In my opinion, 0.05C at those temperatures is not going to cause any issues. The protection for low temperature cut-off however should be part of the BMS, not the charge controller, so you can always see if you can change setting there.
Thanks for the reply. My Batteries are cheap Chinese batteries (no low temp cut off) . They are all the exact same and wired to a bus bar with all the cables from the bus bar to the batteries the same length, so they all charge at the same rates.
Yes. There are multiple threads on exactly that subject…OK, I uderstand well theory over why we should not attempt to charge LPO batteries in cold conditions. But in practical life, how do van owners manage this in winter time? It's cold for few consecutive days, it's sunny, PV produce great amount of power which is being lost? Heating baterries would help?
Look in this section of the forum.OK, I uderstand well theory over why we should not attempt to charge LPO batteries in cold conditions. But in practical life, how do van owners manage this in winter time? It's cold for few consecutive days, it's sunny, PV produce great amount of power which is being lost? Heating baterries would help?
The info starts off "With the complex material system used in LIBs".This study summed it up nicely:
Source: https://www.sciencedirect.com/science/article/pii/S1002007118307536
...I glazed over...Soooo far past me...... but, does this mean that the claims of the -20F chargeable LiFePO4 cells could be true?
If the battery cells are at 32 deg F, you should not charge them. They can be held above the ambient temp of the air in the vicinity by adding heat to the batteries as noted. The ambient temp in the area might be -20, but the cells can be held above freezing with heated plates inserted between the cells/batteriess in the battery bank.In fact, in the cold winter in the north, the temperature is likely to be below 0 ℃. In order to solve this low temperature problem, some manufacturers provide two solutions:
One is to replace the conventional electrolyte with low-temperature electrolyte, so that the LiFePO4 battery can be used at low temperature.
The other is to add a heater plate between each battery cell and formulate relevant procedures in the battery BMS, so as to ensure that the overall temperature of the heated LiFePO4 battery is above 0 ℃.
So it is also possible to charge/discharge LiFePO4 Batteries at -20F, mainly depending on the scheme provided by the manufacturer.
Reading through this thread to get a grasp on LFP thermal issues, I notice the unfortunate fact that almost every data point mentioned talks about 'the temperature of the battery', ignoring that what they are reading is actually the temperature of a sensor and rarely mentioning where it is placed.[emphasis mine]
due to thermal mass of battery and placement of temperature sensor resulting in the temperature sensor leading rather than lagging the cell material temperature?
less than 1C = 1CAmpere?
Can anyone here explain the internal thermodynamics of these prismatic cells better?
Here's a graph I posted over in my build thread that does explicitly show the temperature difference at different points on the cells.Reading through this thread to get a grasp on LFP thermal issues, I notice the unfortunate fact that almost every data point mentioned talks about 'the temperature of the battery', ignoring that what they are reading is actually the temperature of a sensor and rarely mentioning where it is placed.
Looking at rocket mass heaters, it takes about an hour for the heat from a high temperature (~200-300*F) gas to conduct through an inch of the solid mass it flows through. Not trying to draw a direct comparison between current flow and heated gas, rather looking at how long it takes heat to conduct through a mass. Can anyone here explain the internal thermodynamics of these prismatic cells better?
My Seplos BMS has a function to heat the battery while charging at low temperatures, but not a function so first heat and then charge. The only option is heating and charging at the same time, it seems to use about 3A for the heating pads and limit the current going to the battery to 6A (which for a 280Ah battery means 0.02C). Is it acceptable to charge the battery at such a small rate in subfreezing temperatures (while it heats up)?
No, with the Seplos BMS there is no way to heat the battery before charging; it can only activate the pads during charge, not during standby or discharge. I have to chose between charging at a slow rate and heat up the battery at the same time or no charging at all and wait for warmer weatherCan you set the temperature at which the BMS tries to heat the battery and then a separate parameter to turn off all charging when the temperature falls below that value? Are you sure that the BMS is limiting the charge amps? With most BMS, charging is either on or off, not throttled. I'm not familiar with the Seplos BMS.
I took the approach that I didn't want my batteries to charge at all below about 35°F. My heating pads are configured to keep the battery between 35°F and 45°F. This does not rely on the BMS. Within the BMS the low temp charge cutoff is set for about 32°F. This would kick in only if the warming pads were offline or unable to keep up. The charging devices (solar charge controller, inverter/charger) have similar settings to not charge below 32°F.
That seems like a pretty odd way for Seplos to manage the heating, but I'll take your word for it.No, with the Seplos BMS there is no way to heat the battery before charging; it can only activate the pads during charge, not during standby or discharge. I have to chose between charging at a slow rate and heat up the battery at the same time or no charging at all and wait for warmer weather
The Seplos bms can throttle charging current; despite requesting 3kW charging, the inverter is only sensing 9A to the bms, then the Bms sends 6A to the cells and 3A to the pads
It has:Does it have low-temperature cutoff and a different temperature for heating?
You might set it to start heating at 40F, cut off at 35F it temperatures keep dropping.
Of course, need a temperature difference it won't pass through during the night.
Use insulation.