Two MPPT controllers serving one battery bank

James · April 2015

Hello folks,

I realized I had my question here posted in an incorrect sub topic forum, so, I'll set it here;

I'm wondering if there are any issues I should be aware of when connecting two MPPT controllers to one battery bank.

I currently have Kyocrea KC130 panels wired in series (24vdc) handled by an Outback MX60 controller.
I will be adding SolarWorld 24 volt, 230 watt panels to my system. These panels will be handled by an Outback 80 controller.
I'm aware that the array open circuit volts will be different in each of these arrays/controllers.

Since I am planning on using a Outback 80 MPPT unit with my new (used) panels, the difference in open circuit voltage ratings should not be an issue, correct?
The spec sheet open circuit voltage rating on the SolarWorld 230 units is 37.4 vdc.
My actual open circuit voltage readings on the existing series panels (2 Kyocera 130 units) is aprox. 39.5.

Also, when I set up the new system expansion with the Outback 80 unit, should I adjust the charge set points to match the other controller (Outback MX60)?

I've often wondered about two controllers serving the same battery bank.....Do the controllers experience some cycling or "hysteresis"?
If so, is there a specific way to adjust these controllers to play well together?
thanks much

inetdog · April 2015

There are rarely any problems with two MPPT CCs driving the same battery bank. What does not work well is having two MPPT controllers connected in parallel to the same panel array.

waynefromnscanada · April 2015

I've been feeding one battery bank with two different MPPT controllers for going on 6 years. Each controller, one a Morningstar TS-MPPT-60, the other was an outback MX-60 until I retired the MX-60 and replaced it with a Midnight Classic. Sure the two controllers don't always fully agree, but they never fight and the end result is awesome. I have adjusted their settings to be as close to each other as possible and all is fine. One may go to float a bit sooner than the other, but so what. BTW, reason I switched out the Outback was because of it's unfortunate quirk of allowing output voltage to go totally uncontrolled while it sweeps. When the batteries were fully charged, or almost fully charged, this would too often, briefly drive battery voltage beyond the over-voltage point of my inverters, shutting them down, especially in Winter. This does not happen with the Midnight which I consider it a more advanced, refined and better design than the MX..

BB. · April 2015

I highly suggest that you run separate cables from the battery bus to each charge controller ("star" or "home run" wiring, each controller have its own fuse/breaker in the + cable run), and do not send one cable from battery bus to controller A then to controller B (daisy chain).

The controllers will not "see" the voltage drop/noise from the other controller, but see the actual voltage from the battery bank directly (shared cabling can have "noise" from one controller confuse the second controller a bit).

-Bill

waynefromnscanada · April 2015

Well said Bill, totally agree and that is how I have mine wired.

Dave Angelini · April 2015

The one problem you may l have if you want exact voltage (AGM or tight voltage requirements) is the older MX60 would not network with the newer outback gear. For the controllers to really work well together they should be networked.
Be sure and breaker each controller in and out.

James · May 2015

Thanks much for the replies. I was planning on using independent controller battery feed lines. Always a good idea. And yes, definitely will use breakers on input and output of the 80. If anything, I tend to go overboard with breakers.
I am planning to upgrade the MX60 controller to a Outback 80 and Mate system as soon as funds become available.

froggersix · May 2015

mate does not control two controllers only midnite classic can connect two so they work like one -0 i asked someone who knows.

waynefromnscanada · May 2015

But remember, the two controllers, while it's nice to know they're operating in master-slave mode, they don't have to operate that way. If their settings are reasonably close, they'll do an awesome job of looking after your batteries and they won't fight. One of them will likely switch to float sooner than the other for example, but in practice that's not a big deal. Been doing it that way for almost 6 years now and all is awesome.

Dave Angelini · May 2015

froggersix wrote: »

mate does not control two controllers only midnite classic can connect two so they work like one -0 i asked someone who knows.

This is not true

Dave Angelini · May 2015

waynefromnscanada wrote: »

But remember, the two controllers, while it's nice to know they're operating in master-slave mode, they don't have to operate that way. If their settings are reasonably close, they'll do an awesome job of looking after your batteries and they won't fight. One of them will likely switch to float sooner than the other for example, but in practice that's not a big deal. Been doing it that way for almost 6 years now and all is awesome.

True but you do not have the need for exact voltage that some batteries (like some AGM) require. One of the beta tests on the MX-60 when Outback was setting up the networking of controllers
was for trying to keep the AGM battery folks happy. They went to the FM series and the XW controllers did one even better by just having a network cable needed for the contollers and no other hardware.

