"24V" PV & 12V battery charge controller?

20 Posts
Jan 12, 2011 06:02 pm
"24V" PV & 12V battery charge controller?

We are getting a donation of a 215Wp panel for a small school system in Nicaragua.  That's great, but the Voc is around 40V (Isc ~8A).  I browsed a number of controllers on the site, but it wasn't clear to me what the best (cheap but reliable) choice would be.   Our standard approach would be to have one 12V battery, 12V DC for lights, and an AC inverter for TV/DVD or computer/monitor, and in the past we've used Phocos or Morningstar 10 or 20A controllers, but those require the same voltage on PV side and batt/system side. I saw on this site that the Xantrex C35 can be configured for system voltage of 12 or 24V and accepts input voltage up to 55V.  Is 55V an admissible peak only, or would it work continuously with 40V?  An alternative woould be to have tww smaller batteries in series, keep all sides at "24V", and run everything in AC from a 24VDC to 110VAC inverter.  But again all small inverters I've seen are 12V to 110VAC (or 220).
Should be fairly simple, but I wouldn't want the extra cost of the system to cancel the value of the free panel.
Thanks for any insight.
220 Posts
Jan 13, 2011 05:20 pm
Re: "24V" PV & 12V battery charge controller?

 Hello Michel,

 i applaud your efforts helping out in some of these "third world" places! cheers sir!!

 kindly take the following as just one guys opinions before you make your final choices.

 trying to run a small system at 24 volts when we have many others at 12 volts is a royal PITA, especially considering that we want to run some dc loads..we lose our ability to swap equipment out with other stuff we have at hand, and some of the 24v stuff goes at a premium to purcha$e.

 if this is an older 24v nominal module it may have the option to be rewired to a 12v nominal system..do you have the specs?

 barring a friendly "j-box" that allows us to change over to 12 volts, and use a regular 12v controller. i see a few options.

 we can do the c-35 and run it at 12 volts and lose half the pv capacity (it will run as a ~100wp module)

 or we can go with an MPPT type controller that will accept the higher voltage and "buck it down" to our 12v system voltage.

 this unit is up to the task.. and not too bad on the wallet at just over $100.


 looks like AltE has started to stock some of these? (shows one in inventory) otherwise a few weeks of lead time to set you up.

 there is another unit called the BZ250 that would do the trick..currently not carried by the store due to some early technical problems.

 lots of other nice equipment out there to, but these are more budget minded for your application.

kind regards, dave

edited to add: that solar converters controller will current limit at about 10 amps. but should still produce more amphours into the battery per day than the c-35..the BZ250 will run full speed ahead without an upper limit (might even see ~20ish amps) cheers!
« Last Edit: Jan 13, 2011 05:51 pm by david ames »
20 Posts
Jan 13, 2011 06:46 pm
Re: "24V" PV & 12V battery charge controller?

Hi David,

Thanks for your help.  The panel is a brand new "blem", and the j-box is sealed, with just 2 cables coming out (I haven't seen it yet, just looked online at a datasheet).  So I'm reluctant to hack at it and remove the epoxy or whatever.
The charge controller you recommended is interesting, but quite different from the products I've worked with so far (I'm quite willing to learn):
- There is no DC load connection, no LVD, no low-charge LED or indication, no state of charge LED or display, just a "charging" LED , and a "Full" LED.
In my experience, as we need to train the users to monitor their battery and not let it discharge too much so that it will last long enough, this lack of informational display is a problem.  The models I prefer allow the user to manage their use to keep the SOC around 75-80%, for a long battery life even without relying on the LVD feature.
Are these designed for systems that will be all in AC, with an LVD in the inverter itself?  For some reason, LVD's on (small) inverters always seem set way too low, like 10.5V, whereas LVDs on Morningstar or Phocos controllers are set around 11.4V.  At 10.5V, the battery is dead, not just discharged.  That's a whole bag of worms I'd love to understand as well some day, but for now I'll focus on just the charge controller.
Other question on the product you recommended (I looked up the user manual): the spec says Max output 10A, regardless of using 12V or 24V.  So if I connect a 215Wp panel, with say, 31Vmp and 7A  at 1kW/m2,  it takes that input but only gives me 10A *12V = 120W Huh That hardly sounds like intended MPPT performance.  What am I missing?
Where we work, i.e. the tropics, we actually get more than 1kW/m2 on good days, so I want to make sure I use all I get.

220 Posts
Jan 14, 2011 02:03 am
Re: "24V" PV & 12V battery charge controller?

 sounds like there is no changing the voltage output of that sealed module. and i see your concerns about training others to be able to look after the system in a reliable way. gonna need those displays and load control LVD to help with that.

 100% agree on the issues with the inverter cut-off set points set too low on most of the economy inverters!..by the time the things alarm out we are into the permanent damage zone (voltage wise) with our batteries!

 looks like we need to step up to the higher priced models of MPPT with a LVD load controller built in.

 this one is just a hair undersized with a rating of 200wp max. (your call on that.. i might roll the dice if it were up to me)

probably want (need) the optional display as well.

 we are starting to get into some real money to jump up to the next size controller...just a side thought here? any chance of maybe trading that donated 24v module with someone for a better suited 12v one?

