poida Guru Joined: 02/02/2017 Location: AustraliaPosts: 1425
Posted: 10:19am 19 Mar 2024
A lot depends on the load each mppt sees. Let's say you have 2 mppt that are pumping 6kW into the battery. Their Battery voltage calibration are quite close to exactly what the battery is. And you add one more mppt, but this time, it's calibration sees the battery as maybe 0.2V higher than the other 2 mppts. Once the battery gets to ABSORB, the first 2 mppt will be putting in a lot and the 3rd one not a lot at all, if any, since IT sees the battery at ABSORB or even 0.2V over ABSORB voltage.
In my system I have two mppts, one taking power from a North facing array (3kW) and one taking power from an East facing array of panels, again 3kW. Most days, when I have clear skies, I see ABSORB around 10am or a bit later. One mppt is doing ALL the work while the other is doing nothing much, maybe 100W or less. This is due to the low power mppt thinking that the battery is exceeding ABSORB voltage and so it throttles back to near zero. As it should.
In your case I think you now have 4 mppts, with 4 arrays. (or maybe 3, I am not sure).. anyway, if you are getting to ABSORB voltage then expect one or more mppt to throttle back before the last one does. But when they are all in MPPT mode, then it's different. If the battery voltage is well below ABSORB, then they all should be trying to output full power from what is available from the panels. Of course, one array of panels might well be oriented in a different direction than the other arrays and so it should not be able to produce as much.
For best results from the mppts, we need to keep the voltage difference of the battery and the solar array to less than a factor of 2. Maybe even 1.5x. The DC-DC conversion is most efficient when this is the case. It MUST be well above the battery ABSORB voltage though. Edited 2024-03-19 20:23 by poida