Well if you want to save time to get to the bottom line, Lithium is not ready for Solar with the exception of a very small scale systems mainly for toys. But if you want to know more and the challenges read on.
Face the facts lithium batteries are very expensive and do not have the cycle life of lead acid batteries. Not only that are fairly dangerous with high fire risk involved which require some very complex Batty Management Systems to monitor the batteries for charge and thermal management. Coupled with short cycle life, high expense of batteries and BMS they cannot compete with lead acid batteries. Well that is the way it use to be and still holds true today with one exception of the LiFeP04 (aka LFP) lithium family of batteries that deserves a second look and holds some promise in the future.
LFP or lithium Iron Phosphate uses a iron phosphate crystal on the cathode of lithium batteries. I won't go into all the details because it is not important to understand. What is important to know is it makes the LFP battery fairly stable or safe and does not require expensive Active BMS and can use rather inexpensive Passive BMS and no thermal management is required. More on that in a minute. The other important factors are its lower manufacturing cost and cycle life. Manufactures claim up to 2000 cycles which has yet to be proven, but if holds true is about on par with good quality Lead Acid batteries. The other advantage is significantly lower Internal Resistance which means efficient charge/discharge eliminating Peukert Effects to the point it can be ignored, and enables very high charge rates up to 1C and discharge rates of continuous of 3C and burst up to 10C for short periods of times measured in seconds. That is huge for the DIY Electric Vehicle Markets. Another huge benefit is the battery can operate without loss of cycle life between 20 to 100% SOC where lead acid is limited to 50 to 100%. They can also operate in PSOC ranges which are no good for Lead Acid Batteries. All PSOC means is the battery dies not require it to be fully recharged after every use to extend battery life. It can spend days below 100% days down to 20%.
Now the down sides. They are still sensitive to over discharge. Go below 20% and they become a Brick. All it takes is one mistake and they are toast, very expensive toast. They are not as sensitive to over charges as the other Lithium chemistry in that they can take some over charge without bursting into flames, but they can still be turned into bricks from over charges. So with that said some form of BMS is required to protect your investment, most notable protection from over charging which can be done with Passive BMS. Cost is still problematic for LFP. Chi-Com made LFP's are in the range of 40 to 50 cents per watt hour compared to 18 to 22-cents for good 5 year FLA battery and about the same as AGM. Some of that cost can be offset because you can use a smaller LFP AH than FLA. It takes roughly a 70 AH LFP to equal a 100 AH FLA usable capacity. Having said that the Chi-Com LFP quality is questionable at this price range. There are other quality Nano-LFP batteries out there made by companies like A123 but will cost you in excess of $1/wh or 5 times what a FLA will cost you with no real added benefit.
OK now that is out of the way, can you use LFP today. Well the answer is sort of on a very small scale toy size. Why do I say that? Because there is only a single manufacture that makes a Solar Charge Controller for Lithium batteries. That company is GenSun and they only offer 3 LFP controllers. One is made for a 12 volt Kyocera 140 watt battery solar panel (10 amps), 60 watt 12 volt battery panel (5 amps), and a 36/48 volt battery for golf carts using up to a panel with 8 amps under 40 volts Vmp. Not much to work with yet. Why you ask? Its the market folks as there is no demand yet for Lithium solar charge controllers. That is just the way it is for now.
But what if you just cannot wait for a manufacture to make lithium compatible controllers. Well the answer is you had better know WTF is going on, otherwise stop reading because this is above your head and you would be playing with fire. So read at your own risk. LFP can be charged using either CC or CV method. I will not define what that means because if you do not know, you should not be reading this and playing with fire. A LFP cell is considered fully charged using a CV method of 3.65 volts and the current tapers down to .05C. In a solar system there is no reason to charge faster than C/4 although it can go as high as 1C with thermal management using a BMS. Armed with that you should be able to figure things out by modifying a conventional charge controller. With 3.65 voltage would mean you need a simple Float Type charger set to 14.6 volts if you want to go to 100% SOC. You wil have to figure out a way to shut off the charger when current trickles down to .05C.
I also know one person is going to argue this but to get to 100% SOC you are going to have to use at least a Passive BMS. A Passive BMS is really nothing more than a simple circuit board attached to each cell that turns on when the battery reaches 3.65 volts and puts a BLEEDER RESISTOR across the cell to bypass and discharge the cell and remains turned on until the voltage falls below a set level. The cost of such a board runs about $12 each and will accommodate a 50 to 120 AH cell. Passive BMS is a different animal that is used with EV's where it takes power from higher energy cells and transfer it to lower energy cells. much more expensive and complicated.
The other PITA ass with using Passive BMS, or even if you do not use a BMS is you must initially balance all the cells and periodically balance the cells. After several cycles the balance grows to the point that forces you to balance them and you run the risk of destroying the batteries if you do not monitor them.. Remember I said just one over discharge below 20% SOC will turn it into a BRICK. Without a BMS and monitor you run a very high risk of doing just that.
