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--- electrical:12v:drop-in_lifepo4 [2024/02/11 12:22]
frater_secessus [myth: you can't use a combiner to charge batteries of different chemistries]
+++ electrical:12v:drop-in_lifepo4 [2024/04/12 23:02] (current)
frater_secessus [bluetooth]
@@ Line 45: / Line 45: @@
   * the flat voltage curve makes gauging SoC by voltage extremely challenging, and battery "gauges" designed for lead chemistry batteries will not work. An amp-counting [[electrical:12v:battery_monitor|battery monitor]] will be more useful with Li.
+===== sizing the bank =====
+Other than cost, there is little downside to having a larger LiFePO4 bank.(([[electrical:12v:directcharginglfp|direct-charging from alternator]] would require some thought))  Upsides include:
+  * greater [[opinion:solar:sizing.walkthrough#days_of_autonomy|autonomy]] when charging is absent or limited due to poor solar conditions
+  * supporting larger discharge current - [[electrical:12v:battery_capacity|0.5C]] is a common max spec.  So a 200Ah LFP could support 100A loads((0.5 x 200Ah)) while a 100Ah could only support 50A.
+  * safely accepting larger charging currents when charging time is limited.  Charging specs vary between 0.2C to 0.5C;  [[electrical:12v:drop-in_lifepo4#an_example_of_long_life|0.4C]] seems to be a good balance between longevity and charging time.  200Ah could accept 80A from the alternator while 100Ah could only accept 40A.
@@ Line 317: / Line 323: @@
   * **Equalize duration** - zero, or as low as the controller will allow.  Will make no practical difference when Veq is set to Vabs.
   * **Temperature compensation** - Lead needs different charging voltages at different temperatures but Li does not.  Change setting to **0**mV/cell.((lead defaults are something like -4mV/cell))
+Note:  if you are willing to babysit, even a single-voltage power supply would work.  Stop charging when the voltage hits your desired setpoint.
 ==== myth:  you shouldn't Float lithium ====
@@ Line 360: / Line 368: @@
 === testing your isolator with Li ===
-see [[electrical:12v:directcharginglfp#assessing_your_setup_for_direct_alternator_charging|this section]]
+see [[electrical:12v:directcharginglfp#assessing_your_own_setup_for_direct_alternator_charging|this section]]
 ==== but that Victron video ====
@@ Line 508: / Line 516: @@
 Charging is disabled for a few different reasons:  temperature extremes, high cell voltage, overcurrent.
-People who camp in cold weather may want to select a battery that has "low temperature cut-off", which disables charging near freezing.((Lithium is permanently damaged by charging below freezing)).  The belt-and-suspenders solution is both low temperature cutoff and a way to keep the batteries warm.  Some pricier batteries have internal heating, or you may [[https://diysolarforum.com/threads/lifepo4-heating-pad-for-cold-temperatures.5/page-26|DIY a heater]].
 Note that the BMS overcurrent protection kicks in only at the limit, typically 1[[electrical:12v:battery_capacity|C]] (100A for a 100Ah battery).  See the section on the [[:#an_approach_to_greater_longevity|benefits of even gentler charging]].
+=== low temperature charging cutoff ===
+People who camp in cold weather may want to select a battery that has //low temperature cutoff//, which disables charging near freezing.((Lithium is permanently damaged by charging below freezing)).  The belt-and-suspenders solution is both low temperature cutoff and a way to keep the batteries warm.  Some pricier batteries have internal heating, or you may [[https://diysolarforum.com/threads/lifepo4-heating-pad-for-cold-temperatures.5/page-26|DIY a heater]].
+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.
+Note that some chargers have built-in low-temp charging cutoff already.
@@ Line 570: / Line 589: @@
 Some batteries have BMS with bluetooth or other forms of connectivity.  With these you would be able to see more information, like individual cell balance, statistics, setting and the status of any protections.
+Having visibility into the state of the BMS/cells might be useful for tech geeks, but often muddies the waters for non-technical customers.  Lack of BT is not a dealbreaker.

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