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I’d love to see a serious dive into the effect of low-temperature charging. Obviously, 32F has no specific significance, since there is no water in the battery. The data I have seen suggests that low temperature lithium plating is an important factor at C=3 and even at C=1 (recharge 20-60 minutes–not with solar, no way). But at C=0.05 or less, the effect becomes far less important, perhaps no more than lithium charging at C=1 at 20C. EVs, cell phones, and power tools recharge at C=1 or more. It’s convenient. Boats charge slowly.
I see very little research posted at the low recharge rates relevant to sailboat solar systems. For example, 300 W with 450 Ah of batteries is C=0.051. My trimaran charges at C=0.02 on a good day and C=0.01 in the winter, when it matters (low sun angle).
So what is the effect of lithium plating at very low charging rates, for example, C=0.01? Not that it matters, since all of the BMSs that I have seem cut-off charging at about 38 F anyway. But it’s good to know that some charging at relatively low temperatures will be harmless.
I’m thinking the “never below freezing” is a broad rule to save us from fast charging damage.
Intuitively, that has to be correct but I cannot find a single research paper that has actually tested that theory.
I cannot post links here. Google “lithium temperature charging rate” and you will find several studies that compare cycle life with charging rate at a range of temperatures. Reseachgate and others. It’s not clear, in some cases, which chemistry they are testing. However, what does seem clear, is that side reactions and plating are much less important at low charging rates. The reason plating takes place at low temperatures is that ion diffusion slows, and so logically, if you charge very slowly, the lithium ions have time to diffuse rather than plate out.
As I said, this would require a deep dive. The reason it is relevant is that, compared to most users, sailors charge slowly. C=<0.03
Of course, the BMS will block sub-freezing charging anyway. It's just good to know if charging close to the low limit is harmless, for those who leave panels hooked up all year. I think the answer is less.
Interesting, thanks. I guess you are thinking of solar charging during the winter, presumably in the off-season, when you are not around. A key issue is whether the BMS simply stops allowing charging at freezing temperatures (until such time as the ambient temperature rises) or fully shuts down the battery. If the second of the two, solar alone is unlikely to ‘wake up’ a shut down LFP (as the solar chargers require power from the battery to operate). If you are away, off the boat for an extended time, you could return to find the LFP discharged. In that case, there is a strong argument for getting the LFP off the boat for the off season, if you dont have access to shore power. Happily I don’t have those problems in the tropics.
My battery (LiTime) only blocks charging at 32F; I think this is normal. I have not read of a battery BMS that triggers a shutdown below freezing, and certainly, that would be stupid engineering. Sub-freezing temperatures are not an extrodenary event in much of the country, rather they are an expected part of the service environment. I have been through 2 winters with air temperatures as low as the teens and frozen water bottles in the cabin. I do not know the minimum, because I’m not there on cold nights!
Removing the battery is impractical for me because it runs a sump pump and because I sail all year, skipping only snow and a week of two of frozen water.
The wake-up problem is interesting. I did have a BMS shut down once (I stupidly left some high-draw items on for a week). I arrived, and the battery voltage was low, the solar considerably higher, but no charging. I shut everything off, swung the boom aside so that solar would have full sun, and in a few minutes it woke up and began charging. The rate of voltage increase vs. solar input over the next 8 hours matched the capacity of the battery for the temperature. When I returned 5 days later, the battery was at 100%. I was concerned about lack of umph for wake-up, but it was not a problem (C=0.03). I’m sure that can vary. It was 100% my mistake. The battery appears to have recovered 100%, based on output on a long cloudy day. Thank goodness for BMS.
In the absence of a communications port and a common communications channel (eg a more sophisticated system) an internal BMS really only has one tool available to it, which is to shut itself down, when detecting what regards as a fault condition. So, it is unclear to me how your LFP can operate in that way – if your LFP manufacturer’s manual explains how that is achieved, please post the summary.
However, there may be one of two other things going on:
An internal BMS in an LFP can not distinguish berween separate charge and discharge currents – it would only see net flow, at its terminals. If you were serving a parasitic load at the same time as you are charging, and your charging current is less than that, the BMS wouid read the net flow as it being discharged, not charged.
Again, in the absence of an external shunt device with integrated comms, an internal BMS cannot read current accurately. As a proxy for a current reading, it measures the volt drop across an internal resister. That simple approach is good enough for detecting large fault currents but notoriously inaccurate for small currents (that is why the SOC data, provided through Bluetooth to a smart phone app is often quite inaccurate). With the very small charging currents you were describing, it may therefore not even be detecting that current.
If anyone is aware of BMSs that trigger a complete shutdown when there is solar charging applied at sub-freezing temperatures, that would be worth sharing. I think a great many sailors, even if the boat is on the hard, leave the solar on through the winter. Probably more do than do not.