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--- opinion:frater_secessus:self-heated_lifepo4 [2024/09/12 13:45]
frater_secessus [belt and suspenders]
+++ 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 84: / Line 85: @@
 ===== DIYing a solution =====
-If your preferred battery has everything but heating you might want to [[https://diysolarforum.com/threads/lifepo4-heating-pad-for-cold-temperatures.5/page-26|DIY a heater]].  It is typically much cheaper than built-in heat and offers more control.  It typically involves:
+DIY solutions include warming and/or charging cutoff.  Multiple approaches can be combined into a "belt and suspenders" solution.
-  - a [[https://amzn.to/4d5MFii|low-wattage warming pad]], as one finds for reptile pets, animal comfort pads, etc.  [[https://amzn.to/4e1KGww|RV tank heating pads]] will work but the wattage is much higher (60w?) and might result in less even heating or overheating.
-  - a [[https://amzn.to/3zjubNp|temperature controller]]
-Both pads and controllers are available in 12vdc and 120vac.
+==== warming ====
+If your preferred battery has everything but heating you might want to [[https://diysolarforum.com/threads/lifepo4-heating-pad-for-cold-temperatures.5/page-26|DIY a heater]].  It is typically cheaper than built-in heat and offers more control.  It typically involves:
+  - a [[https://amzn.to/4d5MFii|low-wattage warming pad]], as one finds for reptile pets, animal comfort pads, etc.  [[https://amzn.to/4e1KGww|RV tank heating pads]] will work but the wattage is much higher (60w?) and might result in less even heating or overheating. Higher wattages will be required in colder areas.
+  - a [[electrical:temperature_control|temperature controller]] to turn the warming off and on at a given temp ([[https://amzn.to/3zjubNp|examples]])
+Both pads and controllers are available in 12vdc and 120vac. In moderate climates the bank can likely just sit on a mat.  Colder conditions might require pads adhered to all sides, and/or insulating the batter.
@@ Line 99: / Line 102: @@
-==== belt and suspenders ====
-Multiple solutions can be combined into a more failsafe setup.
+==== charging cutoff ====
@@ Line 119: / 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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