I would like to suggest an additional feature for the MPPT charge controllers/Venus devices: they should indicate the surplus solar power available at any moment.
When the MPPT solar charge controller is in MPPT mode, the DC current output is the same as the maximum solar power available, and this maximum power is known. But when it is not in MPPT mode, for example in absorption or float mode with no inverter or direct DC consumption, when little DC current from the solar charger is required, the solar charger output is much less than the maximum possible solar power - that is the moment when excess power is available. The problem is that this maximum solar power and thus the exact amount of surplus solar power is not known.
A possible solution to get that information: I guess that the MPPT algorithm that scans regularly for the absolute maximum power point during "MPPT" mode could also be applied when the controller is not in MPPT mode, just to get the information about the maximum power available at a certain moment. For example if the real DC output of the MPPT controller is 200W, but the maximum solar power that the solar charger could deliver is 3000W, so the available surplus or excess power can be calculated, in this example 2800W.
When the available surplus power is known, it could be used as an additional parameter for more precise excess energy handling. For example turning on a water heater or pump of 2000W at the moment when solar surplus energy exceeds 2000W, in order not to discharge the battery (or not to consume from the grid) for big loads of lower importance.
When an excess energy load with variable controllable consumption is available (electric water heater with a consumption between 0% - 100% controlled by a Variable Frequency Drive, or a PWM Solid State Relay), you could start using those loads earlier, as soon as any excess energy is available. This would allow to harvest nearly 100% of the surplus energy if necessary.
If all "useful" destinations of excess energy are satisfied (water is hot etc), then you could use it just for fun with a pump to power a water fountain - whose height would be a nice analogue display of the surplus energy ;-)
Water pumps usually require some minimum speed defined by the producer to assure lubrication of seals/bearings. The minimum speed of submersible pumps is usually 30Hz, at which they consume about 30% of their nominal power. So you would have to wait till you can meet this 30% minimum power demand with the excess energy before turning the pump on.
In order not to overload the inverter in case that loads of high importance are suddenly turned on, the control logic or let´s say the condition for controlling excess consumption should take into account also the inverter output power as a second parameter, and turn off/reduce quickly the excess energy load when the inverter reaches/exceeds its output limit, in case the excess energy is used through the inverter and not directly in DC.
If the Victron system knows the excess energy, it would be useful to communicate it somehow to third party products. The data communication methods like d-bus or mqtt require a high level of expert knowledge or programming skills, which most installers/users including myself do not have. On the other hand, many devices which allow the gradual regulation of power, like variable frequency drives and heating control devices, use standard industrial analogue control signals of 0-10 V or 4-20 mA, or can receive a PWM input signal. Those who know how to use an external controller (Raspberry etc) might then control a VFD or PWM consumer somehow, either by data communication (Modbus etc), or by creating a PWM or analogue control signal.
Therefore my second feature request: The Venus-device (or Multiplus, MPPT or any part of the Victron system) should have an analogue output for 0-10V and 4-20mA and/or a PWM output, so that those devices which are wide industry standard can be connected by a normally skilled electrician/installer, without innecessary additional control hubs in between. By the way, this analogue/PWM output might then be used also for communicating any other variable within the Victron system, like inverter power, AC Input power etc.
Maybe the MPPT controller itself could create a PWM signal output in the TX port to indicate the available excess energy, for use in a small system, even without a Venus device. I think the hardware in the MPPT is there as it has already a PWM light dimming control implemented. It just would need a software update?
For those who like the idea of knowing the excess solar energy for load management, but who do not want to wait if and when this is being implemented, and who have programming skills, I have the following suggestion: Add a cheap self-made solar radiation sensor and connect it to the analogue input of the Multiplus instead of a current sensor. I have done this with a small 3W panel, short circuited over a shunt which will deliver about 3V at the maximum panel power(current). The analogue input of the Multiplus can receive up to 4,5V. I configured it as current sensor for a PV grid inverter. I get the curve of the solar radiation during the day, but unfortunately in VRM there is not much flexibility for labelling and scaling the data, so it shows up as a PV inverter, and adds non-existent PV power to the monitoring. But it could be used to calculate surplus energy. If you read the radiation sensor directly in the external controller instead through the Multiplus, you don´t falsify PV power information in VRM.
For the meantime, I will try to learn using a Raspberry to do the missing link. I got one yesterday. Help is welcome :-)
And sorry for the long post...
