. This is the battery life setting. There might be a short overshoot depending on the PV power at that mo

ment.

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