LiFeP04 Batteries for Solar & BMS
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When the voltage jumps, it goes up fast, I was yanking the charger off. All I did was turn my back for a moment.Leave a comment:
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When the voltage jumps, it goes up fast, I was yanking the charger off. All I did was turn my back for a moment.Leave a comment:
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That means that every time you fully charge the battery which might be on a daily basis with an off-grid setup that you subject the weakest cell to 0.125 extra voltage. I think it is generally accepted that increase in voltage decreases the life of LFP batteries, 0.125 volts doesn't sound like much but over a ten year plus lifespan that one hopes one gets from these expensive batteries I think it could have an impact. Worst still is that you are subjecting the weakest cell to the extra stress.
Here is a discharge curve of a Calb 180 AH cell that is extremely popular in EV and some solar systems.
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If you are off-grid you have to keep your battery as close to 100% full as possible during sunny weather as you don't know how long you will have to live off your battery storage when cloudy weather occurs. What voltage would you suggest that you charge an LFP battery to under these circumstances.
No they don't all have the same capacity, the weakest cell only has 100ah and the strongest cell has 120ah. Using your example, the weakest cell is charged to 95% of its capacity while the strongest cell is only charge to 79% of its capacity. With bottom balancing you only charge the stronger cells up to the same level as the weakest cell's fully charged state, so you loose the extra 20ah of the strongest cell at the top end. With top balancing it is exactly the same but the 20ah is lost at the bottom end.
In a Bottom Balanced pack all cells have the exact same capacity at all times. They all reach 0% at the same time thus eliminating the possibility of over discharge. You do not give up capacity, just minimize risk of damage.
Top balancing does not mean that you charge them all to a high voltage >3.6 volts/cell. It means that they are all balanced at the voltage that you do charge them too. You do not run a very high risk of over discharging the cells in an off-grid system as the battery spends the vast majority of its time more than half full and as an emergency backup you have some sort of cell monitoring which will shut down your load or notify you when the weakest cell is flat.
Look I do not care if you Top or Bottom balance. What I can tell you is your controller is not capable of doing either one without modifications. It is much easier to Bottom Balance than to Top Balance. In a Top Balanced system you have to over charge the crap out of the weaker cells to get there. LFP and all Lithium batteries two cycle life killers:
1. Over Charging. In a Top Balanced system you over charge them every time you charge except for one battery, the strongest one. In a Bottom Balanced system you never over charge. You only get close to 100% on one cell only, the weakest. In a 100 AH system only 80 AH are usable. You get 80 AH out of either method. One is just less stressful and extends battery life.
2. Over Discharge is the biggest killer. Top Balanced systems greatly increase your risk of over discharging two fold.
A. It is very possible and 90% of all failures result is the stronger cells driving weaker cells into reverse polarity. Weaker cells reach 0%, but dumb equipment does not know that because the pack voltage still is above Low Voltage Cut-Off and keeps on going because the stronger cells still have capacity and keep the pack voltage above LV Cut-Off . In a Bottom Balanced systems cell voltages are equal at the bottom and Low Voltage Cut off monitoring pack voltages disconnects just before the voltage falls of the cliff, thus protecting the the cells from over discharge.
B Top Balanced system use Vampire Boards aka Bleeder Boards to Top Balance. Bleeder Boards aka Vampire Boards failure mode is Shorted more times than Open. When shorted completely discharges and destroys a cell or cells. Bottom Balanced systems do not have Vampire Boards to fail.Leave a comment:
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Using the EQ setting to do the balance is a great idea.
I tried to bottom balance @ 2.6 on 4 cells. As I recharged them I found that once the voltage gets above 3.460 one cell would climb very fast while the rest were 3.2 -3.3 v.
The other problem is that even if you do shut the controller down with some sort of cell monitoring system that at shutoff the weakest cell will be at 3.425 volts and the stronger cell might be at 3.3 volts. That means that every time you fully charge the battery which might be on a daily basis with an off-grid setup that you subject the weakest cell to 0.125 extra voltage. I think it is generally accepted that increase in voltage decreases the life of LFP batteries, 0.125 volts doesn't sound like much but over a ten year plus lifespan that one hopes one gets from these expensive batteries I think it could have an impact. Worst still is that you are subjecting the weakest cell to the extra stress.
