RVwiki for vanfolk and others who live in vehicles

User Tools

Site Tools

Trace:
opinion:frater_secessus:self-heated_lifepo4

Differences

This shows you the differences between two versions of the page.

Link to this comparison view

Both sides previous revision Previous revision
Next revision 		Previous revision
opinion:frater_secessus:self-heated_lifepo4 [2024/09/12 13:35]
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 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. 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 ====
  
Line 84: Line 85:
 ===== DIYing a solution ===== ===== 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. 
  
-=== solar charge controller setpoints === 
  
  
 +==== charging cutoff ====
  
-=== automatic intervention === 
  
-It may be possible to add low temperature cutoff to a battery whose BMS lacks that feature.  It requires the charging source that can be [[electrical:12v:alternator#disabling_alternator_charging|disabled on demand]].  In this approach the we are reversing the logic so that **the default state of the charging source is disabled** and it is only **enabled when battery temps are warm enough** to safely charge.   
  
->> For belt-and-suspenders you could add a $10 12v temp controller or NO thermal switch in series [with the disabling method].   Stick the probe (or switch) on the bank and only complete the [disabling ] circuit when the ignition is on **and** measured temp >0C or >2C or whatever you prefer. + 
 +=== 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_lifepo4.1726148113.txt.gz · Last modified: 2024/09/12 13:35 by frater_secessus

Page Tools

affilliate disclosure | the RVwiki runs on a virtual server from NetCup

Except where otherwise noted, content on this wiki is licensed under the following license: CC Attribution-Noncommercial-Share Alike 4.0 International
CC Attribution-Noncommercial-Share Alike 4.0 International Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki