Idea

Lukas avatar image
Lukas Posted ·

Battery Discharge Current Limit

Idea:

Add the possibility to limit the discharge current of the battery in the venus settings.


Rationale:

ESS installations are often overcomitted in terms of installed PV power. During seasons with lots of sun, it would be beneficial to sell as much power as possible to the grid or use it on-site. When the system is on battery, it might be necessairy to limit the battery discharge current to enhance battery lifetime and get more Ah out of a smaller battery bank.

From my point of view, this could be done by introducing a new config option in the ESS menu:

Limit inverter power (discharging) -> 0W - XXX W (maybe the existing limit can be renamed to "Limit inverter power (charging)

or by introducing a distinct discharge current limit alongside the charge current limit, maybe in the system settings where the charge current limit is found.


In this way, the multi can convert as much as it can to AC during solar peaks and either sell it to the grid or use it locally, and limit the output to a much lower value once the system is on battery, to power for example essential loads with lower discharge C-ratings applied to the battery.

Venus GX - VGXVenus OSbattery
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Hendrik Coetzee avatar image
Hendrik Coetzee Posted ·

Ditto for all of the above.

My goals seem simple :

  1. maximize (expensive) battery life by preventing large (probably quick) discharges while grid is available
  2. maximize PV generation/consumption as much as possible (complicated by the fact that grid feed-in is NOT an option in my area - Gauteng, South Africa)

I fully agree there are hodge-podge work-arounds possible using (for example)

  • custom, scheduled scripts
  • integrations with MQTT
  • scheduled charging

Just seems that I could achieve exactly what I want with one simple little limit setting (battery-only discharge limit) - that by default is turned of, so that it does not bother anybody that does not want /need it.

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Jaco Reinecke avatar image
Jaco Reinecke Posted ·

Have also asked for similar feature i.e. limit the discharge from the battery (DC) and not the inverter overall, and not from the MPPT's, when it is AC coupled.

Would be a great feature for some of us, like when Utilities are off, who match the loads to the batteries and not the batteries to the loads so we can use smaller battery banks.

Have a 3.5kw array on a 3kva Multigrid with 225ah 24v bank.

Currently the only way I can solve this:
a) Get more batteries.
b) Or Keep always charged.

Limiting the inverter power is not an option as I am grid tied and need as much power from the panels as I can get and not from the batteries unless it can be limited.

Have suggested before that a BMV could be the point of measurement for DC draw.

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Elimac avatar image
Elimac Posted ·

Hi,

To address this issue, VENUS OS actually alread has an option in

Start/Stop Generator -> conditions

That should trigger action based on the battery current:


My Venus relay is set as "Generator Start/stop". This should suffice to activate a contactor to either siwtch on the GRID or disconnect AC loads, to aliviate the DC current from battery. Things are really close to work.

The problem however that i faced is that for the time being, in a non-ESS, DC coupled system like mine, with only Victron Inverter and Victron MPPT and no BMV, Venus OS is not subtracting the MPPT current from the total current. So, Battery Current is in effect the Inverter input Current. During day this is of no use..., it triggers the relay even if battery is not providing current.



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Nicholas Wessels avatar image
Nicholas Wessels Posted ·

I would also love to know when this "Limit discharge current" feature will be implemented on the CCGX/Venus, if ever.

In fact in the early days of ESS (when it was still called Hub1/2/3) this functionality was available but it was removed due to some compatibility issues and never re-added.

Just to clarify, what we are trying to do is limit how much current is discharged from the batteries when the system is running ESS and the Grid is available. This setting would not apply when the system is inverting during a grid outage.

The aim of this setting is to prevent battery damage by discharging them too quickly, especially in the case of lead acid batteries.

The the “Limit Inverter Power” setting that is available now is not useful in this situation because it also limits PV power.

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ezra avatar image
ezra Posted ·

This is the only downside of the easysolar i have so far, would love to see this go live, and not use a dirty workaround.

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Keith Arnold avatar image
Keith Arnold Posted ·

Wondering why there is such a big reluctance to put in a separate battery discharge value.

also seems some have difficulty understanding that inverter limit is not the answer. In fact why limit the inverter at all unless for overload or over temperature it should automatically protect itself. On grid with battery and mppt inputs to the inverter it might be tricky to control discharge current while allowing full mppt throughput.

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Daniël Boekel (Victron Energy Staff) avatar image
Daniël Boekel (Victron Energy Staff) Posted ·

Hi @Lukas

Have you tried the 'limit inverter power' setting? this does what you want (as long as there is a grid connected)

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Sure I did.

But as far as I've noticed, the inverter power is then always limited, right?

