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How does an MKS 4 mix PV, grid, and battery power?


Ben Harper
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I have:

  • Axpert MKS 4 (5.6kw)
  • 1 x Pylontech UP5000 (4.8kw)
  • 2.2kw of PV

Lately I've been wondering if I'm in danger of loading the battery too hard, even when there is plenty of PV power available. The battery specs say that continuous discharge is OK around 2.4kw.. but sometimes I have a load of e.g. 3.5kw, for 5 minutes.

I have my own software which can switch between SBU and SUB mode, depending on PV, load, etc, so I want to do the right thing to maximize the life of my battery.

My questions are about how exactly the MKS inverters mix the power inputs:

  1. If the mode is SBU, and there is 2kw of PV available, and the load is 3.5kw, will the battery be drawing 1.5kw or 3.5kw?
  2. Is it normal for SUB mode to use zero PV when there are substantial loads, but the battery is full? My previous VM III inverter would clearly use PV inputs to power loads when in SUB mode, and just use grid power to "top up" the shortfall, but this doesn't seem to be the case with my MKS. The MKS doesn't seem capable of mixing grid and PV power in SUB mode?

I'm sure these questions have appeared numerous times on this forum... wouldn't it be great if there was a place for a FAQ somewhere?

Thanks,

Ben 

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3 hours ago, Ben Harper said:

1. If the mode is SBU, and there is 2kw of PV available, and the load is 3.5kw, will the battery be drawing 1.5kw or 3.5kw?

It should be 1.5 kW. All inverters should be able to blend battery and PV power.

3 hours ago, Ben Harper said:

2. Is it normal for SUB mode to use zero PV when there are substantial loads, but the battery is full?

It should not, but it seems that recent Voltronic firmware in many models, it seems that this is the case. Especially if using "the cable" so that the BMS controls the charge. When the battery is full, the BMS tells the inverter "charging forbidden", and the inverter (either the display firmware or the inverter's DSP or both) misinterprets this as "PV power forbidden". So the display firmware seems to be telling the inverter to make the charge power negative, when it should be setting it to zero (i.e. PV power should be allowed to roam up and down up to but not exceeding the load power plus losses). Instead, the inverter seems to turn off all PV power when the battery is full, so then the battery discharges up to a certain point, at which time the BMS removes the "charging forbidden" flag, and the PV is allowed to come back on again, to power the load and recharge the battery. This is needlessly cycling the battery, reducing its life, and is wasting available solar power that could be powering the loads.

I hope that Voltronic realises this bug soon, and comes out with a slew of updates to fix the problem on all affected models. Alas, their track record is not great in this regard. They still think that the premature float bug is a figment of our imaginations, for example.

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1 hour ago, Ben Harper said:

Wow.. that's shocking.

At night time, if I'm on SUB, then about once every 45 minutes, my inverter charges at about 1kw for 2 minutes. 

Any recommendations for pestering Voltronic?

I think a Axpert is great as a UPS but not so good at saving and cycling. The 1kw used from grid is charging the battery up to setting 13 and then using battery power to setting 12 and then repeating the charging. About 10% of the grid power used for charging is lost by charging the battery. I still think if this goes on it is better to use only grid power as in bypass as a UPS with the battery full in case of LS.

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3 hours ago, Scorp007 said:

 I still think if this goes on it is better to use only grid power as in bypass as a UPS with the battery full in case of LS.

OK.. I get your point. But how do you actually achieve that?

I can't find any setting on my inverter that will keep the lithium ion battery at a floating charge.

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13 hours ago, Ben Harper said:

OK.. I get your point. But how do you actually achieve that?

I can't find any setting on my inverter that will keep the lithium ion battery at a floating charge.

Depending which lithium you have it might not reach 28V as bulk or 27.8V for float. That is what I found on my similar Axpert.

Go to settings and choose no 1 and set to Utl . Battery is then kept full for when there is LS. When grid is off loads will be powered from battery.

Setting 05 allows you to select USE and then you can adjust the various settings for lithium.

Edited by Scorp007
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23 hours ago, Coulomb said:

It should be 1.5 kW. All inverters should be able to blend battery and PV power.

It should not, but it seems that recent Voltronic firmware in many models, it seems that this is the case. Especially if using "the cable" so that the BMS controls the charge. When the battery is full, the BMS tells the inverter "charging forbidden", and the inverter (either the display firmware or the inverter's DSP or both) misinterprets this as "PV power forbidden". So the display firmware seems to be telling the inverter to make the charge power negative, when it should be setting it to zero (i.e. PV power should be allowed to roam up and down up to but not exceeding the load power plus losses). Instead, the inverter seems to turn off all PV power when the battery is full, so then the battery discharges up to a certain point, at which time the BMS removes the "charging forbidden" flag, and the PV is allowed to come back on again, to power the load and recharge the battery. This is needlessly cycling the battery, reducing its life, and is wasting available solar power that could be powering the loads.

