need help with my off grid system

[dk1393]

I have an off grid system at my cabin. I am in need of a new charge controller. I am borrowing one at the moment.  I was thinking of a MPPT controller.  I am not sure of what would work best with my setup.

Also, fromwhat I have been learning it seems my Panels are not enough to properly charge my batteries. I can get another 180w panel. If you think that would help.

My cabin is used most weekends, year round. I had figured on 480w a day usage when I did the calculations a few years ago.

My system is wired for 12V.

I have 4 6V L16 360AH batterries

2 180w panels wired in parellel.
Vmpp 24.5V
Impp 7.47A
Voc 30.4V
Isc 8.45A

I have a Samlex 1500W Pure si wave inverter.

Also what are the benefits as far as wiring the panels in series or parallel?

[MountainDon]

series vs parallel ;  what is the distance from PV modules to the charge controller?  Series has the benefit of the possibility of using smaller gauge wire. If the distance is not too great that is of lesser concern.  Series also makes it possible to go to a 24 VDC (battery) system.

The CC would have to be able to handle the higher voltage of course. Voc 30.4 x 2 = 60.8 x safety factor and cold weather spike. Maybe as high as 95 VDC.

A chance of wanting to increase the number of panels in the future affects  equipment choice.

Does the present system have the ability to recharge the batteries to where they will go into float every normal day? That should be the goal. If it regularly does not reach float then more PV capacity would be a great idea.

Do you know how much the batteries are depleted each normal day?  (to what DOD, depth of discharge)
Do you use a hydrometer or go by volts?
Location; hours of good sun in worst time of year?

[dk1393]

Thank you for the reply. 

On a normal weekend in the winter. It is probably an average of 2-3 hours a day sun. I am in upstate NY.

I do have a 90a IOTA charger in the system. I use my generator to top off the batteries. I would like to get away from having to do that every weekend. On a normal weekend. It is 14v when I arrive. It will drop to 12.2 by the end of the day if it's overcast all day and the tv has been on a few hours.  It will drop under 12v if I don't charge it up the next day.

I normally go by the voltage to keep an eye on things. Once a month I will check the specific gravity. It is normally about 1.24 on all cells. The batteries were installed in June 09. I have them located in an insulated box under the camp.

My inverter is 12v so I would like to keep the system 12v.

[MountainDon]

Assuming the batteries are still okay and assuming the voltage readings are NOT taken when under load but when sitting idle for a while, I'd guess you don't have enough battery storage for the use, as well as insufficient PV capacity.

At this point (battery age) I don't know if it is a good thing to add new batteries to the old. The new will not perform as new. Their apparent performance will be degraded by the older batteries. If the old batteries have been well maintained maybe that won't be too bad. Note that batteries sitting under charged will sulfate quicker than fully charged ones.

The 14 VDC figure sounds like what you might see when the system is floating. 12.2. not under load or charge) is down to 60% of battery capacity; shortening life.

Have you done an accurate re assessment of the use? If all loads go through the inverter you could try a Kill-A-Watt meter on the output to log actual use, then apply efficiency loss for the inverter itself.

I think I'd start with reassessing use and capacity before deciding what charge controller to get. I am partial to Outback and upper level Xantrex as first choice.

A note on charging with a generator and separate charger...  It is my opinion that when you know the sun will not provide enough PV power to fully charge the batteries it is a better use of the fuel the generator uses and better use of the time on the generator, to charge with the generator first thing in the day and then let the available sunshine slowly bring up the charge in absorb mode. When a generator is used to finish the charge you get much less charge for the fuel used. That's what I did when we RV'd as our PV capacity was not much good for a deep bulk charge rate.


Oh, the 1.24 sp gr reading = about 80% charge depending on temperature.  If that is as high as the batteries are getting at any time that is not good for longevity, assuming the temperature of the electrolyte was about 75 F.

Here's a table you might find useful
http://countryplans.com/smf/index.php?topic=6059.msg162277#msg162277

[dk1393]

The 14.? volts is when I first arrive at the cabin. 12v reading is under load. I will be back at the cabin on the 29th and I will get a better check on things. I will also reavaluate my loads and get some numbers for you.

Also the specific gravity has been at colder temps this winter. I notice on the link you sent that the temp they are using is from the electrolite?  Never thought to check that but I will.

Thanks for the tip about my generator. I don't always run it in the morning. That makes a lot of sense.

My system was origiunally designed by a solar shop where I bought the system. It was set up with the thought that the cabin was going to be used a few weekends a month. The system has never performed up to par. We also use the cabin a lot more. You cant beat a nice getaway.

I am slowly understanding how everything works. Hopefully I will get it better this summer.

[MountainDon]

If you are using voltage as a charge indicator try to read when the system is not charging or discharging. Low use like a small lamp is not going to cause the voltage reading to be much under but many lights plus TV will. I go by the morning reading as the system hasn't been used overnight and that reading will be a pretty good indicator.

Yes, use electrolyte temperature not the air temperature. There can be a large difference. NAPA sells a hydrometer with a thermometer built in. It reads the electrolyte temp and has a small attached chart for how much to correct. It's got cheap rubber parts and lasts much better if rinsed with distilled water after use.

[dk1393]

I did some readings this weekend. The system was fully charged when I got there. It was at 14.2V.  We spent the night and in the morning it was 13.2V.
I checked the batteries . The specific gravity was 1.250-1.260 for all the cells. The electrolite temp was 39'. 

after a days use it dropped to 12.2V and I used the gen to charge them back up.

Using a load calc. chart I estimate my daily watt hrs per day to be 937. That is figuring a loss factor of .8.

All the reading were take under no load conditions

I hope this info helps. 

Thanks for your time.