Charge controllers in series?

[inetdog]

I'm sorry but I'm zero technically, but the picture on the first post shows how my system is wired: one wire coming down from the panels, into the first CC and then another wire connecting it to the second CC.

Sorry for being a dimwit but I don't understand how it would drive the MPPT insane: wouldn't the total voltage get divided and each CC take 50% (assuming both banks need charge) until the smaller bank is charged, then one CC reduce its demand automatically?

What you are not taking into consideration is the way that an MPPT CC works. Rather than appearing to the panels as a constant resistance load, instead each CC will try changing its effective resistance until it finds the point where it *alone*, since it has no information about the other CC, is getting the most power from the panels.

When two CCs are in parallel and one CC tries to draw more current, letting the voltage drop correspondingly, to see if it gets more power from the panels, it will see that it is getting more, since it is stealing power from the second CC. When both battery banks need charging, the two CCs must share what is a constant, limited amount of power from the panels. So if one CC tries to draw more, by dropping its input voltage, the other will fight back by dropping its own input voltage. The result of this will either be the voltage and current point of the panel output bouncing up and down and the power draw bouncing back and forth between the two CCs.
In the worst case, both CCs will go to the lowest possible voltage in their MPPT range, and will therefore be wasting half or more of the available panel power.

[yzahmad]

Thanks for all the feedback, I've scheduled a session with the installer and he will make this right by separating the panel arrays that will feed independently into the charge controllers.

Which brings me to my next great question: I figured when I'll be feeding both banks from an equal number of panels, why not replace my existing setup (2 x 24v inverters) with a single 48v system? The problem, thanks to my wonderful planning, is that I now have 2 MPPT charge controllers that are 24v.

I've read on other forums that two 12v chargers can be used to charge a 24v system (+ and - of each charger connected to each battery), so following the same logic could I use these two charge controllers to charge a 48v battery bank?

[bstedh]

I may be wrong about this but I think the simple solution would be to drive the two inverters from a larger common battery string. this would allow the use of one charge controller (if the amp ratting is enough) simplifying the setup. If needed you could probably sell the two CC for a single higher amp CC.

[Sunking]

Which brings me to my next great question: I figured when I'll be feeding both banks from an equal number of panels, why not replace my existing setup (2 x 24v inverters) with a single 48v system? The problem, thanks to my wonderful planning, is that I now have 2 MPPT charge controllers that are 24v.

Well you can certainly do that but you will need a new controller.

[yzahmad]

Well you can certainly do that but you will need a new controller.

Aww shucks, I had read on other forums that you can charge a 24v bank from two 12v chargers, and a 48v bank from four 12v chargers (important for the four to be connected to isolated panels to prevent a short +/- short). So following the same analogy, I had figured after seperating the panels/controllers I would be able to look at 2 batteries as a seperate 24v bank and charge them seperately.

Sigh...

[inetdog]

Aww shucks, I had read on other forums that you can charge a 24v bank from two 12v chargers, and a 48v bank from four 12v chargers (important for the four to be connected to isolated panels to prevent a short +/- short). So following the same analogy, I had figured after seperating the panels/controllers I would be able to look at 2 batteries as a seperate 24v bank and charge them seperately.

Sigh...

There are definite problems with what you describe. although it is not completely out of the question. Those problems include:

1. You would have to be confident that the two panel arrays (one per CC) are very well matched, to avoid a situation in which you run one half of the bank down to a too-low SOC while the other half keeps the voltage at the inverter above the Low Voltage CutOff level set for the 48 volt bank.
2. Or you would have to add an external balancer circuit which tries to keep the voltages of the two banks equal. (Such devices are available, but I have not seen one advertised for 24/48 systems.)
3. If either CC failed, your system would be dead. (Manually switching one CC between the two banks is not a long term option!)

Using two separate chargers is more practical when the they are identical mains-fed chargers which are always both able to deliver their rated output at the same time.

[yzahmad]

There are definite problems with what you describe. although it is not completely out of the question. Those problems include:

1. You would have to be confident that the two panel arrays (one per CC) are very well matched, to avoid a situation in which you run one half of the bank down to a too-low SOC while the other half keeps the voltage at the inverter above the Low Voltage CutOff level set for the 48 volt bank.
2. Or you would have to add an external balancer circuit which tries to keep the voltages of the two banks equal. (Such devices are available, but I have not seen one advertised for 24/48 systems.)
3. If either CC failed, your system would be dead. (Manually switching one CC between the two banks is not a long term option!)

Using two separate chargers is more practical when the they are identical mains-fed chargers which are always both able to deliver their rated output at the same time.

Thanks for the reply, what I had planned was to connect 50% of the panels to each charger controller (all panels are in a straight line so should be receiving roughly the same amount of light). After this, my options would be to (1) connect each charger to a 24v bank, or (2) wire the output of each CC in series to create 48v.

Now I had tried to connect the existing two CCs using option (1) after forming a 48v bank, but it was creating a short (probably because the CCs are connected to each other and hence we cannot join the + and -). So once the CCs are independently wired to the panels, I guess this should work?

Reason why I'm trying to hang on to these CCs is that the readings show their input from the panels at 37v (that would mean the excess volts are converted to amps, right?), and if I get the PWM CC (only type available here for 48v systems) I'm afraid I will lose quite a bit of power going into the batteries.