"10A *12V = 120W  That hardly sounds like intended MPPT performance.  What am I missing?"

 your right on with those points...the one advantage would have been to have that 10amps charging current for a longer period of the day..closer to 10A *14v = 140w (i think we see now that, that controller is a no-go for this application)

 we should think about our battery sizing with this system as well..if we go too small we will be in regulation much to early and won't harvest all our available power. i would shoot for somewhere around a 300-500ah battery bank if possible to be able to store all we can.

kind regards, dave  de kb1mzf

20 Posts
Jan 14, 2011 01:41 pm
Re: "24V" PV & 12V battery charge controller?

Hi Dave,

Thanks for all this.  The Morningstar MPPT 15A seems like the way to go.   I don't think we would use the RM, because the users wouldn't be able to make sense of the info.  The built in battery status LED should suffice.
Unfortunately, the panel is part of a larger donation, and it comes with a commitment to report on how it's being used, so I don't think any swap for a 12V is feasible here.

About inverters LVD vs. controller LVD.  Maybe we should start another thread for viewers benefit.  BUt in short, would it be possible to use the controller LVD by installing a relay on the controller load side, and use the relay to switch the inverter input on & off? 
I'm an ME and haven't looked into relays since school a long time ago, but there must be relays that consume current only at the time of activation, right?

[Otherwise, one advantage of the Phocos controller(s) is that they can sound an alarm as well as give a visual signal when LVD is activated.  That would help tell the user something need ot be done]

Any idea why the inverter LVD's are set so low?  Car batteries (the main market for those things) supposedly are even less tolerant of low voltage than deep cycle, so I find this really puzzling.

220 Posts
Jan 14, 2011 06:32 pm
Re: "24V" PV & 12V battery charge controller?

Hi Dave,

About inverters LVD vs. controller LVD.  Maybe we should start another thread for viewers benefit.  BUt in short, would it be possible to use the controller LVD by installing a relay on the controller load side, and use the relay to switch the inverter input on & off?

Any idea why the inverter LVD's are set so low?

Great topic Michel, we put up a new thread with some methods that a diy guy might find fun..(the inspector might not be amused though)Smiley

cheers, dave

22 Posts
Jan 18, 2011 04:25 pm
Re: "24V" PV & 12V battery charge controller?

Hi guys, great conversation going on here.�  This is a common dilemma we deal with here at the AltE Store.

The problem with using the Morningstar 15A MPPT charge controller is that the 15A is max output.�  So if you've got 24V 8A in, and 12V out, that's going to double the output current to 16A out.�  That's too much.�  Also, the max wattage for a 12V battery bank is 200W, so the 215W is too high.�  So alas, that charge controller is just a little too small for this use.

You could use the Solar Boost 3024iL for $338.�  The Auxiliary Output serves as 20 Amp Load Controller.�  http://www.altestore.com/store/Charge-Controllers/Solar-Charge-Controllers/MPPT-Solar-Charge-Controllers/Blue-Sky-Solar-Charge-Controllers-MPPT/Solar-Boost-3024iL-Solar-Charge-Controller-No-Display/p6784/

Give me a call and we can work out some details.

Solar Thermal Queen
AltE Store
« Last Edit: Apr 14, 2015 05:24 pm by Amy Beaudet »
220 Posts
Jan 18, 2011 06:53 pm
Re: "24V" PV & 12V battery charge controller?

good info Amy,

soo many products, each with their own quirks.

 agree it's "officially" too small..no danger though, perhaps limits itself on occasion and we lose a bit of power. never could do this with a 12v nominal module as it could overload the inverter input. output on this one is self protected.

from manual:

4.5 Protections
Solar Overload
(No LED indication) The SunSaver MPPT will limit battery
current to the 15 Amp maximum rating. An over-sized
solar array will not operate at peak power. The solar array
should be less than the SS-MPPT nominal max. input power
rating for optimal performance. See Section 7.0 Technical
Specifications for more information.

cheers, dave

edit: "overload the inverter input" should read "overload the controller input" Smiley

some more good info:->


"It is important to note that exceeding the maximum array wattage for a given controller/
nominal voltage combination will not damage the controller. Any wattage in excess of the max array
wattage will simply be lost. (i.e. Using a 300W array in a system where the max array W is only 200W
will not damage the controller, but the 300W array will effectively act like a 200W array and 100W of
power will be lost.)
IMPORTANT: MPPT controllers can be used with off‐grid or on‐grid modules. PWM controllers should
only be used with off‐grid modules.
Maximizing Efficiency
Morningstar TrakStar™ MPPT controllers will operate at slightly different efficiencies depending
upon the nominal battery voltage being used, the Vmp of the array, and the total wattage of the array.
These efficiency curves are printed in the appropriate manual for every Morningstar MPPT controller.
This data can be used to optimally size your solar array for best performance and maximum energy

« Last Edit: Jan 18, 2011 09:16 pm by david ames »

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