Charge controllers in series?

[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.

[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]

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...

[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.

[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.

[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?

[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]

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?

[Sunking]

Yes sir that is correct, could you help me understand why it is incorrect so I may convince the installer to fix it.

Are you certain?

It is fine to have 2 MPPT controllers feeding a common battery bank, where say one controller has 1000 watts of solar panel connected to its input, and the other has another 1000 watts of panels connected to its input. Then the the output of the two controllers paralleled connected to a common battery.

You cannot parallel inputs on a MPPT controller as it would drive the MPPT tracker insane.

[inetdog]

Do I understand you correctly that you have two Charge Controllers in parallel connected to a common solar panel array? If that is correct is insane and will not work worth a darn.

Not just two CC, which might work, but two MPPT CC, which surely will not.

[yzahmad]

Yes sir that is correct, could you help me understand why it is incorrect so I may convince the installer to fix it.

My present setup is two inverter/ups each powering one floor, both having their own two batteries (24v aystem). The solar panel ray is coming in and is connected to both controllers as shown in the first post. Each controller is connected to the batteries of one inverter.

The installer claims it is setup this way so that the entire array is feeding both the battery banks, so if one bank is near full charge the other bank will pull the maximum power the controller can channel from the panels.

[Sunking]

Do I understand you correctly that you have two Charge Controllers in parallel connected to a common solar panel array? If that is correct is insane and will not work worth a darn.

[yzahmad]

OK the installer came today to finish up some wiring, I told him this setup doesnt make sense why would you not split the arrays and wire into separate charge controllers.

So he said under the present setup the entire array is connected to both battery banks, so you will not face a situation where one bank is fully charged and the other may require more power but doesnt have the unutilized panels attached to it. I hope he is right about this!

At the time, one battery bank was fully charged while the other was nearing complete charge (90%+). He asked me to add load on the the bank that was fully charged, so I powered four ceiling fans and a bunch of lights. When I came back, the panels were giving 10 volts/amps (I'm not sure ) to the first CC, and the same amount was going to the second CC; the second CC was sending 13/14 v/a to the battery bank (again, not sure what the numbers meant just that the MPPT CC was sending out more than it was receiving). The first battery bank (with higher load) had consumption of 25amps and getting charged in 22/23amps, and its voltage was now at 24v instead of the previous 29v at full charge. The second battery bank (with negligible load) was only drawing 0.5amps, and was showing voltage of 24v. (I'm sure I've messed up alot of the units here but I'm sure you guys get it )

So I guess while not an ideal setup, it seems to be working. Now I will get that device he had (something that clips around a wire and shows the voltage/amperage going through) and observe if the condition inetdog mentioned occurs or not.

Thanks for all the help!

[Sunking]

Sounds to me like an incompetent installer.

[inetdog]

It's the first time he has tried this with mppt controllers, which is why I was concerned. So I guess I should has him to split the arrays going into separate controllers?

That would definitely be my advice, yes. You can try it first to see how it works. Look at the voltage and power indications on both CCs together, then turn off one of them and see what the voltage and power indications are. This will only be a useful test if you have loads connected or the batteries are so discharged that the CC selects the Bulk charging stage though. Otherwise the power input to the CC will be limited by what is being accepted by the batteries.

Anyway, what I would worry about would be a situation where the two CCs are repeatedly changing power settings, with one or the other first taking more power then less power, or a situation where the sum of the power taken by the two CCs is less than the power taken by one CC with the other turned off.

It is too bad that it is set up this way, since you will not be able to use the combined array power to charge just one battery bank if it gets more use than the other bank.