Multiple MPPTs

[DeepBass9]

I would appreciate if the learned on this form could comment on this, and let me know if it is a problem or something to be ignored.

I have 2 MPPTs, one with 3x2 300W panels, and another with 2x2 300W panels. Today being a good solar day, the batteries charge up quite quickly but eventualli I end up with this situation: One MPPT and array ends up doing all of the work, while the other is trying to float. Is this a problem, or should it be ignored?

MPPT 1:

MPPT2:

 

[Mark]

No problem at all... I do set my float voltage slightly lower for the smaller string (my Microcare 40A MPPT - 480W) and have that very close to the bulk level.  That way I bulk and equalise with the larger string (facing NE) (Axpert 1800W) and the smaller string (WNW facing) comes in as support when the voltage drops to the float of the second MPPT.

If you have them both the same they will get confused and they will jump between the 2. 

[DeepBass9]

The settings on both are identical, but the larger array ends up being the one that does the final charge. I guess that while the settings are the same, there are slight differences between the two units.

[Guest]

I had same when I had 2 x 200w on a MPPT and 3 x 310w on a PWM controller and both where set identically.

In the end I split the systems for as you, one worked harder than the other. Wasted resources.

Off course if the system is under load, then the controllers will produce, but how much effort is that?

[Mark]

The one will work harder to a point and then they will both work harder as the sun sets.  I really think the solution is to face the strings in different directions so you spread the work and power generation across more of the day.  My solution anyway.  On a cloudy day they will both work - the nature of the deal I guess.

[DeepBass9]

The problem only occurs once the batteries reach 59.2V.  Right now both controllers are on float,

[___]

I don't think it is a big problem. I basically foresee three scenarios.

1. The second array (the one that remains in absorption/bulk longer) is strong enough to hold the voltage up, so while the second array has gone to sleep and the other one does all the work, nothing bad happens. Eventually it goes to float anyway. One might say the second controller is the "master", it decides when you're done.

2. The second array is not strong enough to hold the voltage up. It remains in bulk for that reason. If the voltage drops below float, the other controller wakes up and holds it, so you effectively have the bank in float even though one controller doesn't know it. Still not a bad situation. One might say the first controller is the master here.

3. The second array can't hold the voltage up long enough to reach float, but it can hold the voltage higher than float, somewhere in the middle. Now you have a bank that should have been floated but instead remains at a higher gassing voltage, and cannot get out of this deadlock without the help of the other array (which is doing nothing). This could be bad depending on your setup, but once again I'm not overly concerned. If it is too small to get the voltage up, it is probably too small to do much damage.

[Mark]

I have my Microcare set as the following:

• 13.4 (therefore 53.6V) Float
• 13.5 (Therefore 54V) Bulk

(manual equalise)

My Axpert is at:

• 54.6 Float
• 56.4 Bulk

The MC is therefore sitting doing not much until far past the point the bank has been bulked and is at float.  It then slowly adds value until I get 7A from 480V at 17:00 in the afternoon.

[___]

As I recall the MCs don't go back into bulk once it's gone to float on the day, so it is definitely less of a problem with them.