James · May 2015

hi folks,

just to be clear, the Outback Mate system does not "control" or "sync" two Outback 80 controllers, it just monitors them?
I do have an AGM battery bank if that matters.
EDIT: I meant to state one Outback FX80 and one older MX60 controllers.

Dave Angelini · May 2015

James wrote: »

hi folks,

just to be clear, the Outback Mate system does not "control" or "sync" two Outback 80 controllers, it just monitors them?
I do have an AGM battery bank if that matters.

From the mate manual which you can download from Outback. Do you really think that it just monitors them? Almost all Outback, Midnite, Morningstar, Schneider MPPT controllers that you can buy are networked these days. --Dave

"The OutBack Power Systems MATE serves several functions: • Displays and configures the system and its components—the FX Series Inverter/Charger, the FLEXmax 80 and FLEXmax 60 Charge Controllers*, and the FLEXnet DC • Coordinates system operation, maximizes performance, and prevents multiple products from conflicting • Permits adjustments of your power system through a series of convenient display screens, which allow switching among different components, viewing the status of each and changing settings * MX60 Charge Controller—remote controls the AUX function and displays, but does not control, all other functions When connected to an OutBack HUB communications manager, a single OutBack MATE can: • Link to as many as ten FX Series Inverters/Chargers, OutBack Charge Controllers and additional future OutBack Power System products. • Issue a global Bulk or EQ recharging command which includes the Charge Controller’s charging function. This manual will show step-by-step use of the OutBack MATE to best run a power system"

James · May 2015

Thanks Dave. I have not done any research on the Mate system. It was suggested here that I might get the mate to use with my two Outback controllers after my 80 is installed. I guess I was confused with some of the posts; some seemed to be stating two different things.
Initially, I will not have the budget to add the Mate system. I just wanted to be clear on what functions it had. I will download and read the manual for the Mate. Your quote from the manual seems to indicate it cannot control the MX60, but can control multiple Flexmax units.
Please forgive my confusion. To be honest, I have mild dyslexia, and have difficulty with some written word and instructions.

Sun-Wind-Keeper · May 2015

I have an Outback Flex Max 60 and 80 with the Mate2...no problems with the "Global" control features. I also programmed my two 3524 inverters in the Outback Classic Parallel Stacked 120 VAC configuration. (Master/Slave Setup).
The firmware of all the components needs to be compatible as there have been changes over the years...They usually send a loaner while you send the component for firmware upgrade. I'm 100% satisfied with the FM 60 & 80 & Mate2 setup.

James · May 2015

Thank you Sun-Wind-Keeper for sharing your experience. I haven't really done much research into my MX60. I was able to afford that type of MPPT controller just as the newer updated models came out. I bought the MX60 not even knowing there was update models out. I've leaned my lesson and I am trying to do some more research. I'm guessing by the time I could upgrade my MX60 to another Outback 80, they will have new models once again. Some of us out here are always a step behind with the technology. For instance, some on this forum might think i'm crazy for running a Xantrex SW2024 inverter with so much new stuff out there. My 4024 has worked flawlessly for 6 years+. I do understand that I'm on borrowed time with that unit, and as I have read here, less and less places will repair them and there may be part availability issues. When the inverter dies, i might, if i'm lucky, be able to afford today's technolgy. I guess I'm ranting here and I'm off topic, my point is I'm still learning and have made purchase mistakes. I'm trying to purchase more wisely with my very limited budget. Thanks again to all for viewpoints and info.

Sun-Wind-Keeper · May 2015

You may want to consider purchasing an Outback Hub4 to link the charge controllers and MATE "properly"...this will also help when you eventually replace the SW4024, and if you go Outback with a inverter.
I hear the Mate and 2 may be replaced with the 3 soon..so stocking one now might also be a good investment...the 3 is considerably more expensive.

James · May 2015

thanks for the tip

Sun-Wind-Keeper · May 2015

Hub ususally mounts on the side of a surface ..like a Flex Ware metal AC-DC enclosure and is a "limited energy circuit".
Once the Cat5 RJ45 plug is inserted make sure NOT to bump it! The jack port is delicate and when broken leads to a $90 Outback repair charge.

SolaRevolution · June 2015

waynefromnscanada wrote: »

I've been feeding one battery bank with two different MPPT controllers for going on 6 years.... Sure the two controllers don't always fully agree, but they never fight and the end result is awesome. I have adjusted their settings to be as close to each other as possible and all is fine. One may go to float a bit sooner than the other, but so what.....

Hey Wayne,

Curiously, which settings are you talking about? Just voltage points? The TriStar and the Midnite have very different charg-timer algorithms, no? Are you disabling the End Amps on the Midnite?