OK I think that is enough for now.
Face the facts lithium batteries are very expensive and do not have the cycle life of lead acid batteries. Not only that are fairly dangerous with high fire risk involved which require some very complex Batty Management Systems to monitor the batteries for charge and thermal management. Coupled with short cycle life, high expense of batteries and BMS they cannot compete with lead acid batteries. Well that is the way it use to be and still holds true today with one exception of the LiFeP04 (aka LFP) lithium family of batteries that deserves a second look and holds some promise in the future.
LFP or lithium Iron Phosphate uses a iron phosphate crystal on the cathode of lithium batteries. I won't go into all the details because it is not important to understand. What is important to know is it makes the LFP battery fairly stable or safe and does not require expensive Active BMS and can use rather inexpensive Passive BMS and no thermal management is required. More on that in a minute. The other important factors are its lower manufacturing cost and cycle life. Manufactures claim up to 2000 cycles which has yet to be proven, but if holds true is about on par with good quality Lead Acid batteries. The other advantage is significantly lower Internal Resistance which means efficient charge/discharge eliminating Peukert Effects to the point it can be ignored, and enables very high charge rates up to 1C and discharge rates of continuous of 3C and burst up to 10C for short periods of times measured in seconds. That is huge for the DIY Electric Vehicle Markets. Another huge benefit is the battery can operate without loss of cycle life between 20 to 100% SOC where lead acid is limited to 50 to 100%. They can also operate in PSOC ranges which are no good for Lead Acid Batteries. All PSOC means is the battery dies not require it to be fully recharged after every use to extend battery life. It can spend days below 100% days down to 20%.
Now the down sides. They are still sensitive to over discharge. Go below 20% and they become a Brick. All it takes is one mistake and they are toast, very expensive toast. They are not as sensitive to over charges as the other Lithium chemistry in that they can take some over charge without bursting into flames, but they can still be turned into bricks from over charges. So with that said some form of BMS is required to protect your investment, most notable protection from over charging which can be done with Passive BMS. Cost is still problematic for LFP. Chi-Com made LFP's are in the range of 40 to 50 cents per watt hour compared to 18 to 22-cents for good 5 year FLA battery and about the same as AGM. Some of that cost can be offset because you can use a smaller LFP AH than FLA. It takes roughly a 70 AH LFP to equal a 100 AH FLA usable capacity. Having said that the Chi-Com LFP quality is questionable at this price range. There are other quality Nano-LFP batteries out there made by companies like A123 but will cost you in excess of $1/wh or 5 times what a FLA will cost you with no real added benefit.
OK now that is out of the way, can you use LFP today. Well the answer is sort of on a very small scale toy size. Why do I say that? Because there is only a single manufacture that makes a Solar Charge Controller for Lithium batteries. That company is GenSun and they only offer 3 LFP controllers. One is made for a 12 volt Kyocera 140 watt battery solar panel (10 amps), 60 watt 12 volt battery panel (5 amps), and a 36/48 volt battery for golf carts using up to a panel with 8 amps under 40 volts Vmp. Not much to work with yet. Why you ask? Its the market folks as there is no demand yet for Lithium solar charge controllers. That is just the way it is for now.
But what if you just cannot wait for a manufacture to make lithium compatible controllers. Well the answer is you had better know WTF is going on, otherwise stop reading because this is above your head and you would be playing with fire. So read at your own risk. LFP can be charged using either CC or CV method. I will not define what that means because if you do not know, you should not be reading this and playing with fire. A LFP cell is considered fully charged using a CV method of 3.65 volts and the current tapers down to .05C. In a solar system there is no reason to charge faster than C/4 although it can go as high as 1C with thermal management using a BMS. Armed with that you should be able to figure things out by modifying a conventional charge controller. With 3.65 voltage would mean you need a simple Float Type charger set to 14.6 volts if you want to go to 100% SOC. You wil have to figure out a way to shut off the charger when current trickles down to .05C.
I also know one person is going to argue this but to get to 100% SOC you are going to have to use at least a Passive BMS. A Passive BMS is really nothing more than a simple circuit board attached to each cell that turns on when the battery reaches 3.65 volts and puts a BLEEDER RESISTOR across the cell to bypass and discharge the cell and remains turned on until the voltage falls below a set level. The cost of such a board runs about $12 each and will accommodate a 50 to 120 AH cell. Passive BMS is a different animal that is used with EV's where it takes power from higher energy cells and transfer it to lower energy cells. much more expensive and complicated.
The other PITA ass with using Passive BMS, or even if you do not use a BMS is you must initially balance all the cells and periodically balance the cells. After several cycles the balance grows to the point that forces you to balance them and you run the risk of destroying the batteries if you do not monitor them.. Remember I said just one over discharge below 20% SOC will turn it into a BRICK. Without a BMS and monitor you run a very high risk of doing just that.
OK I think that is enough for now.
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