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opinion:frater_secessus:self-heated_lifepo4 [2023/09/03 16:17]
frater_secessus [when it's a wash] 		opinion:frater_secessus:self-heated_lifepo4 [2024/11/24 16:28] (current)
frater_secessus [robust charging] added blog post
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.   
  
 +===== when self-heated might be preferable =====
 +
 +When you want a packaged solution.
 +
 +If you run your bank very low habitually((stop doing that!)) and the warming load is enough to trigger BMS discharge shutdown.  This problem does not occur with self-heated because heating only runs when charging is present. 
 +
 +
 +===== when DIY warming might be preferable =====
 +
 +==== robust charging ====
  
-===== perfect storm for self-heated batteries ===== 
  
 With robust charging (alternator and/or bigger solar) there can be charging opportunities lost with self-heated batteries in freezing conditions.   With robust charging (alternator and/or bigger solar) there can be charging opportunities lost with self-heated batteries in freezing conditions.  
  
-Imagine +Imagine the following scenario
  
-* freezing overnights followed by a half-hour drive in the morning with a 30A DC-DC.  +  * freezing overnights followed by a half-hour drive in the morning with a 30A DC-DC.  
-* a battery with warming pad that consumed 60Wh overnight to stay above freezing, vs +  * a battery with warming pad that consumed 60Wh overnight to stay above freezing, vs 
-* a self-heating battery that is below freezing in the morning+  * a self-heating battery that is below freezing in the morning
  
 The externally warmed bank accepts current immediately and takes **15Ah of recharge**.  So after recouping the 60Wh used overnight it's **net +132Wh**. The externally warmed bank accepts current immediately and takes **15Ah of recharge**.  So after recouping the 60Wh used overnight it's **net +132Wh**.
Line 55: Line 63:
 With 50A charging it's 260Wh net vs 107Wh net. . With 50A charging it's 260Wh net vs 107Wh net. .
  
-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 ====
  
-===== perfect storm for DIY warming =====+You may want more control over the temperature setpoints.   
 + 
 +Note from Secessus:  "since reading [[https://diysolarforum.com/threads/why-you-cannot-charge-lifepo4-below-0-degrees-celsius.2912/page-2#post-199774|this cell spec sheet data]] I keep my bank at toasty 50deg F."
  
  
-You run your bank very low habitually (don't do that) and the warming load is enough to trigger BMS discharge shutdown.   
  
 ===== how to think about the decision ===== ===== how to think about the decision =====
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   * how much charging would I miss out on because the bank is taking only 60w to run the heaters instead of taking everything I can throw at it?; and   * how much charging would I miss out on because the bank is taking only 60w to run the heaters instead of taking everything I can throw at it?; and
-   * how does this compare to the Wh required to keep the bank above freezing when no charging is present? +  * how does this compare to the Wh required to keep the bank above freezing when no charging is present? 
-   * 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? 
 + 
 +===== 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.  
 + 
  
 ===== further reading ===== ===== further reading =====
opinion/frater_secessus/self-heated_lifepo4.1693772228.txt.gz · Last modified: 2023/09/03 16:17 by frater_secessus

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