Could add a grid charger. Kind of misses the whole point, but a quick fix to keep from ruining the batteries.  7.6v many times will ruin them.

to run your sunfrost first dig out the specs on it find out how many watt hours per day it draws and then size your battery bank 6x that size (3 days of running with no charging without discharging the batts below 50% capacity) + your other loads like lighting ect. off the top of my head i am thinking 10 t105's or 8 L16's would about do it.

take the watts/hours per day the sunfrost uses and add to it your dayly watt hour usage for other stuff. now multiply that number by 3  that is how many watts of solar your gonad need + a good wind Gen for low sun days.

"I guess my question should be how many of you out there have these sunfrost fridges and are running them on 12 volts?"

We own two SunFrost units. One is a 12 cf frig/freezer and the other a 4 cf frige/freezer. We stated out with the small one, then later went to the larger one moving the smaller unit out to a guest cabin. Both of them are 12v.

These things draw very little power, but my guess is one of three items if not all of them may be your problem.

(1) You are not providing your batteries enough power to run your SunFrost.

(2) You do not have a large enough battery bank, you need more batteries.

(3) Your battery bank is pretty well shot and needs replaced.

From past experiences with our own system, it's a guess that it could be all of the above. Also too small of wire, or corroded connections can cause your Frig to see lower voltages than normal, something to definately check out as well.

We finally out grew our 12v system. Our 12v PV array produces 90 amps at 12 volts ( about 1000 watts) on a sunny day ! So we have a lot of copper 0000 size cable (size of garden hose), and even 2ga welding cable routes around the house in specially designed accessible conducts !

I like you, started out with modest loads, but as the size of the system grew things got out of hand. If your frig will run on 12 or 24vdc you have the latest, the earlier ones like ours will not convert up to run on 24vdc without replacing the entire compressor.

Somebody jump in and correct me here, but if I recall, just jumping from 12v to 24v will allow you to run 4 times smaller wire sizes etc.

After dealing with copper cable almost the size of garden hose, we decided to keep this 12v system, but we decided our next system would be a higher voltage. I opted for 120vdc because a lot of brush motors (shop tools) would run directly off of it w/o an inverter. 120vdc is lethal voltages and being dc makes it even more so. you can strike a arc with it and hold the arc to about a space of one inch or so! I quickly found that unless your shop tool is rated as ac/dc you fry the switch the first time you try to use it!

What I didn't think about, is that 120v inverters are only available as commercial units, and not considered to be sold to your average consumer, So they are priced out of sight ($6,000 to $7,000 ) for starters ! I'll never be able to afford one, and if I did, probably not be able to afford to have it serviced if it would quite working !

So there are draw backs as well as advantages to higher voltages. Depending on how large of a system you have in the future, 24v and 48 volts are nice modest options.

The problem is, once you start a system, you tend to purchase so many devices that will only run on those voltages, that you end up with such a large investment in all the appliances, lights etc. That you can't afford to upgrade to a higher voltage system. you are very fortunate that you have a frig that will let you bump up to 24v just in case your system should outgrow your expectations.

We fly-in fish in Canada. Propane 'fridge, 40 gallon water heater, lighting, and stove. Supplied from a 100# tank (I think. About 5' hi, 16" dia.). Most years, they are partly used when we get there, and we don't have to change tanks for the week.
Amazing to me how much you get from a pound of propane.

Most inverters are designed to shut off at about 10-10.5 volts to save your batteries. Running down to only 9V will damage them faster, less life. If your hitting 9V cutoff alot, then you are not producing as much power as you are using. Going to a higher voltage won't help, you still would not be producing enough.
 So what I think is you need something to produce that power and keep the batteries well above 11v anyway. If what you have now aren't reaching full charge then adding more won't help either, you'll just have more dead ones.

 Either that or you produce alot of power in a short time, but don't have enough batteries to store it and then when the production stops right away you use up what you have stored and need more batteries to store more. But only if your reaching full charge on what you already have.

Last thing is if your batteries are shot already, then they need replaced. I have some junk ones I am playing with I can charge to 13V but letting them sit over night they are dead in the morning, and that's with no load. SO If I were using them, they would look fully charged sometimes but might last an hour at most with a small load on them.
 When I take the charger off them I can watch a digital meter drop about .01V per second, and that's nothing connected but the meter.

Some advantages to higher volts yes. Like smaller wires, less loss to resistance. But if running an inverter for power, then use the same 120VAC wires and run them to the inverter near the batteries instead of 12vdc so far to the inverter. But if running DC then you have no choice but to run battery power over larger wires I geuss.

I'll be doing both eventually.
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nothing to lose