question

I am looking for exactly the same answer with 400w SP & 200ah of lithium

BUMP

a recent video from Victron is showing this feature, see link below

Trigger a dumpload

Hi all, well in short, all Victron MPPTs can switch on excess solar, how depends on the model. Beside this there are of course multiple ways which lead to the same as always.

In Ve Connect you can program the MPPT to switch the load output (if there is one) or the relay (if there is one ) or the Ve direct port (if there is one).

Sometimes the dump load is more powerful then the MPPT can switch directly, as the current is limited. So then the MPPT controls a relay on/off, and that relay controls the dump load.

Easiest way to program is to have the Ve connect controls switch the dump load ON when the battery voltage is at a level of a full battery. Beware that this can result is a on/off switching as the dump load may require to draw more power then the PV can deliver. so set the OFF voltage at just a little lower then the ON voltage. this does mean the dump load will not be on when the MPPT is in float. depending on the battery type (lithium for example) the OFF level can be set to just below float. Another way is controlling the dump load on SOC, for this a BMV is needed.

A dump load is basically just another consumer. If you monitor your system through vrm and turn on a load you should see an increase in production from the pv panels. Approximately the same charge rate on the batteries.

I have 2 small systems. On my 12v system (max 320w from pv panels) with a mppt 100|20 charge controller. This controller has the 10A load output. A relay on the output turns a small inverter on and off. The inverter run my fridge ( the camping type without compressor). In the victron connect app I can set voltage thresholds for the load output to go on and off. This way I get the most possible from this small system.

On the bigger system I have a cerbo with the large venus os enabled. I enabled node-red and have my hot water heater turned on/off (wifi smart house relay) based on charge state and predicted pv production. This solution requires a bit more involvement than the previous one.

Maybe not the answer you wanted, but I hope it gives you a few ideas to how you can configure your system to get the most from it.

I must have a look at node red as don't know what that is.

But to try to explain better: When the PV could be 4000w it sometimes isn't because the battery is at 90% and will not accept all potential available power. Unless I add loads to use the potential, the MPPT throttles back.

These are lithium batteries and I know it can be done by SoC but if you imagine a water heater switching on at 90% SoC and off at 85%, the battery stays within this range theoretically for ever if hot water is used. If it is triggered only when the MPPT throttles back then other loads that come on here and there regardless of SoC will use the potential and allow water heating whenever those other loads switch off regardless of SoC.

If I use SoC to trigger then if the batt hits 90% and there is only just enough PV to keep it charging at absorbtion levels, then the hot water switches on then we will use battery power and cycle down to 85% for example.

If the MPPT triggers hot water only when the MPPT is throttling back, then the batt charges as required even up to 100%. If I switch on the dishwasher and that uses enough power to stop the MPPT throttling, the hot water will switch off automatically, no need to wait for the batt to descend to a certain SoC.

But in a system where things are set up to use as much solar as possible automatically, they may never get to 100% if I use a SoC low enough to turn on HW before throttling!

But if I use a higher SoC, I will lose production.

Also, I have to say it is the MPPT that is doing the throttling. During bulk charging the batteries can use as much current as the PV can give, then on absorbtion, the batteries reduce their current draw, the current has nowhere to go, the MPPT responds by reducing production. If there were no batteries it would do the same when the output couldn't be all used

You are probably right that I am overthinking this but ...can you tell me if the MPPT would sense (or even cause) higher panel voltage when throttling? If so, I can set the relay and let you know how it works out.

Looking in the RS450 documentation it says "When the MPPT switches to float stage it reduces battery charge current by increasing the PV power point voltage". Nearly what I need but really I need to know what the MPPT does when it switches to absorption stage.

I'll wait for the next sunny day and study the PV string voltages when the thing throttles back.

A bit late but as I know getting as much as possible from small pv systems is interesting to many I'd like to point you all to this diy solution:

https://www.youtube.com/watch?v=qcn4b3xWygU

Requires a bit of soldering and sourcing suitable parts.