I would love beeing able to set charge and discharge current limits, so my BlueSolar MPPT will always operate on its maximum power point. When on battery, the inverter power should be limited (I do not want to supply 12kW for cooking/ovens from battery, ouch... in my case, powering only the basic loads in the house would be enough when on battery, which is around 200Wac to 450Wac peak). But I want to use the full potential of the PV to power large loads during the day when the MPPT delivers max. power which would exceed the power limit set (for example to 400W).

Or does this reflect the current implementation?

Edit: The grid setpoint should not be ignored, so technically, the MPPT does still not always operate at it's max. power point, but tries to reach produce as much as possible while the grid setpoint is not met. If feed in is allowed on the other hand, it should operate at it's max and feed in what's leftover accordingly. Just to also consider the case where "Feed-In Excess Solarcharger Power" option is set.

Hi, was this ever resolved?

Nope, not yet...

I also wanted to limit discharge current so I was excited about your inverter limit. Implemented it but all is good until the load is high. Then the inverter draws up to 4 kW from the batteries (2x 100 Ah li-ion Huawei). My system is AC coupled Fronius on AC out1 of Multiplus. No dc charge controller yet.

In my thinking the grid can supply the increased load, it does not have to all of a sudden come from the inverter. The inverter should control at the set point unless the load is lower. Inverter a contributor to the load not the main supply to the load, while grid is on.

Drawing 4 kW from them hits the low voltage, then vrm just shows painted line between -4kW and zero until the soc limit.

I will be adding DC MPPT and then I would expect the inverter could supply increased load through the inverter while maintaining the set discharge limit from the batteries.

From other replies I see this is not going to be the case.

@Daniël Boekel (Victron Energy Staff)

I have exactly the same problem it would really help if Victron added a "Limit the discharge from the battery and not the inverter"

I have a 10Kva inverter with 6.5kw array and 7.2Kwh battery bank. The return on investment on batteries just does not make sense so I have a small battery bank which lasts most of the night as I have gas stoves and heat large amounts of water during the day for underfloor water heating and showers. As soon as the battery is charged I get a maximum of 2.5kw from my PV panels and as Jaco says I cant feed the energy into the grid in South Africa.

Is there anything on the road map or do I need to spend another $5k on batteries or write a bash script?

Lukas avatar image
Lukas Posted ·

Hi @Daniël Boekel (Victron Energy Staff),


any news on this feature? Will there be an update?

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Hi Lukas,

if / when this is possible, we will let it know :)

cristipurdel avatar image
cristipurdel Posted ·

Would it be possible to have the "Limit inverter power (discharging" as an firmware upgrade for the BlueSolar Chargers (pricier model should have higher wattage) or at least for the BMV-712 ?

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Lukas avatar image
Lukas Posted ·

@Daniël Boekel (Victron Energy Staff) anything new on the reply above?

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Hi @Lukas

Did you actually try what I wrote in that answer?

The option already exists in ESS mode, and if you use a CANbus BMS, the BMS also sends a Charge / Discarge Current Limit which is used.

Only when the grid is disconnected the discharge limit can not be held when AC loads are too high.


John Hagtharp avatar image John Hagtharp Daniël Boekel (Victron Energy Staff) ♦♦ ·

I've wished for the same thing recently.

Limiting inverter power is not the same. The input to the inverter is battery + solar chargers. Ideally a limit could be placed on just the battery discharge component.

This limit would only apply when the grid is available.

I use lead acid batteries so there is no BMS to assist.

Lukas avatar image Lukas Daniël Boekel (Victron Energy Staff) ♦♦ ·

@Daniël Boekel (Victron Energy Staff)

I do. This is my daily routine since I got a multiplus II: At around 9/10am I login in and disable the current limit - there is already enough sun since it's summer. In the evening, I enable the power limit and set it to 400W or 500W. It works, basically but for a system as sophisticated as the vennus/ccgx, it is strange to have this manual process after all.


As @John Hagtharp menitioned, the inverter power limit is total current, not battery specific.


I also do use lead acid batteries...and currently there is no BMS available from victron for that chemistry. But I was thinking to develop one myself (I'm an electronics engineer and functional safety guy)...would victron be interessted in such a device? Smart Lead Acid BMS for 12V blocks including a balancer and CAN interface?

Let me know what you think...

Aha so it is a DC coupled PV system!


If the battery is too weak to supply the loads, I would to set the minimum SOC limit in ESS to a high value, like 80-90%
This way your battery is not used much, but when grid fails you still have full capacity from the battery.

Most systems have batteries sized to supply all loads, so this is not a very common problem.

Lukas avatar image Lukas Daniël Boekel (Victron Energy Staff) ♦♦ ·

@Daniël Boekel (Victron Energy Staff)


The size of the battery is not the problem, don't get me wrong. However, I do have this set to limit the discharge depth for the lead acid bank to get at least some cycle life out of it.

