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--- opinion:frater_secessus:self-heated_lifepo4 [2024/11/24 21:28]
frater_secessus [robust charging] added blog post
+++ opinion:frater_secessus:self-heated_lifepo4 [2026/10/03 20:07] (current)
frater_secessus [Self-heated vs DIY warming]
@@ Line 1: / Line 1: @@
 DRAFT
-====== Self-heated vs DIY warming ======
+====== LiFePO4 charging and freezing temps:  self-heated vs DIY warming approaches ======
 ===== TLDR =====
-  * LiFePO4 cannot be charged when the cells are below ~freezing, or discharged well below freezing
+  * LiFePO4 cannot be charged when the cells are below ~freezing, or even discharged at substantially lower temps.
-  * some kind of warming is required before charging
+  * therefore when cells are at ~freezing temps some kind of warming is required before charging
      * self-heated:   easier
-     * DIY:  more control, more charging under certain conditions, usually cheaper
+     * externally-heated (usually DIY):  more control, more charging under certain conditions, usually cheaper
-  * in practice, warming is combined with other forms of protection:  BMS low temp charging shutoff, solar or DC-DC low temp charging cutoff, etc.(("belt and suspenders" approach))
+  * in practice, warming is combined with other forms of protection:  [[electrical:12v:drop-in_lifepo4#bms_features_and_specs|BMS low temp cutoffs]], solar or DC-DC low temp charging cutoff, etc.(("belt and suspenders" approach))
 ===== how the approaches work =====
@@ Line 126: / 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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