150V 45A MPPT - roll your own


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poida

Guru

Joined: 02/02/2017
Location: Australia
Posts: 1425
Posted: 10:19am 19 Mar 2024      

  rogerdw said  
I fitted another dozen 250 watt panels on the weekend and hooked up the third mppt. Only thing that's a little disappointing is that the output is quite a lot lower than the other two that each have a dozen 275 Watt panels.

While the first two might be pumping out 2,800 watts each, the third one might only be supplying 1,800 watts. Now I'm weighing up if I fit the other 12 x 250 watt panels I have here  ...  or sell them and buy some more 275 or 300 watt panels instead.


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