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Idea

Anders suggested Oct 14, '20

BMV's SOC 100% relay on/off

I have an off grid solution where the BMV SOC relay function is used to harvest excess PV energy.

The BMV control a Smarthome switch. Switch off = SOC 100%, Switch on SOC <98%

This information together with sunset and sunrise is used to control 1000W of electric heating.

Herein lays the problem. As long as the PV's produce more energy than 1000W, SOC stays at 100%. When PV production drops, energy starts to be pulled from the battery, and as much as 2% has to be consumed before the SOC relay on the BMV it triggered. It is acceptable but not ideal.

So the idea is rather simple.

Please Introduce a function to the BMV so the SOC Relay can trigger on anything but 100%.
If necessary it would be ok to introduce decimal steps so the relay can trigger on 99.9%

As a good alternative (or maybe in combination) could a NET battery current trigger do the trick.

Relay OFF if NET battery current is positive
Relay ON if NET battery current is negative
even better,
Relay OFF if SOC = 100% and NET battery current is positive
Relay ON if SOC = 100% and NET battery current is negative
Relay ON if SOC <100%

This would together with simple Smarthome tech be a very useful function to harvest excess PV energy

BMV Battery Monitor

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trip-tracks commented Oct 15, '20

I assume you are trying to only supply the heating while the batteries are at 100% and charging is still occurring. Is this correct?

Most people would be happy with something similar to the rules you have already set - ie 100-98 or even more, but you could put your control circuitry in series with the solar regulator relay load relay? Assuming you have a model with a relay...

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Anders commented Oct 15, '20

Yes I am

I currently harvesting 7Kwh a day with the solution, energy which otherwise would have been lost. PV's is typically delivering max 1500W in Norway now, and the days are getting shorter.

Yes I have a SmartSolar Charger MPPT 150/100, What relay function would you use, battery voltage high or low?

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Mark commented Oct 16, '20

While there are many different parameters and logic conditions that can be referenced/implemented to control (switch on/off) a fixed power dump load - even with the existing relay control functionality in the BMV & larger size MPPTs (which have an inbuilt relay), you can acheive quite a good control system.

This is what I would recommend:

BMV

Invert relay function - to obtain required dump load control
Include 'Low SOC %' condition, set on @ ~90% & off @ ~95% - there is nothing wrong with running the dump load during the final period of charging if there is enough solar power (remember this will be inverted)
Include 'Low voltage' condition, set off a little lower than the MPPT float voltage and on a little lower again - if there is insufficient solar power available to both charge/maintain battery voltage and power the dump load then the dump load will be turned off (remember this will be inverted)
Note that both conditions need to be satisfied for the relay to be turned on
Include 'Minimum closed time' condition, set to ~5min - because the relay output is inverted this condition will actually control the delay between dump load activation events (so it's not repeatedly switching on and off unnecessary, there will be a delay before 'trying' again)

MPPT

Physically wire the BMV relay in series with the MPPT relay
Set the MPPT relay mode to 'Panel high voltage'
Set on a little lower than the PV MPP voltage and off a little lower again
With this extra condition, if solar conditions are poor then the dump load won't even 'try' to be activated, since it is already known that there is insufficient PV power available to run it without draining the battery

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