But the problem persists, even if you would limit the system to 95%, without a discharge current limit you could, say, discharge with 5kW very rapiply but then you hit the SOC barrier.

I think what's needed is just a very simple additional setting apart from a potential BMS to limit the discharge current from the battery. If this value is eventually overridden by a BMS - even better. But the general setting should be there just like the charge current limit. Limiting the charge current is also a nonsense option for a large battery bank, which can potentially sink hundreds of Amps I20 once it's large enough. Yet, the option is there (which I do not use btw with a 48V 200Ah Battery).

Again, a discharge limit would enable everybody to max out their inverters while on solar, but keep the load on the battery during discharge at a reasonable level, resulting in more Amp-Hours beeing available especially from lead-acid. Depending on the quality and size of the battery (and also the general type), this can be quite substantial.


Just out of curiosity: why a lead-acid battery? is it purely for back-up purpose with a little bit of extra self consumption as a bonus?

I'll ask internally if an option to limit discharge current is feasible.

Lukas avatar image Lukas Daniël Boekel (Victron Energy Staff) ♦♦ ·

@Daniël Boekel (Victron Energy Staff)

Thanks for the effort, highly appreciated!


You're right, it's a self-consumption booster, but with daily cycle use.

I did choose lead-acid because it was cheap and the other lithium options did not seem safe to me. I work with high-voltage DC systems in electric (aerial) vehicles as a side business and the effort to really make it safe is quite substantial. There was no real option on the market for 48V which met all my requirements to be installed in my basement.

With lead acid, I simply bridge the gap between lithium-titanite is cheap enough or lead-carbon has matured, which I also consider a possbile option.

But even with a super-powerful LTO battery, I would have asked the same question, to ignore high-power loads in my ESS when on battery.

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RainerB avatar image RainerB Daniël Boekel (Victron Energy Staff) ♦♦ ·

Hi,

with my smaller battery bank (24V/330Ah) I also had the issue with a missing "Max.battery discharge current" setting.

The "Limit inverter power" setting also limited the MPPT power at daytime. On the other hand, the daytime setting was too high for the night.


My workaround was programming a bash script that sets a suitable value for the "Limit inverter power" depending on my additional Fronius PV power (on Mulit ACout) and the Battery SoC. This script runs every 10 minutes on my venus GX and I'm very happy with it.


In the meantime I installed a much bigger lead-acid battery bank (1200Ah/24V OPzV) and therefor the Inverter setting is the most time at max. power.


--------------------

By the way, if weight and volume is not an issue, good solar lead-acid batteries can still be a good and affordable choice for a house. My 1200Ah Sonnenschein A602 battery bank provides the following:

  • Nominal capacity: 28,8kWh
  • 4.250 Cycles according to IEC 896-2 (40% discharged / 60% left) -> 11,5kWh daily use!
  • 5.750 Cycles according to IEC 896-2 (30% discharged / 70% left) -> 8,6kWh daily use!
  • Design life time 20 years

And yes, I like lead-acid batteries for its simplicity :-)


Regards

Rainer


Hi, I just read your comment and it's what I'm really looking for, a decent alternative, until Victron implements a battery discharge limit. I would like to know if you could pass me that script and as I install it in my ccgx, I would solve this serious problem that if the user is not pending it lowers the life of the batteries drastically

Hi Paco,

attached you can find my bash script for a Multiplus 24/3000, Fronius PV at ACout and a MPPT Smart solar. I'm quite sure you will have to adjust it to your specific hardware. Maybe you need other ModBus values too.

This script is started by a cronjob every 10 minutes.

maxinverterpwr_mod.zip


Regards

Rainer


Nazareth Kevorkian avatar image
Nazareth Kevorkian Posted ·

Hi @Lukas, @Daniël Boekel (Victron Energy Staff) and all the others,

@Lukas

I think you pinpoint the problem in the wrong direction. I understand your concern to maximize PV consumption and limit battery consumption when off-grid; for this a solution shall be found. But this cannot be done by limiting battery discharge power when mains/generator or other reliable sources are off, but rather by power management and system architecture. Whether during on or off-grid, the connected power sinks (loads) will demand power at their request, you can only stop them at load connection level or reduce their consumption with EE measures. But as long as they are connected to your Multiplus or Quattro and load demand is there, they would request power. In case you limit the power at battery level, theoretically said, VE products do not have the choice but to shut-down completely to protect the battery based on your settings. This resembles low-voltage dynamic cut-off feature of VEConfig; Multi/Quattro can shut down not because the batteries are totally empty, but because you have configured the low voltage settings to enable shut-down on custom settings.

