Inexpensive AGM Battery

A little math (to make sure I understand your setup):

• 34 amp*hours * 1/48 hour period * 24 hours per day = 17 AH @ 12 volts
• 17 AH per day * 2 days * 1/0.50 maximum discharge = 68 Amp*Hour @ 12 volt battery bank minimum recommended.

Guessing your solar panel is:

• 140 Watt * 17.5 volts Vmp = 8.0 Amps Imp

If you assume an average pretty bad gloomy winter day of 1.8 hours of sun per day:

• 17 AH per day * 1/8.0 Amp panel = 2.1 Hours of sun per day minimum to "break even"

I would suggest that you double the panels to two 140 Watt panels in parallel on a PWM controller--Just to give you a larger margin of safety--Unless this is installed in some relatively sunny region.

Battery bank should be charged at 5% to 13% rate of charge. Say you have a 200 AH @ 12 volt battery:

• 200 AH * 14.4 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 187 Watt array minimum (good for seasonal/weekend use)
• 200 AH * 14.4 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 374 Watt array nominal (daily off grid usage)
• 200 AH * 14.4 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 486 Watt array "cost effective" maximum

So--A 200 AH battery seems to be a bit of over kill--But it really depends on how local conditions (latitude, local weather conditions, etc.). Notice the size of the solar array not only depends on daily loads and hours of sun per day, it also depends on the Amp*Hour rating of the battery bank. Larger battery banks should have larger solar array to properly/fully recharge.

AGM batteries are much more efficient than flooded cell batteries--So, you can get away with a smaller battery bank than the "nominal" solar array I have suggested above... But, I would still suggest that you run at least 2x 140 Watt panels in parallel. This will help keep the AGM battery bank happy (solar panels have never been cheaper, and AGM batteries have never been more expensive).

AGM batteries (even very good ones) still tend not to last as long as as decent flooded cell batteries with good maintenance.

But, for remote/unattended/automated systems, you run a risk of the batteries being over discharged in bad weather (or load failure--say pump "stays on instead of cycling"... Lead Acid batteries are not usually easy to estimate state of charge based on battery bank voltage--But for this system, you do have a choice... Let the system run until the batteries die, or get a PWM controller (or voltage based switch) that turns off at ~11.5 to 12.0 volts, and turns back on at ~13.6 volts (when battery has been recharged some).

One thing that lead acid (and AGM) batteries do not like is being discharged below 20% state of charge (and in general, try to avoid discharging below 50% state of charge). So, having a large solar array and some sort of low voltage/low state of charge cutoff will help extend the life of the batteries themselves.

-Bill