Before you object, hear me out.
Low temp protection is certainly important with LiFePO4. Charging below 0* C is not smart. Charging at moderately high C-rates anywhere close to 0* C is not smart. LFP battteries are not cheap, its worth protecting this investment. And in many situations it makes perfect sense to stress low temp protection.
That said, I feel there are two issues with how we generally think about/talk about low temp cutoff.
First, we rarely acknowledge context and if we do usually only in passing. I would argue that low temp cutoff is a secondary function for a BMS and one that is not relevant for many people. Obviously there is no problem erring on the side of caution, and I personally want my BMS to be capable of low temp protection, but it should be acknowledged that low temp protection is only necessary in certain contexts. When we don't acknowledge this (even if we implicitly understand it) we potentially cause newbies to spend more than they need to for features that won't benefit them and/or rule out reasonable choices because it doesn't have a feature they might not need anyways. If our batteries live inside with us, in a climate controlled space, or in a temperate or warm climate, low temp protection is non-essential. But this is not my main point/thought.
Second, and this is my main point, we assume that it should be the BMS that protects against low temperature. I would encourage you to question that assumption. Ask yourself, where does it make the most sense to manage charging.
A common question in the beginner forum is something along the lines of "why do i need a BMS and a SCC?" or "can't my BMS handle charging / can't my SCC/inverter handle HVD/LVD?)" And my usual answer is something along the lines of a BMS and an SCC have overlapping roles and capabilities, but they serve different purposes and have different specialities/roles. The fundamental and unique role and specialization of a BMS is cell level management and protection. The fundamental role of an SCC is safely and efficiently managing charging / supplying power from the PV array. The BMS has the advantage of cell level awareness but isn't well suited to control charging beyond a very basic level, whereas the SCC lacks cell level awareness but has much more sophisticated/granular capabilities for controlling charging.
So the question is, where should the low temperature disconnect reside?
In a multi-charge-source system the answer might still be the BMS, or "the BMS and..", but in a single charge source system, I would argue that it is more sensible to locate low tempcutoff protection at the device that controls charging (the SCC). The SCC is better equipped to make more meaningful use of temperature data. At a minimum, it could do what a BMS' low temp cutoff does--disconnect when temperature hits a certain threshold--but ideally it could adjust charge rate based on temperature. As discussed above there is no hard limit or magic number for temperature/c-rate, we talk about 0*C because its a nice round number, but realistically its a gradient. Charging at a super high C-rate at 5* C is worse than charging at a trickle at -2* C. Ideally there would be no binary 'low temp cutoff', charge rate would be scaled back as temps dropped. A BMS doesn't have that level of control over charging, but an SCC might (it occured to me midway through writing this that I don't know for sure that an SCC is actually capable of this, but I think that it is possible).
What are your thoughts? Am I missing something? rambling? missing something and rambling? rambling but onto something?
Low temp protection is certainly important with LiFePO4. Charging below 0* C is not smart. Charging at moderately high C-rates anywhere close to 0* C is not smart. LFP battteries are not cheap, its worth protecting this investment. And in many situations it makes perfect sense to stress low temp protection.
That said, I feel there are two issues with how we generally think about/talk about low temp cutoff.
First, we rarely acknowledge context and if we do usually only in passing. I would argue that low temp cutoff is a secondary function for a BMS and one that is not relevant for many people. Obviously there is no problem erring on the side of caution, and I personally want my BMS to be capable of low temp protection, but it should be acknowledged that low temp protection is only necessary in certain contexts. When we don't acknowledge this (even if we implicitly understand it) we potentially cause newbies to spend more than they need to for features that won't benefit them and/or rule out reasonable choices because it doesn't have a feature they might not need anyways. If our batteries live inside with us, in a climate controlled space, or in a temperate or warm climate, low temp protection is non-essential. But this is not my main point/thought.
Second, and this is my main point, we assume that it should be the BMS that protects against low temperature. I would encourage you to question that assumption. Ask yourself, where does it make the most sense to manage charging.
A common question in the beginner forum is something along the lines of "why do i need a BMS and a SCC?" or "can't my BMS handle charging / can't my SCC/inverter handle HVD/LVD?)" And my usual answer is something along the lines of a BMS and an SCC have overlapping roles and capabilities, but they serve different purposes and have different specialities/roles. The fundamental and unique role and specialization of a BMS is cell level management and protection. The fundamental role of an SCC is safely and efficiently managing charging / supplying power from the PV array. The BMS has the advantage of cell level awareness but isn't well suited to control charging beyond a very basic level, whereas the SCC lacks cell level awareness but has much more sophisticated/granular capabilities for controlling charging.
So the question is, where should the low temperature disconnect reside?
In a multi-charge-source system the answer might still be the BMS, or "the BMS and..", but in a single charge source system, I would argue that it is more sensible to locate low temp
What are your thoughts? Am I missing something? rambling? missing something and rambling? rambling but onto something?
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