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--- opinion:frater_secessus:self-heated_lifepo4 [2024/09/12 13:58]
frater_secessus [warming]
+++ opinion:frater_secessus:self-heated_lifepo4 [2025/06/24 18:52] (current)
frater_secessus [backing into a minimum temperature setpoint]
@@ Line 5: / Line 5: @@
 ===== TLDR =====
-  * LiFePO4 cannot be charged when the cells are below ~freezing.
+  * LiFePO4 cannot be charged when the cells are below ~freezing, or discharged well below freezing
   * some kind of warming is required before charging
      * self-heated:   easier
@@ Line 65: / Line 65: @@
 If one was charging by alternator only and only driving to/from work while stealthing in freezing weather it's conceivable the self-heated batt would get //never get charged at all// because it wouldn't have time to get out of the heating phase.
+[Note:  those who want to charge vigorously might want to [[https://mouse.mousetrap.net/blog/2024-11-23-backchannel--charging-rates-and-self-heating-batteries-.html#temperature-and-charge-rate|keep the battery bank quite warm]]]
 ==== specific configurations ====
@@ Line 125: / Line 126: @@
 Chargers which lack cutoff but can be switched OFF/ON can be controlled by a signal from a thermal switch (nonconfigurable) or temp controller (configurable).  In this approach the switch or probe is attached to the bank.  Below a given temperature the control circuit is disrupted and the charger turns off.
+==== backing into a minimum temperature setpoint ====
+There are two pieces of information needed here:  a datasheet for your cells, and a realistic assessment of your maximum charging current ([[electrical:12v:battery_capacity|C rate]]) under best possible conditions.
+=== what the datasheet tells us ===
+The cells inside [[electrical:12v:drop-in_lifepo4|your LFP]] are made of individual cells.  These cells are usually made by a big cell manufacturer, who publish datasheets related to those cells.  You may have to google to find out the actual cells inside your battery, or watch a teardown video.  Then you can google to find the datasheet for those cells.
+The datasheet contains much information but the part that concerns us here is the acceptable charge rate (C) at different cell((not ambient))  temperatures.  Here is the relevant info from secessus' REPT 280 cell datasheet:
+^    ^   ^
+| Temp (T)   | Charge rate (C) |
+| <0°C | No charging allowed      |
+| 0°C  T <10°C | 0.1C charge to 80% SOC   |
+| 10°C  T <15°C | 0.2C   |
+| 15°C  T <45°C | 0.5C   |
+| 45°C  T <55°C | 0.2C  |
+| >55°C | No charging allowed   |
+=== C rate at full charging power ===
+In secessus' use case his charging can  max out at ~55A((and usually much lower)) with solar and alternator combined.  55A/280Ah = ~**0.2C**.
+=== putting it all together ===
+Because his charge rate can reach **0.2C** he chose a warming setpoint of **10°C (50°F)**.

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