How not to go about selecting batteries!
Ok, so I admit, I have a tendency to go cheap and improvise until I know that doesnt work. Well, on batteries I have learned the most expensive batteries are cheap (used) batteries. This knowledge cost me $2000.00. Here is my journey and lessons learned.
Pricing batteries (sorta) apples to apples
The most important lesson I have learned is to not simply look at the price of a battery. It really pays to do your homework here. Before I get into the nuts and bolts of comparing apples to apples let me reiterate something you will find in ANY alternative energy battery posting on the net. DO NOT BUY CAR STARTING BATTERIES. Click HERE for a google search that will tell you all about why this is a waste.
With that out of the way, lets look at what is important in pricing a battery. First off is the total storage capacity of the battery. This is the (Kilo) Watt hours the battery will hold when new. To get this figure multiply the voltage of the battery and the rated amp hours of the battery.
Example:
12 volt battery with 255 amp/hour capacity AGM type. Capacity 3.060 Kwh. Cost $339.00
12 volt battery 100 amp/hour capacity LiFePO4. Capacity 1.2 Kwh. Cost $342.00
Think the choice is obvious? Not by a long shot! To give us a more fair comparison, lets break that down some more. How much does 1 Kwh of storage cost us. For that we do the following calculation:
(Price / Capacity) / 1000
With this the AGM comes in at $110.78 and the Lithium at $285.00. The choice is still looking obvious in favor of the AGM to the consumer. Now we add some "art" to this.
The AGM battery cannot be discharged on a regular basis below 50% of capacity without shortening the life of the battery. AGM in general is good for 2000 cycles before they should be replaced. With this in mind we have roughly 3.060*.50 (50%) or 1.53 Kwh available to us for approximately 5.4 years. With that in mind our battery will have cost us about $63.00 per year.
Now we turn to the "expensive" battery, Lithium. A Lithium battery cannot be discharged 80% of capacity without shortening its life span. Lithium is also good for at least 4000 cycles before you need to think about replacing them. So here we have 80% usable battery or roughly 1.2*.80 (80%) or .96 Kwh available for us to use for approximately 10.9 years. This brings the cost of our battery to $32.00 a year.
Now the last piece of the puzzle is the storage capacity. Maybe you noted that for our $63.00 / year we had 1.53 Kwh available and for our $32.00 we only had .96 Kwh available. Because of this, and to keep things simple, lets buy 2 lithium batteries to our 1 AGM.
We have now doubled our capacity to 2.4Kwh (total with 1.92 usable Kwh) of battery. We still have the 10.9 year lifespan but we have doubled the cost. With this in mind our yearly cost is now $62.00/year.
We come out $1.00 per year BUT we have 0.39 Kwh/day or 142 Kwh/year available for us to use (if we use each battery to its full capacity daily).
I have intentionally not calculated industrial batteries. The reason for this is while the overall cost MIGHT be competitive, they weigh in around 1000 pounds per pack. This makes them pretty difficult to move around. Additionally, I do not consider them safe for residential or RV indoor use and would not recommend that (been there done that).
My thoughts on used batteries
First off dont buy used! You have no real idea how much life is left in the battery. When I bought my panels, they guy selling them also had some beautiful looking AGM type batteries. They were Interstate 12MQ2400 12 volt rated at 94 amp/hour each. Cost $100.00 each (I bought 10). This came out to $88.65 per Kwh. The problem is that Ive had the full pack now 2 years. 2 of the batteries have a shorted cell and had to be pulled out of the pack. The 8 remaining batteries can now barely get me through 24 hours with a 1 Kw/day load without needing recharged. I estimate them to be at about 25% of their new capacity.
I dont consider my mistake to be all that earth shattering. The system was built as an experiment / learning tool and not because I absolutely need it. All of the mumbo jumbo I just wrote may not mean a lot to the reader. Because of that I will just say this, if your system is critical, DO NOT BUY USED OR EVEN "RECONDITIONED" BATTERIES. It is a complete roll of the dice.
Where do I go from here?
What all of this has basically come down to is what do I do now that I will have to replace my battery pack soon. I used to be a firm believer in lead acid technology (AGM is a specially constructed lead acid battery!). I had turned my back on Lithiums as being too expensive and too high tech.
A couple things changed that idea. First among those was actually crunching the numbers. Next I came across this video. The video outlines how to create a litium battery pack from individual 3.2 volt cells. From there I turned to Amazon and China to price cells. Basically without being "conservative" I can do lithiums for approximately $250.00 per Kwh or less using individual cells.
Basically what it has come down to is two choices. You can either spend less now but more over time, or you can spend more now and less over time. As for me the lifetime cost and the weight factor, has finally pushed me into the lithium battery made up of individual cells court.
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