It's been a little while since I looked into this issue. I was curious after talking with someone who has tried several different CCs into the same battery bank. They see that the TS logs higher daily production but I wonder if that is a result of the way the TS determines it's absorption time vs the settings used in other controllers.

attkumar · June 2019

BB. said:

I highly suggest that you run separate cables from the battery bus to each charge controller ("star" or "home run" wiring, each controller have its own fuse/breaker in the + cable run), and do not send one cable from battery bus to controller A then to controller B (daisy chain).

The controllers will not "see" the voltage drop/noise from the other controller, but see the actual voltage from the battery bank directly (shared cabling can have "noise" from one controller confuse the second controller a bit).

-Bill

Dear Bill.
You have given fantastic solution. I have stacked with above issue. I have connected the A and B controller with "Daisy chain" method. One of my controller A will cut off the current when connect controller B. I will do your solution this week end.
Thanks lot
Kumara from Sri Lanka

BB. · June 2019

Kumara,

Good luck, I hope it works for you... You should have relatively heavy copper cables between the batteries and to the "battery bus"... (low voltage drop at charging and high load current)...

For loads, (there are for 12 volt battery bus) suggest a maximum of 0.5 volts drop for loads... For charge controllers, a 0.05 to 0.10 volt drop maximum at full charging current from charge controller to battery bus.

Too long of cables and/or to small of cable diameter, and you will have less than ideal charging (the extra voltage drop makes the battery look "fuller" than it really is).

Do you know how to calculate voltage drop (i.e., 20 amps by 2 meters and xxxmm or mm^2 diameter cable)?

Here is one example in USA units as an example (calculator link has some sample numbers and results):

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=12&phase=dc&noofconductor=1&distance=2&distanceunit=meters&amperes=40&x=78&y=21

-Bill

attkumar · June 2019

Dear Bill
I used a 16mm wire for load. 8mm wire for charge controller. Both controller wires connect to load wire in connection box. load wire nearly 4 m log from batteries . Now Ii am going to get 2 separate wires from charge controller to battery terminals.Is it OK sir?

NANOcontrol · June 2019

I have two MPPT controllers feeding one battery. Actually three, one is turned off because it did not play well at all. A charging system will always come up with a number. If your charging system doesen't sense current out of the battery, charging is dubious. With a lot of panels and a lot of battery it all works fairly well.

BB. · June 2019

Good day Kumara,

Can you confirm if you have 16 mm or 16 mm^2 wire? 16 mm diameter cable is very large. 16 mm^2 is relatively heavy for a "typical" solar charge controller (which is not a bad thing--Heavy cable for low voltage DC power systems is pretty standard).

Also, can you tell us a bit more about your system? The Voltage and AH rating of your battery bank, Watts of Solar array and Vmp/Imp/series and/or parallel wiring of array, perhaps a bit about your charge controller (PWM or MPPT, Amp rating, brand/model, etc.).

And, 4 meters from charge controller to battery bank is usually less than ideal... In general, it is better to have the Solar Charge Controller mounted close to the battery band and longer cables between the array and the charge controller. And run the +/- cables bundled together in pairs... You want to avoid "loops" and separate +/- pairs... This causes them to act like antennas. The cabling can pickup electrical interference (light nearby lighting strikes) and can also broadcast electrical interference (making AM and HAM radios whine and click).

When you have two charge controllers in parallel--It is common for them not to transition from charging to float at different times (their voltage sensing is always slightly different between controllers). This is not a problem. The charge controller with the "highest voltage setpoint voltage" usually "wins".

-Bill

gnoeld · July 2022

I have two Charge controllers connected to one battery bank using buss bars.
One CC is a Renogy 100/40 and the other is a Renogy 150/60.
The challenge is they read the battery % differently. The 40 will read 77% and the 60 will read 92% and etc.
Can I get them to read the same or how can I tell which one is accurate?
Thanks

BB. · July 2022

Welcome to the forum Gnoeld,

I would suggest that you create your own discussion--That way we do not confuse with the origintal poster's questions and system design.

Also, can you tell us a bit more about your battery bank? Flooded cell lead acid, AGM, Lithium LiFePO4 or what?

Also the size of the solar arrays, the size and length of cable from the two charge controllers to the battery bank, etc.

As far as I know, the Renogy controllers do not have the ability to "sync"--So they will never be in-sync with each other.

How are the controllers behaving? Do they stop charging because the sun goes down, or do they stop charging (enter float charge) in the middle of the day?

-Bill

Two MPPT controllers serving one battery bank

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