SimonLeave a comment:
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You are missing the big picture and still stuck using lead acid battery techniques. Keeping LFP batteries at or near 100% shortens their lives, not maximize as you mistakenly said. That is a lead acid world thing. Keeping LFP at 100% are taking them their daily accelerates their aging. Keeping LFP cell voltages equal are not important at the top as it does not tell you anything about them or has no benefit.
If you are off-grid you have to keep your battery as close to 100% full as possible during sunny weather as you don't know how long you will have to live off your battery storage when cloudy weather occurs. What voltage would you suggest that you charge an LFP battery to under these circumstances.
Again you logic is flawed and stuck in a Pb world. In a Bottom Balance system every cell has the same capacity, and the system is charged up referencing the weakest cell in the string. In a Top Balanced system the capacity of the pack is determined by the weakest cell. Example in a 100 AH system your AH capacity, cell values are going to range from 100 to 120 AH. The weakest is 100 AH. Bottom Balance them at at any point every cell has the same capacity. You charge them up using the weakest cell. Say 95 AH.
If you Top Balance not only do you stress every cell every time you charge, you run a very high risk of over discharging and destroying the weaker cells. In a Bottom Balanced system you eliminate those risk.
Regular basis? What are you talking about? You do it one time initially at the beginning.
SimonLeave a comment:
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Might be worth changing the 'Rebulk Voltage' in the Advanced menu to 13.0 volts to get the FM60 to go back to 'Bulk' charge at a more realistic battery SOC, 12 volts (3.0 volts/cell) is nearly fully discharged for an LFP battery. Could you tell me what your 'Absorbing' and 'Float' voltages are from the Charger Screen and also 'Absorb Time Limits' and 'Absorb End Amps' from the 'Advanced Menu', also what are the 'Equilization' settings.
I tried to bottom balance @ 2.6 on 4 cells. As I recharged them I found that once the voltage gets above 3.460 one cell would climb very fast while the rest were 3.2 -3.3 v. As I watched them the high cell went to 3.785 and the lowest was still at 3.4 v. That let me know that the safest high charge voltage would have to be 3.4 and even that would be to high to be safe. So if I used 3.3v that would make my bulk at 13.2 v and to me that is way to low and giving up to much capacity, so I decided to top balance. I have no idea how temperature effects voltage, I am watching them close until I get a handle on them. I am trusting the BMS, it's better than losing a cell. I have not cycled them yet to see what the lowest safe voltage is yet. It won't be that low, since the lowest it can go is 12.7 before it will recharge off another system.
Real life trumps theoretical BS.Leave a comment:
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Might be worth changing the 'Rebulk Voltage' in the Advanced menu to 13.0 volts to get the FM60 to go back to 'Bulk' charge at a more realistic battery SOC, 12 volts (3.0 volts/cell) is nearly fully discharged for an LFP battery. Could you tell me what your 'Absorbing' and 'Float' voltages are from the Charger Screen and also 'Absorb Time Limits' and 'Absorb End Amps' from the 'Advanced Menu', also what are the 'Equilization' settings.Leave a comment:
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Firstly, In off-grid systems you want to charge the battery to as close to 100% when the weather is sunny while also maximising the lifespan of the battery, a tricky compromise. Usually this means fully charging the battery on a daily basis whenever there is sunshine. As has been mentioned in previous posts, bottom balancing a battery will mean that the individual cell voltages will be unequal every time the battery is fully charged which in this situation will mean on a daily or if not daily a very regular basis. I would think that this imbalance could lead to some cells ageing faster than others over the hopefully 10 year plus life of the battery.
Secondly, off-grid batteries relying on solar power to charge them spend most of their time above 50%SOC as you want to have as much power stored as possible for cloudy weather. It is only very rarely that a battery in a well managed off-grid systems gets to anything near 0% SOC where the protection given by bottom balancing has any impact. If you have cell level monitoring of the battery, imbalance in cell SOC at the bottom end is not an issue.
If you Top Balance not only do you stress every cell every time you charge, you run a very high risk of over discharging and destroying the weaker cells. In a Bottom Balanced system you eliminate those risk.
Regular basis? What are you talking about? You do it one time initially at the beginning.Leave a comment:
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The CC is a FM60 It's like the system just doesn't want to reset because the voltage is to high and it wants to stay in Float. Sometimes integrated systems aren't all that they are cranked to be. It could be something in the algorithm that is fixed. I said 12.5, it's hard to tell exactly, it could be 12.7. I'll have to watch it again to see. Just getting everything calibrated has been a pain within the voltage range of LiFep04 batteries.
