We've entered a new era in energy storage for automotive applications, which used to be burdened by the old, heavy and low cycle life lead acid batteries by the creation of the Lithium Iron Phosphate batteries (LifePO4). The coming of these new lightweight and high energy-density cells has not been without problems. The cells to suffer when they are charged to an excess voltage or discharged excessively.
There are two streams of thought amongst LifePO4 battery users:
There are also folks who have no clue, but are persuaded by one of the two main groups at some point. I struggled long before deciding to try my first large lithium pack with a BMS, thinking that I could take it off and maybe sell it if it didn't live up to the hype. There are so any accounts on each side of the debate that a novice who needs a LiFePO4 battery pack is in a dire pickle. If there are two equally sized sides proclaiming opposite requirements, how can you justify significantly increasing the cost of your project when there is no sound advice? Should you take the advice of the manufacturer or other retailers who definitely stand to profit from the suggestion? It's worth noting that there is also a side that profits from advising against the use of a BMS: EVTV sells products to support the bottom-balancing of cells, which conveniently costs about the same as a BMS.
I have researched these questions and examined the motives of each side, attempting to be as unbiased as possible. I also purchased several packs from different manufacturers. The only necessary test is to use these new cells without a BMS and see what happens. I'd like to point out that I am not well equipped to do any optimal tests, but I do have an understanding of the limitations of the cells, which I acquired from reading everything I can find and killing a few cells along my way. I am really only able to test the cells in real world situations, as traction packs or auxiliary power supplies.
I have noticed through using my cells that each manufacturer's cells behave a bit differently (as could be expected). The difference to me really seemed to be quality and consistency of the cells; Thunder-Sky cells showed much more than 5% difference in capacity in cells purchased in the same batch. GBS cells appeared to be more closely matched than Thunder-Sky, but still not to an acceptable level. CALB seem to be the most consistently matched cells- I've been using 36 of their 40 Ah cell in series for my VW Beetle traction pack without a BMS and I see very little imbalancing. The Thunder-Sky cells were so bad that I could no longer use them as traction batteries, instead placing the in service as UPS backup batteries and other low current applications where they seem to work much better.
All of the LiFePO4 cells that I have used seem to become more out of balance as your current demand increases, which can be attributed to the internal resistance. The cells may be the same capacity at low current draw, but those having higher internal resistance will dissipate some of their capacity as heat as load is increased to that of traction pack levels (I am pulling up to 300 battery amps in my EV). The problem I can thus see is inconsistencies of internal cell resistance, which is much less of a problem in my CALB cells, but it's still noticeable.
My observations lead me to believe that the cause of the internal resistance inconsistency is a lack of proper quality control in the manufacturing process, which is we hobbyists are experiencing in differing degrees . I don't doubt that the manufacturers are trying to make the cells better, though I also think that we could do without BMSs if they would tighten up their operations, even at the risk of costlier cells. If y theory is correct, the problem for hobbyists lies in not being able to predict which batch of cells is going to work and which ones won't. A BMS will only accommodate so much of a difference in cell performance, so the worst cells I have used (the Thunder-Sky) are so far apart that a BMS won't help. Instead, they are doomed to service in UPS power supplies (leaving me frustrated at the huge cost for poor performance). Please don't take this article to mean that all Thunder-Sky cells are bad, I am just observing that mine did not perform well.
I bet some people are wondering, in response to my theory, why manufacturers are using BMSs if they do have better quality control for the cells manufactured for their cars. I think the answer is that they have to employ a certain amount of redundancies into their vehicles to reach compliance with vehicle standards and avoid lawsuits.
Even though these cells are wrought with problems, they are still much better than lead acid and I now consider the best LifePO4 cells to be the only choice for use as traction batteries.
Notes:
Before using any of the cells that I bought, I did either top or bottom balance the prior to use. All I can say about bottom is that it's a bad idea for most people, as it's far too easy to kill cells. Top balancing is the most practical way if you have some means of protecting the cells from over charge (either a smart charger that cuts off at high voltage cutoff or an intelligent Ah meter from lightobject.com or an actual BMS).
Over discharging cells does not always kill them, but it does reduce their performance in terms of capacity and cycle life. I discharged a few of my GBS 20Ah cells to .5 volts each and all but 2 of them are still in service and have no glaring problems since. I charge and discharge them at currents lower than 1C.
