Should I use Trojan or Interstate batteries?

Ken:
Well its a 48 volt system, and L16's are 6 volt, so yes that would require 8 batteries.

I guess the main point of my question were:
1) Are the Trojan batteries worth the extra cost? Rechecking its $55 a piece extra. Will they last longer, be more reliable, will I get more cycles than I would with the Interstate batteries?
2) Will my 1300 (eventually to be 2000 watt) array be enough to maintain L16's? or should I use a smaller battery like a T145? Keep in mind I'm located in Central NY, a good day for me this time of year is 2Kwh, and zero production days aren't uncommon.

System setup is handled. I only have select non-essential loads tied to the outback power panel. I'm not trying to get off the grid, the solar just supplements my electric supply. Between the solar and conservation I have cut my grid dependence in half, imagine if everyone could do or did that...
 
Right now I have the inverter set to run solar until the batteries fall to 50% when it will then seamlessly switch the loads to the grid, once the batteries reach at least 75% then it automatically toggles back to the batteries. And note that I do not charge the batteries from the grid, the loads just switch to grid power and stay there until the solar brings the batteries back up.
Once I get new proper batteries I will probley bump up the set points to a shallower depth of discharge so to take it extra easy on the batteries and get as long a life span as I can from them.

So if anyone has any opinion or data on Trojan vs. Interstate bang for the buck it would be appreciated.
Thank you.

Dave:
Yep, I've got my homepower subscription, recent loyal subscriber. I was looking at the battery buyers guide in the Oct/Nov 2008 issue, and it failed to mention Interstate, but I do see now that it lists US Battery. On the surface it looks like the Interstate / US Battery has better specs as far as number of cycles.
But once I get home tonight I'll be sure to pour over the host of info you supplied.
Thank you so much.

Thanks for some more good info.

The Trojan dealer I spoke with also carried US Battery, but they didn't have eight in L16 or a golf car size, and since they had to buy them by the truck load they didn't know when they would have at least 8. Plus they claimed the Trojan's were a much better battery. But who knows, that could just be them steering me towards what they had.

At this point I'm leaning towards the Trojan's, partly for the convenience of a local dealer, vs. having to borrow a truck and blowing half a day going to get the Interstates.
But I don't want to go with 16 batteries, one, I only built a battery box big enough for 8, and two, that would be quite a change to come up with.

I got the system on-line mid September, and at that time I only had 1000 watts worth of panels. Even then I was pulling close to 4KWh a day, and that was with a small shading issue, a pine tree that will be removed soon.

Thanks again for all the info.

Dave:

Your question is a good one.
I think that before going into it, I want to say that the numbers we calculate are only as good as the data we have to start with.  We are making estimates, and looking for ballpark answers.  While we might come up with 21% DOD, that answer is often more precise than the data we started with. We often find that the actual depth of discharge does not match the original calculations.  We just hope the real numbers come within a few percent of the estimate.  If it isn’t, the problem will generally be in the load estimates that we are given.

Having said that:
If somehow your load matched the generation perfectly, The battery would not be in use as long as that condition was maintained. The impact of that would result in a DOD that is less than calculated. E.g.  we calc’ed for 20%, but you used 5% worth as it was generated, so the actual DOD would be 15%.  But that almost never happens.

The battery is usually charging or discharging.  If our system is generating exactly 800W, what are the odds of having exactly 800W worth of load during the day?  Usually, we will be under that figure, so the battery is charging, in spite of some of the 800W being used. If we go over the 800W (perhaps using the stove for lunch) we will experience a short minor discharge cycle in the middle of what is normally a “charging period”.  My experience is that the minor discharge cycle either doesn’t count, or you could say that it is somehow averaged into the numbers.  The figure that is going to count the most, is the greatest depth of discharge that occurred during the 24 hour period, or since the last major charge cycle.

And here, we are really talking average depth of discharge over many days.  If you run 30 days at a 20% DOD, and then have one to 50%. What does that work out to ?  20.96774% average depth of discharge over 31 days.  (How is that for an overly precise number ?) The bottom line is that you averaged somewhere in the ballpark of 21% over the 31 day period.  (And the 20 percent figure for the first 30 days was also probably bouncing around in say the 18-22% range).

A couple of percentage points really doesn’t matter much. What we do know is that when your “about 20% DOD” drops to “about 30% DOD” your battery life will be shorter. And it get shorter for about every 10% it drops.

I hope that answered your question. 

Ken