Looking for recommendations

97TJ

I'm going to upgrade my small off grid system that is 10 years old.  Currently a 12 volt system with 600 watts of panels but I purchased 200 watts more so I will have 800.  PWM controller and Crown lead acid batteries.  The Crown batteries have finally given up the ghost so I want to go lithium with a mppt controller.  So, with 800 watts of panels, what size battery bank of lithium would be appropriate.  I had (4) 6volt 230 amp in series parallel.  Could I purchase a couple of lithium batteries then add another in about 6 months?  I'm thinking maybe a 60 mppt controller with (3) 100 AH batteries in parallel. 

Photowhit

What is your final goal?

If just 800 watts of array (panels), you could stay at 12 volts, but if you want to grow more you might consider going to a higher system voltage and go to an all-in-one inverter MPPTcharger.

Another thing to consider is do you live near this system or is this for an off grid system that will be exposed to below freezing temperatures?

97TJ

800 should be enough for what I have.  Small fridge, lights and a TV.  I have been running the lead acid set up for 10 years and really just going to lithium.  Can I add a additional 100 AH battery in 6 months without issues.  Seems like when building a lead acid system you were told to buy your batteries at the same time because the older batteries would be degraded. Will a 800 watt array be sufficient for three 100 AH batteries in parallel?

706jim

97TJ said:

 Will a 800 watt array be sufficient for three 100 AH batteries in parallel?

Yes I think so.

Wheelman55

You can add another lithium in 6 months no problem. 

BB.

Without looking at your location, hours of sun, Watt*Hours/Amp*Hours of loads specifically... I like to use the 5% / 10% rule of thumb as a sanity check... For a "weekend system" (mostly summer usage, etc.)... 5%+ minimum rate of charge. For a full time off grid system, a 10%+ minimum rate of charge:

• 300 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 282 Watt minimum array (weekend)
• 300 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 565 Watt minimum array (full time)

This is based on charging needs for FLA batteries... But seems to work well with any off grid system.

You Li Ion batteries will work "better" than the FLA/AGM batteries--And since you had a long life with the original lead batteries--The Li Ion (with proper care/watching minimum/maximum bank voltages) should be very happy too.

-Bill

97TJ

And will 60 amp 12 volt mppt charge controller be adequate.  I've seen some 60 amp 12 volt charge controllers rated for a 750 watt array.

BB.

Assuming a "good quality" MPPT charge controller, you can "over panel" them and they will work safely and reliably (as always, need good cooling/airflow to keep electronics happy).

An 800 Watt array on an MPPT controller typically (rare cool/clear day near solar noon) would output:

• 800 Watt array * 0.77 panel+controller deratings * 1/14.5 volts average charging = 42.5 Amps charging a 12 volt battery bank.

PWM controllers should not be "over paneled" (always refer to manual)... So they will be designed to manage a "worst case" design (i.e., the panels should never supply more that rated current)... And "the powers that be" use an 80% rating for solar power:

• 60 Amps * 18.0 Vmp typical array * 0.80 NEC solar derating = 864 Watt array "max" PWM charge controller

For a 60 MPPT controller, an "optimum" "over paneling" would be (clipping would be "rare" on "perfect" solar noon days):

• 60 amps * 14.5 volts * 1/0.77 panel+controller derating = 1,130 Watt array "optimum" over paneling for MPPT charge controller

-Bill

97TJ

Thanks!