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====== LiFePO4 charging and freezing temps:  self-heated vs DIY warming approaches ======

===== TLDR =====

  * LiFePO4 cannot be charged when the cells are below ~freezing, or even discharged at substantially lower temps. 
  * therefore when cells are at ~freezing temps some kind of warming is required before charging
     * self-heated:   easier
     * externally-heated (usually DIY):  more control, more charging under certain conditions, usually cheaper
  * 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 =====

**Self-heated batteries**

  - sense ~freezing temps 
  - shut off charging((normal BMS shutoff))
  - allow incoming power only to the internal heaters, typically ~60w
  - when cell temps are >freezing charging is re-enabled
  - 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. 


**DIY warming solutions**

  - use an external temperature controller and temperature probe
  - use the lowest-wattage warming mat practical for conditions((for gentlest, most-even heating))
  - and turn the mat on/off with the controller

This means //warming happens whether or not charging is present//.  This may or may not be what you want. 
  
===== when it's a wash =====


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. 


===== 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 ====


With robust charging (alternator and/or bigger solar) there can be charging opportunities lost with self-heated batteries in freezing conditions.  

Imagine the following scenario

  * 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 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 self-heated battery shunts 60w to the heater (leaving 10A of charging unused) until the warmth equalizes up to the temp sensors.  Let's say this takes 20 minutes.  So now we have only 10mins of actual charging and the bank gets **5Ah of recharge**, **net +64Wh**.

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.

[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 ====

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."



===== how to think about the decision =====

There is no one-size-fits-all solution but outside cost considerations, the essential questions are 

  * 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?
  * 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. 


==== 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 =====

  * [[https://diysolarforum.com/threads/lifepo4-heating-pad-for-cold-temperatures.5/page-2|DIY Solar Forum thread]] on DIY warming
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