Imagine for example you limit the battery discharge to 1kW and PV is providing 2kW and your load is 2kW. So far no problem. Suddenly a load increase happens, now 3.5kW, but PV power stays the same (or even worse drops from clouds passing), so you have 0.5kW of load demand that you cannot take from anywhere. This technically and practically cannot happen. It is as if demanding a power cable to not heat up, catch fire without protecting it by circuit breaker. The solution for such a case would be to disconnect/efficiently use some loads to prevent fast discharge or to continue using battery power until minimum SOC has reached, in that moment start a backup generator or else shut down the system on dynamic cut-off low voltage limits.

Long story short, if you want to protect the battery from deep or quick discharges and simultaneously maximize PV power consumption, you can opt for AC-out coupling with on-grid PV inverters and to connect the secondary loads (non-priority) to the AC-out 2 of the Multi/Quattro and program VEconfig to disconnect AC-out 2 on desired conditions. Another option is to use the few relays on Multi/Quanttros to drive external power control. This will become a power management issue at load side. With PV connected on AC-out, if you "limit inverter power". then when mains is there, it would use for PV and then the batteries with a limit only on the batteries, but when Mains is off, you only have the option to disconnect to prevent deep discharges until PV or Mains power is back to charge the batteries back.

@Daniël Boekel (Victron Energy Staff)

In my suggested solution, the only problem is the active power control of on-grid PV inverters AC-coupled with battery based VE products to protect the batteries from overcharging. VE products control Fronius inverters with frequency shifting, but this has never been done so well, batteries in such cases are not charged well, and inverter power is derated sometimes more than needed. Its optimization process is complicated, needles to mention the flickering issue and voltage regulation issues that on-grid inverters have in such power control measures. That's why in lots of cases it is better to go with DC-coupled MPPT chargers for better PV power and battery charge control. So in such cases, similar to Lukas's request, but only when on-grid with mains connected (I explained above what happens in off-grid scenarios), is it possible to send separate commands to Blue/SmartSolar MPPT chargers, to not limit their production when 'Limit Inverter Power" is enabled? If that's not possible, may be a second DC input on Multis/Quattros would solve the case (though its development's financial feasibility and market share should be considered)!

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Hi there,

@Nazareth Kevorkian

You're right for off-grid operation, but this feature request is only needed for the on-grid scenario. For off-grid, the limit needs to be deactivated. E.g. once the grid cuts out, disable the power limit to supply the critical loads, even if this overrides the discharge current limit. For the always on-grid use-case, I think this can be done since there are already some scripts doing that.

Since I do not own a lithium battery with a supported BMS: What happens if, say, the lithium BMS reports that the discharge current limit is 30A, but the MP-II wants to draw 32A? Will the MP-II reduce the output power (and if the load is too high, stop producing a working output) or will it ignore the limit when the grid has been disconnected?


Regards,

Lukas




Hi,

Sorry I am not an expert in BMS and Li batteries, can't help much. But I guess the MP-II cannot reduce the output on off-grid mode, as long as there is load demand, so it will ignore the limit when the grid has been disconnected.

Hi,

the ESS "Limit Inverter Power" setting is also ONLY effective in on-grid connected ESS systems. The ESS manual says: "This limit only applies while connected to AC-in: In inverter mode, the AC loads determine how much power is drawn from the battery".


The "Battery Discharge Power Limit" (that we want) could or should work in the same way.

My workaround is a dynamic "Limit Inverter Power" setting controlled by a bash-script that runs every 10 minutes on my Venus GX.

Regards

Rainer

Hi RainerB,

I understand from what you just said and previously stated above that this script works only on on-grid scenarios. That's is great for on-grid scenarios.

A suggestion in this: "My workaround was programming a bash script that sets a suitable value for the "Limit inverter power" depending on my additional Fronius PV power (on Mulit ACout) and the Battery SoC. This script runs every 10 minutes on my venus GX and I'm very happy with it."

Why don't you add the MPPT SmartSolar production as dependent input in your function?

RainerB avatar image RainerB Nazareth Kevorkian ·

Hi,

there is a simple reason in my specific configuration why I do not use the MPPT output value for my calculations. Because I do not get money for it, I do not allow grid feed-in of the MPPT Smartsolar production but I allow full feed-in of the AC-coupled inverter.

That is why the MPPT production is throttled down if no load/charging demand is available and therefor the MPPT PV value is not suitable for the calculations.

The AC coupled inverter power value delivers a good estimation of what the MPPT should be capable of (after its quick ramp up to full power again).


Regards

Rainer

spark88 avatar image
spark88 Posted ·

Another one for this request please. Could really do with being able to limit battery discharge.

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