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Author: Subject: Solar - 24 volt array to 12 volt battery bank?
Doug/Vamonos
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[*] posted on 10-10-2016 at 11:37 AM
Solar - 24 volt array to 12 volt battery bank?


Hi. At Bahia now installing my new solar system. I have 8 100 watt 12volt panels going to an outback FM 80 flexmax controller and charging 6 105 amp 12 volt batteries. Everything is wired for twelve volts. Also a xantrex prosine 2000 pure sine inverter. Can I configure the panels for 24 volts to the controller but still keep the battery bank at 12? Or would I need to wire the batteries for 24 and get a 24 volt inverter? My distances are not critical so I can keep it at 12 with the wire I have. Just curious about the 24 volt configuration and if that would be beneficial. As it stands I will max my controller with two more panels if I stay at 12 volts whereas 24 volts has a lot of potential growth.
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BigWooo
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[*] posted on 10-10-2016 at 12:36 PM


Yes, with the FM80 charge controller you must wire the panels at a higher voltage than the battery bank for it to work properly With 12 volt batteries, the minimum voltage for the panels is 24 volts. Here's a link to the instructions for the FM80 charge controller. Read them very carefully. There's breaker and wire size minimums that must be adhered to. Read up on wiring, breakers, and maximum panel watts that can be hooked up to the controller.

FM 80 manual

Not @ edit. FM80 at 12 volts (battery bank) can have a maximum of 1250 watts worth of solar panels. You might want to even consider wiring the solar panels at 48 volts as there is less loss between the panels and the charge controller.

[Edited on 10-10-2016 by BigWooo]
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Bob and Susan
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[*] posted on 10-10-2016 at 12:58 PM


i agree...bring the voltage down from the panels at 48v

let the controller change the voltage to your batteries...

you can always change your batteries later to 24v if you need a new inverter

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larryC
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[*] posted on 10-10-2016 at 04:07 PM


Definitely wire your panels to atleast 24v. My panels are wired to 90v in so I can use a smaller wire gauge. To take advantage of the growth potential of a 24v system you would have to change your battery bank and your inverter to 24v or even 48v to maximize the potential of your FM80 charge controller.



[Edited on 10-11-2016 by larryC]




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[*] posted on 10-10-2016 at 04:46 PM


Thanks ... always good info from folks off the grid
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[*] posted on 10-10-2016 at 05:03 PM


Agree with Larry. The FM80 can take a max of 150 volts input (be careful, that is the Voc voltage not the Vmax) The higher the voltage the better.



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soulpatch
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[*] posted on 10-10-2016 at 05:43 PM


I doubt Doug would ever hit Voc with that array in that environment.

That CC could handle all of what those modules would throw at in in one string. Make it easy on yourself.

Amperage stays low and wire sizing stays small.

With real world heat degradation of output it will never get close.

Series them all, your CC will let you drop it to 12V.

In all actuality they are probably 17V modules at max voltage but you will never get close to that unless you put bowls of ice behind them.
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larryC
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[*] posted on 10-10-2016 at 05:54 PM


If Voc of those panels is 17 then 8 in series would be 136v going into the charge controller. Awfully close to the 150v that Outback says will burn up the charge controller. I would probably wire them in 2 strings of four panels to give an input voltage of around 88v. Doug will have to look at the back of the panels and see what the Voc is for those panels. The panels I have have a Voc of 35v.
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[*] posted on 10-10-2016 at 07:41 PM


Quote: Originally posted by larryC  
If Voc of those panels is 17 then 8 in series would be 136v going into the charge controller. Awfully close to the 150v that Outback says will burn up the charge controller. I would probably wire them in 2 strings of four panels to give an input voltage of around 88v. Doug will have to look at the back of the panels and see what the Voc is for those panels. The panels I have have a Voc of 35v.
Larry


STC is 25°C at 1000Wm2.

I doubt if those two conditions will ever meet.... you might get the 1000W/meter2 but with a temperature derate I can't imagine.

Maybe in January with an array angle of 45°on a very clean module and a low humidity day but that would kill summer production unless you put in an adjustable racking system.... which would be very cool.
Drop them to about 12 degrees in the summer and you'll bump your production.

There is also a derate for age.

However, two parallel strings of 4 would still push your batteries where you want them to go.
It'd certainly be easy enough and would make you rest easy on those colder days when voltage goes up.

Whatever you do put in some circuit protection and put an EGC on the array for surges that may occur due to weather.

It's happened before in Camp Gecko.
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del mar
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[*] posted on 10-10-2016 at 08:02 PM


northern arizona wind and sun is your friend ;)
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willardguy
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[*] posted on 10-10-2016 at 08:25 PM


Quote: Originally posted by del mar  
northern arizona wind and sun is your friend ;)


bingo! :yes:
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ncampion
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[*] posted on 10-10-2016 at 09:17 PM


Quote: Originally posted by larryC  
If Voc of those panels is 17 then 8 in series would be 136v going into the charge controller. Awfully close to the 150v that Outback says will burn up the charge controller. I would probably wire them in 2 strings of four panels to give an input voltage of around 88v. Doug will have to look at the back of the panels and see what the Voc is for those panels. The panels I have have a Voc of 35v.
Larry


Again, agree with Larry. Most "12v" panels have a Vmax around 17v but the Voc is usually 20 - 21v . This would exceed the input rating of the FM80. Fortunately the cc will just shut down with no damage. Use two strings.

[Edited on 10-12-2016 by ncampion]




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Doug/Vamonos
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[*] posted on 10-11-2016 at 08:19 AM


Thanks everyone. I was hesitant because I didn't see anything in the manual specifically addressing "down converting". It does say it can do it but didn't expand on that. Time to go fishing. Yesterday toros were crashing the sardines on the beach right in front of geckos. Too much fun! Play with solar later.
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Doug/Vamonos
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[*] posted on 10-11-2016 at 01:11 PM


I set up two strings of 48 volts each and it seems to be working fine. Thanks.
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larryC
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[*] posted on 10-11-2016 at 02:09 PM


Good for you, as long as none of the wires get hot and the magic smoke stays in the controller then you did good.



Off grid, 12-190watt evergreen solar panels on solar trackers, 2-2812 stacked Outback inverters, 6-2v enersys 125-33 2000 amp hour AGM batteries, FM 80 and MX 60 Outback charge controllers, X-240 Outback transformer for 220v from inverter, 6500 watt Kubota diesel generator.
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Doug/Vamonos
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[*] posted on 10-14-2016 at 01:25 PM


I set up two strings of 48 volts each and it seems to be working fine. Thanks.
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