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opinion:frater_secessus:self-heated_lifepo4

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opinion:frater_secessus:self-heated_lifepo4 [2023/09/03 20:24]
frater_secessus [how to think about the decision] 		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 ===== ===== 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   * some kind of warming is required before charging
      * self-heated:   easier      * self-heated:   easier
Line 18: Line 18:
   - allow incoming power only to the internal heaters, typically ~60w   - allow incoming power only to the internal heaters, typically ~60w
   - when cell temps are >freezing charging is re-enabled   - when cell temps are >freezing charging is re-enabled
-  - optional:  heating may continue until some warmer setpoint, typically ~50F when this feature is utilized+  - optional:  heating may continue until some warmer setpoint is reached, typically 40F-50F when this feature is utilized
  
 This means //warming only happens when charging is present//.  This may or may not be what you want.  This means //warming only happens when charging is present//.  This may or may not be what you want. 
Line 34: Line 34:
  
  
-With "soft" charging like a small solar setup it's a wash.  a matter of preference and cost.    +With "soft" charging like a small solar setup it's a wash.  a matter of preference and cost.   But in some specific use cases one approach might work better. 
- +
-In some specific use cases one approach might be better-suited than the other.  +
  
  
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 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. 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 ==== ==== specific configurations ====
  
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   * do I need greater configurability than the built-in heater allows?   * do I need greater configurability than the built-in heater allows?
   * do I have the interest or ability to DIY?   * do I have the interest or ability to DIY?
 +
 +===== DIYing a solution =====
 +
 +DIY solutions include warming and/or charging cutoff.  Multiple approaches can be combined into a "belt and suspenders" solution.  
 +
 +
 +==== 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.  
 +
 +
 +
 +
 +
 +
 +
 +
 +
 +==== charging cutoff ====
 +
 +
 +
 +
 +=== internal charge cutoff ===
 +
 +It is very common for BMS to have a charging cutoff feature that engages near freezing.  Some will allow you to adjust the cutoff setpoint in an app. 
 +
 +If the BMS lacks cutoff feature (or lacks configurability and you want to disable at a higher temp) see below.
 +
 +
 +=== external charge cutoff ===
 +
 +It may be possible to disable charging externally but //all charging sources// must be considered, and must have a temp probe or other way to measure bank temperature.
 +
 +Some [[electrical:solar:charge_controller|solar charge controllers]] have configurable setpoints to stop charging at a given temp.  
 +
 +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)**.
 +
  
 ===== further reading ===== ===== further reading =====
opinion/frater_secessus/self-heated_lifepo4.1693772697.txt.gz · Last modified: 2023/09/03 20:24 by frater_secessus

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