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--- opinion:frater_secessus:self-heated_lifepo4 [2023/09/03 16:02]
frater_secessus [how to think about the decision]
+++ opinion:frater_secessus:self-heated_lifepo4 [2023/10/17 15:13] (current)
frater_secessus [how the approaches work]
@@ Line 5: / Line 5: @@
 ===== TLDR =====
-  * self-heated:   easier
+  * LiFePO4 cannot be charged when the cells are below ~freezing.
-  * DIY:  more control, more charging under certain conditions, usually cheaper
+  * some kind of warming is required before charging
+     * self-heated:   easier
+     * DIY:  more control, more charging under certain conditions, usually cheaper
+  * in practice, warming is combined with other forms of protection:  BMS low temp charging shutoff, 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.
+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.
-===== perfect storm for self-heated batteries =====
+===== 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
+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**.
@@ Line 31: / Line 63: @@
 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.
+==== 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 =====
@@ Line 44: / Line 78: @@
   * 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?
+===== 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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