I would check that it is going through this cycle each night. I couldn't work out if the battery voltage needs to go below the 'Rebulk Voltage' at night before the FM60 will go back to 'Bulk' mode when it wakes up, maybe someone else can help here.
SimonLeave a comment:
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My MPPT charge controller is homemade so I can program it however I like. I have programmed it to start a new bulk charge at the start of each new day or if the battery voltage goes below 3.25 volts/cell (13 volts for a twelve volt system). What is the make and model of your charge controller?
SimonLeave a comment:
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Do you have any issues with your CC triggering a full Bulk, Absorb, Float cycle until the voltage drops below 12.5. Sometimes my CC will not automatically re-trigger and it will limit the number of output amps and stops at the float setting unless the voltage drops below 12.5. I can set a bulk cycle, but that requires manual intervention.
SimonLeave a comment:
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From my research and experience with an LFP battery that has been part of my off-grid power system for over two years now I am not so sure that bottom balancing LFP cells in off-grid systems and probably most power systems that rely on solar energy as their primary source of power is as good or as practicable as top balancing.
Firstly, In off-grid systems you want to charge the battery to as close to 100% when the weather is sunny while also maximising the lifespan of the battery, a tricky compromise. Usually this means fully charging the battery on a daily basis whenever there is sunshine. As has been mentioned in previous posts, bottom balancing a battery will mean that the individual cell voltages will be unequal every time the battery is fully charged which in this situation will mean on a daily or if not daily a very regular basis. I would think that this imbalance could lead to some cells ageing faster than others over the hopefully 10 year plus life of the battery.
Secondly, off-grid batteries relying on solar power to charge them spend most of their time above 50%SOC as you want to have as much power stored as possible for cloudy weather. It is only very rarely that a battery in a well managed off-grid systems gets to anything near 0% SOC where the protection given by bottom balancing has any impact. If you have cell level monitoring of the battery, imbalance in cell SOC at the bottom end is not an issue.
Thirdly, doing a bottom balance on a regular basis in an off-grid system is difficult if the battery is your main source of power, especially if you do not have an alternate power supply. I think it also subjects the battery to unnecessary stress.
I agree that for EV use bottom balancing cells has many advantages but the charge/discharge profiles, expectations and requirements for an EV battery and for a battery in an off-grid system are very different.
Simon
Do you have any issues with your CC triggering a full Bulk, Absorb, Float cycle until the voltage drops below 12.5. Sometimes my CC will not automatically re-trigger and it will limit the number of output amps and stops at the float setting unless the voltage drops below 12.5. I can set a bulk cycle, but that requires manual intervention.Leave a comment:
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From my research and experience with an LFP battery that has been part of my off-grid power system for over two years now I am not so sure that bottom balancing LFP cells in off-grid systems and probably most power systems that rely on solar energy as their primary source of power is as good or as practicable as top balancing.
Firstly, In off-grid systems you want to charge the battery to as close to 100% when the weather is sunny while also maximising the lifespan of the battery, a tricky compromise. Usually this means fully charging the battery on a daily basis whenever there is sunshine. As has been mentioned in previous posts, bottom balancing a battery will mean that the individual cell voltages will be unequal every time the battery is fully charged which in this situation will mean on a daily or if not daily a very regular basis. I would think that this imbalance could lead to some cells ageing faster than others over the hopefully 10 year plus life of the battery.
Secondly, off-grid batteries relying on solar power to charge them spend most of their time above 50%SOC as you want to have as much power stored as possible for cloudy weather. It is only very rarely that a battery in a well managed off-grid systems gets to anything near 0% SOC where the protection given by bottom balancing has any impact. If you have cell level monitoring of the battery, imbalance in cell SOC at the bottom end is not an issue.
Thirdly, doing a bottom balance on a regular basis in an off-grid system is difficult if the battery is your main source of power, especially if you do not have an alternate power supply. I think it also subjects the battery to unnecessary stress.
I agree that for EV use bottom balancing cells has many advantages but the charge/discharge profiles, expectations and requirements for an EV battery and for a battery in an off-grid system are very different.
SimonLeave a comment:
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