Elimac avatar image
Elimac asked

Trigger generator / change to ACin based on battery current


To protect the battery bank (Lead Acid) from too fast discharge - currents too high for the battery capacity - I have set some conditions to start Generator (which in my case is the grid). The setting that I wanted most is:

The issue that I'm having is that I have just realised that this current is not truly the Battery Current but rather the Multiplus Input current. During the day, this value is the SUM of MPPT output and battery current. My system was triggering generator during the day when I want to take advantage of direct solar use (high loads).

As a workaround, I have set quiet hours and defined 2 values, but this is not ideal since we can have the washing machine working very nicely with 2kw from the MPPTs and suddenly a cloud comes and makes the system draw a lot of current from a small bank.

Is there a way with assistants to start generator from the true battery current? Could this option be changed so we can subtract the MPPT production?

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Elimac avatar image Elimac commented ·

I didn't have any reply to this question but I really need to understand how this "Battery current" works. Below I show a set of graphics of yesterday direct solar use, so you can see there was not current draw from battery.

From left to right

a) AC load - during daylight, plenty of Sun

b) System Voltage and Current - This is what I really want VENUS GX to use as trigger on the "Conditions" to start generator

c) VE.BUS - This seems to be what Venus is using, but is NOT the true battery current, it is simply inverter input current, either from battery of MPPT.

So, If Venus OS is using c) as trigger, this is not very advantageous. It is almost the same as the AC Load, is redundant in inverter mode.

Can Venus use picture b) instead?

Am I missing something?

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2 Answers
Mark avatar image
Mark answered ·

I can see that the 'battery monitor' selected is the Multi - so that is the information it's using.

Change the selected battery monitor to the BMV or similar (assuming you have one) & it should work as you desire.

PS. In earlier Venus OS versions I think that this setting is independent for generator start/stop to the main system/display battery monitor. But with the latest firmware I think it's now linked/the same.

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Elimac avatar image Elimac commented ·

Thanks for reply. I don't have a BMV, unfortunately. But since the information is available - it is reported on to VRM - shouldn't this be feasible without BMV? I mean, picture b) (System - Battery voltage and current) is already computed, so why add more hardware?

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Mark avatar image Mark ♦♦ Elimac commented ·

So how is it calculating the system current on VRM then & is the current shown in VRM the same as the system current displayed on the CCGX (within the battery symbol on the main screen)?

Do you already have it setup to consider the MPPT current & use the Multi as the system battery monitor per instructions below?

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Elimac avatar image Elimac Mark ♦♦ commented ·

First let me thank you for your interest in my case!

Then, the 2 answers, starting with the latter:

- Yes, I have the system set up like that. Only detail that I didn't mention is that I do not always enable the DVCC + current limit control (i have one MPPT still with temporary SW version), but the other options as you pasted are OK.

- Yes, I can see on main screen the "true" battery current. Today I tested again: after 11h AM battery current was approaching zero, absorption was almost complete. Available sun power from MPPT was more than 2kW. I started the washing machine (2kW) and the system drew from battery some residual current (like 5 or 10A at most if some clould passes on). However, the system turned PowerAssist because I had 80A set as Battery Current condition to start generator. This is not useful, current from battery was almost zero...

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Mark avatar image Mark ♦♦ Elimac commented ·

In that case I think that there may be a bug/issue with the exact current reading that the gen start/stop logic is using when a VE.Bus device is selected as the battery monitor - together with current compensation from a solar charger.

I don't think it's the issue, but what time delay do you have on the battery current gen start/stop condition? If it's very short then it could just be the very high startup currents combined with the time it takes for the MPPT to realise & increase its output to compensate... it would be a good exersize to increase the delay to ~60 sec & try again. Note that the VRM log rate may be too slow to capture this occurring.

Maybe also check that all devices are running the latest released firmware.

Hi @mvader (Victron Energy Staff), you/your staff may want to comment or take a look into this. It seems that when a VE.Bus device is used as the system battery monitor with current compensation from a solar charger, only the raw current from the VE.Bus device is being refered to as the battery current gen start/stop condition.

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Elimac avatar image Elimac Mark ♦♦ commented ·

I tested with several seconds time delay. It's needed to have some delay to avoid triggering PowerAssist on AC inrush current loads (fridge for example). But it did never change anything in respect to the trigger value for constant AC loads.

I am using latest FW versions both in Venus (2.31) and Multiplus (459).

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Mark avatar image Mark ♦♦ Elimac commented ·

I don't think that only a few seconds of delay is enough for the MPPT to respond & for things to stabilise.

I recommend to increase the trigger delay to 60 or even 120 sec then retest.

Note that the MPPT knows nothing about the new load turning on except the effect it has on the battery voltage.

It can take some time for the MPPT to see a drop in the voltage and then react by slowly increasing its charge current to bring the battery voltage back up to the charge voltage setpoint and then maintaining it there, while also powering the new load.

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Elimac avatar image Elimac Mark ♦♦ commented ·

Hi Mark. Victron MPPTs are much more fast than that! 60 seconds is an exageration! I've watched the MPPT reaction, in no more than 20 seconds it is usually stabilised. Normally, much less than that.

Just an example, obviously always with a grain of salt due to resolution, etc.

Besides, I have two MPPTs working at same time...

Anyway, I will increase the delay, but I don't believe it will make any difference.

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Mark avatar image Mark ♦♦ Elimac commented ·

I'm not saying that its the issue for sure - but you should 'rule it out' (particularly if your current delay is only a few seconds)

You should not rely on VRM for this check, even with the fastest sample rate of 1/min.

Its not only about only the MPPT responding - its about the whole process as the MPPT will only respond to a drop in voltage & only aims to control/maintain voltage.

My time suggestion is only for the test to prove or prove out the theory (without any doubt) - if it happens to work or if you have a proper log (with like 1 sec sample rate) then you can always back it off later.

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Elimac avatar image Elimac Mark ♦♦ commented ·

Yes Mark, I understand your point. I did test with 70 seconds. Didn't change anything. See my test above with smaller trigger value, definitively without drawing any current from battery. It still triggered generator...

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Elimac avatar image
Elimac answered ·

Ok, I've made a very simple test:

- This morning battery is charging as usual. MPPTs are pumping IN say 1800W to loads and battery , this translates in about 50A to battery. This is positive current going IN TO battery. AC Loads on inverter were about 300W.

- Then, without adding extra AC loads, I just lowered the condition to start generator based on battery current to just 6A. Guess what happened - yes, it turned "generator" ON.

But current IN TO Battery has never stopped flowing IN (just got smaller). Can anyone from Victron tell if this is normal?

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