I hope that Voltronic realises this bug soon, and comes out with a slew of updates to fix the problem on all affected models. Alas, their track record is not great in this regard. They still think that the premature float bug is a figment of our imaginations, for example.

OK now I'm seeing this behaviour very clearly, on a day when there was nobody home.

I'm considering removing the BMS cable until the situation changes. Do you think there's any potential harm in this, provided I input the correct voltages under "User Battery"? I understand that I'll once again be victim to the inverter knowing only the voltage of the battery as a proxy for SOC, but I think that's the right tradeoff.. considering this inverter is just sucking the life out of my new expensive battery!

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I might be wrong by thinking Voltage is just estimated for SOC. The problem without BMS to me is that under load with rhe voltage being lower than no load the Axpert might stop discharging and go to grid at a higher SOC. For this the pay off will be lower discharge level which should provide longer life. It does also mean less saving by using less battery power before PV recharge the next day.

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22 hours ago, Ben Harper said:

Do you think there's any potential harm in this, provided I input the correct voltages under "User Battery"? 

There is a small potential risk, which gets larger as the battery ages. It's that one cell may get into voltage trouble (too low or too high) when the average and therefore total battery voltage is normal. In these circumstances, the BMS and cable can save that cell from damage, but the USEr battery type will not.

Edit: sigh. The BMS will presumably still protect the cell from damage, but it has to resort to cutting off the battery from the inverter with little or no warning, blacking out the loads rather than invoke grid charging (where possible).

Edited by Coulomb
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2 hours ago, Coulomb said:

There is a small potential risk, which gets larger as the battery ages. It's that one cell may get into voltage trouble (too low or too high) when the average and therefore total battery voltage is normal. In these circumstances, the BMS and cable can save that cell from damage, but the USEr battery type will not.

Would the battery BMS not take care of each cell in the bank even if there is no comms to the inverter?

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6 hours ago, Scorp007 said:

Would the battery BMS not take care of each cell in the bank even if there is no comms to the inverter?

Duh, yes it certainly should, my bad. I've corrected my post now. I'm unsure as to how bad it is to disconnect the inverter from the battery when under load. 

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2 hours ago, Coulomb said:

Duh, yes it certainly should, my bad. I've corrected my post now. I'm unsure as to how bad it is to disconnect the inverter from the battery when under load. 

Perhaps just as bad as disconnecting a 3kw geyser that reached its temperature if you are brave to use battery power to run such loads. 🤔🤔 Perhaps some people with 2 x 5kw inverters in parallel might be in the brave category.

My limited experience with lithium is by the time they reach their upper voltage level they are charging close to zero current. DIY and Hubble S-100.

Edited by Scorp007
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Thanks for all the info guys.. super useful as always.

One interesting thing I've observed today, is that my "zero PV after 100% SOC" period lasted only 10 minutes, and then never showed it's head again for the rest of the day. I was a bad scientist as usual, and changed two variables at once: Lowered max utility charging to 2A, and switched on a couple more PCs.. to make sure my baseline load was at least 600W (compared to 300W the previous day, when PV generation just switched off and mostly stayed off). My suspicion is that the higher base load is the relevant factor.

Anyway.. I'll post here again if I discover something interesting.

Thanks for all the help.

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8 hours ago, Scorp007 said:

[ Re: battery disconnecting under load. ] Perhaps just as bad as disconnecting a 3kw geyser that reached its temperature if you are brave to use battery power to run such loads. 🤔

No, it's much worse. Loads come and go all the time; it's expected, and is very well tested.

Taking away the battery, which is expected to stay there and provide a stable, low impedance path for inductive spikes that happen some 19,000 times per second is a different matter. The capacitors (bus side and battery side) absorb most of that inductive energy, but there is also the inductance of the battery cables. The cable inductance would be huge compared to the stray inductance of the switching circuits, so the capacitors that protect the MOSFETs would take quite a hit every time the battery disconnected. I don't have a feel for how significant that impact on inverter life would be. 

 

 

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4 hours ago, Coulomb said:

No, it's much worse. Loads come and go all the time; it's expected, and is very well tested.

Taking away the battery, which is expected to stay there and provide a stable, low impedance path for inductive spikes that happen some 19,000 times per second is a different matter. The capacitors (bus side and battery side) absorb most of that inductive energy, but there is also the inductance of the battery cables. The cable inductance would be huge compared to the stray inductance of the switching circuits, so the capacitors that protect the MOSFETs would take quite a hit every time the battery disconnected. I don't have a feel for how significant that impact on inverter life would be. 

 

 

Thanks for sharing your insight.

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