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Home Forums Technical Discussion Off-Grid Solar energy storage with LiFePO4

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    Dacian
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    I moved Off Grid about two years ago. I was looking in to batteries for storage about 3 to 4 years ago and of course 99% of people use Lead Acid so I was looking in to those.

    I was checking the specifications of Trojan and Rolls since those where the largest with deep cycle battery for solar and I was trigging to compare them to find the best price per energy stored over their life time.

    My calculation was something like this (Cost of battery / Energy stored over life time)

    That energy stored over life time was something like this Capacity[Ah] x Voltage x Nr of Cycles x Depth of Discharge [%] x Charge/Discharge Efficiency [%]

    Back then 4 years ago the price / kWh for some of the best Lead Acid was around 80 cent to 1$/kWh and that seemed realy high for me not to mention this was a bit of a theoretic number best on ideal conditions so I was expecting hinger real life numbers.

    I started to look in to other batteries like NiFe and lithium but still quite expensive to run. Just by luck I seen an article about A123 System at that time and they claimed 3000 cycles at 100% DOD (Depth of Discharge). That was hugely impressive but there was no way to get those batteries.

    Soon after the small 18650 size cells from A123 where on eBay so I decide to by a few and experiment was not sure if they where genuine or fake. I still do not know but if they where fake they where great quality fakes since they where meting the spec in regards to internal resistance and capacity.

    I was contemplating building a large pack out of those small 1.1Ah 3.2V cells but that was quite some work to connect so many of them to build a large pack.

    Not long after that there where some larger cells made in China with the same LiFePO4 chemistry and larger 20Ah A123 cells started to be available in China again even if they where supposedly made in USA.

    Since I had good results with the smaller A123 I took the ricks and ordered 10 of the larger 20Ah cells from China.

    I build a 24V pack with 8 of them and made some tests (again even more impressive than the small cells) I was only able to test the capacity that was about 18Ah from my measurements but I was not going down to 2.8V/ cell just down to 3V.

    Since I needed about 24V 100Ah for my house I decided to go with the large 100Ah cells and not deal with connecting a lot of those smaller 20Ah cells they where not in a hard case and had tabs that where hard to connect.

    I opted for LiFeMnPO4 from GBS 100Ah cells and got a supplier in Canada (was more convenient than to order from china do to weight, shipping cost and import tax).

    So this is what I use now for my OffGrid house. I have 3x 240W PV panels that 2.5kWh,  24V, 100Ah battery made of 8 GBS cells. This allows me an average monthly consumption of 80kWh worst case 60kWh/month in December and I do not realy use more than 90kWh in summer.

    I know this seems small for most of you but I started with energy efficiency and try to reduce as much as possible the energy that I need then sized the battery and PV to met this.

    Anyway forgot to mention. I made a amortization cost for the entire sytem including battery, cables, breakers, charge controller (I needed to design and build that part), PV panels, and inverter.

    I divided all this cost + the cost of another battery after 12 years to a total of 25 years with that average monthly energy use of 80kWh. and the cost/kWh was just 16 cent as good if not better than grid depending on location.

    Is true that I did all the installation so this is just for the cost of equipment not installation but even so is way better than with Lead Acid.

    I will be sure of this numbers once I do a test to see what is the capacity loss on battery after 2 years if is less than 5% that I expect then this 16 cent/kWh is real for the equipment amortization.

    Dose anyone here uses LiFePO4 battery for energy storage? Or something else that can compare ?

    If you are interested I can give some more details but this type of installation is at this point only good for those that have good knowledge about electricity and electronics.

    Here is a nice 7 day production and consumption energy graph I made for my house.

    [caption id="" align="alignnone" width="680"]7 day energy graph 7 day energy production consumption for my OffGrid house.[/caption]

     

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