There are two streams of thought amongst LifePO4 battery users:
- Users that say a Battery Management System (BMS) is necessary
- Those who say a BMS is not necessary
There are also folks who have no clue, but are persuaded by one of the two main groups at some point. I struggled long before deciding to try my first large lithium pack with a BMS, thinking that I could take it off and maybe sell it if it didn't live up to the hype. There are so any accounts on each side of the debate that a novice who needs a LiFePO4 battery pack is in a dire pickle. If there are two equally sized sides proclaiming opposite requirements, how can you justify significantly increasing the cost of your project when there is no sound advice? Should you take the advice of the manufacturer or other retailers who definitely stand to profit from the suggestion? It's worth noting that there is also a side that profits from advising against the use of a BMS: EVTV sells products to support the bottom-balancing of cells, which conveniently costs about the same as a BMS.
I have researched these questions and examined the motives of each side, attempting to be as unbiased as possible. I also purchased several packs from different manufacturers. The only necessary test is to use these new cells without a BMS and see what happens. I'd like to point out that I am not well equipped to do any optimal tests, but I do have an understanding of the limitations of the cells, which I acquired from reading everything I can find and killing a few cells along my way. I am really only able to test the cells in real world situations, as traction packs or auxiliary power supplies.
I have noticed through using my cells that each manufacturer's cells behave a bit differently (as could be expected). The difference to me really seemed to be quality and consistency of the cells; Thunder-Sky cells showed much more than 5% difference in capacity in cells purchased in the same batch. GBS cells appeared to be more closely matched than Thunder-Sky, but still not to an acceptable level. CALB seem to be the most consistently matched cells- I've been using 36 of their 40 Ah cell in series for my VW Beetle traction pack without a BMS and I see very little imbalancing. The Thunder-Sky cells were so bad that I could no longer use them as traction batteries, instead placing the in service as UPS backup batteries and other low current applications where they seem to work much better.
All of the LiFePO4 cells that I have used seem to become more out of balance as your current demand increases, which can be attributed to the internal resistance. The cells may be the same capacity at low current draw, but those having higher internal resistance will dissipate some of their capacity as heat as load is increased to that of traction pack levels (I am pulling up to 300 battery amps in my EV). The problem I can thus see is inconsistencies of internal cell resistance, which is much less of a problem in my CALB cells, but it's still noticeable.
My observations lead me to believe that the cause of the internal resistance inconsistency is a lack of proper quality control in the manufacturing process, which is we hobbyists are experiencing in differing degrees . I don't doubt that the manufacturers are trying to make the cells better, though I also think that we could do without BMSs if they would tighten up their operations, even at the risk of costlier cells. If y theory is correct, the problem for hobbyists lies in not being able to predict which batch of cells is going to work and which ones won't. A BMS will only accommodate so much of a difference in cell performance, so the worst cells I have used (the Thunder-Sky) are so far apart that a BMS won't help. Instead, they are doomed to service in UPS power supplies (leaving me frustrated at the huge cost for poor performance). Please don't take this article to mean that all Thunder-Sky cells are bad, I am just observing that mine did not perform well.
I bet some people are wondering, in response to my theory, why manufacturers are using BMSs if they do have better quality control for the cells manufactured for their cars. I think the answer is that they have to employ a certain amount of redundancies into their vehicles to reach compliance with vehicle standards and avoid lawsuits.
Even though these cells are wrought with problems, they are still much better than lead acid and I now consider the best LifePO4 cells to be the only choice for use as traction batteries.
Notes:
Before using any of the cells that I bought, I did either top or bottom balance the prior to use. All I can say about bottom is that it's a bad idea for most people, as it's far too easy to kill cells. Top balancing is the most practical way if you have some means of protecting the cells from over charge (either a smart charger that cuts off at high voltage cutoff or an intelligent Ah meter from lightobject.com or an actual BMS).
Over discharging cells does not always kill them, but it does reduce their performance in terms of capacity and cycle life. I discharged a few of my GBS 20Ah cells to .5 volts each and all but 2 of them are still in service and have no glaring problems since. I charge and discharge them at currents lower than 1C.
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