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--- electrical:12v:drop-in_lifepo4 [2025/04/19 16:50]
frater_secessus [myth: you have to charge Li to 100%] reset link
+++ electrical:12v:drop-in_lifepo4 [2025/08/28 15:17] (current)
frater_secessus [choosing a drop-in LFP battery] fleshed out
@@ Line 43: / Line 43: @@
 ===== drawbacks of lithium =====
-  * Li is relatively expensive up front
+  * Li has (historically at least) been relatively expensive up front.  Prices started plummeting somewhere around 2024.
   * Li cells need a [[#bms_functions|BMS]] to protect them from damage.((some DIYers run Li "barefoot" (without a BMS) )) For example, Li can be damaged by overvoltage, undervoltage, charging below freezing (32F), etc.  Some batteries have low-temp cutoff and/or internal heating to address the cold-charging limitation.  Most Drop-in Lithium batteries will have a BMS integrated into them, but raw cells do not.
   * Li can be **damaged** by long duration at full charge or high voltage, or high ambient temperatures
@@ Line 55: / Line 55: @@
 ===== choosing a drop-in LFP battery =====
-There are many factors here which only you will be able to assess.
+There are many factors here which only you will be able to assess. Basic due diligence will involve:
+  * reading and understanding the specs (see below)
+  * searching for youtube teardowns and testing on that particular model
+Shortcut:  Will Prowse has [[https://www.mobile-solarpower.com/lithium-batteries.html|a list of recommended, tested batteries]] which can be trusted.
+**Heads up**:  do not use lowest-possible-price as the main criterion for a battery unless there was a test/teardown available for it.
 ==== voltage ====
@@ Line 89: / Line 97: @@
 LFP cells can be damaged by overvoltage.  Charging is typically disabled when one or more cells rises to ~3.65v (~14.6vv for the pack)
+=== low temperature (~freezing) charge cutoff ===
+[not present in all BMS]
+LFP cells are damaged by charging when the cells are at ~freezing temperatures.((discharging too, but the limits are much colder))
+Lack of **low temperature cutoff** is not necessarily a deal-breaker.  Maybe you live in a hot location.  Maybe your chargers have low temperature cutoff.  Maybe you externally warm your battery.
+[[opinion:frater_secessus:self-heated_lifepo4|externally warmed vs. self-heated LiFePO4]]
 === high temperature charge/discharge cutoff ===
@@ Line 107: / Line 128: @@
-=== low temperature (~freezing) charge cutoff ===
-[not present in all BMS]
-LFP cells are damaged by charging when the cells are at ~freezing temperatures.((discharging too, but the limits are much colder))
-Lack of **low temperature cutoff** is not necessarily a deal-breaker.  Maybe you live in a hot location.  Maybe your chargers have low temperature cutoff.  Maybe you externally warm your battery.
-[[opinion:frater_secessus:self-heated_lifepo4|externally warmed vs. self-heated LiFePO4]]
@@ Line 421: / Line 433: @@
 Further reading:  [[https://www.youtube.com/results?search_query=offgrid+garage+absorption|Off-grid Garage videos]] testing various absorption approaches
+==== myth:  you shouldn't Float lithium ====
+Li certainly doesn't need Float voltage (Vfloat) in the sense lead-chemistry batteries do, but the Float setpoint is still useful for Li battery banks.((Li batts with active balancers or other parasitic loads may be drawn down by them))
+Reminder:  lead requires Float because
+  - lead banks need to be held at 100% SoC whenever possible for their long-term health
+  - the self-discharge rate is so high that they lose capacity just sitting there
+Neither of these is true for Li, which dislikes sitting at 100% SoC and has vanishingly-low self-discharge rates.((but see [[https://www.technomadia.com/2020/06/what-killed-our-rv-lithium-batteries-8-5-years-of-lifepo4/|this cautionary tale]] about add-on balancers depleting/killing a $4,000 bank)) So **with lithium Float is used for a different purpose**, as a **voltage floor**.  It is a voltage below which the charger shouldn't let the bank fall while charging is present.  Without Vfloat (or a very low one) the bank would charge then fall until reaching the "re-bulk" setpoint.((when a fresh charge cycle begins)).  After initial charging loads would run off the battery instead of the charging source.  Having a sane Vfloat allows Li to "relax" after charging while retaining the desired amount of Ah/Wh capacity.
+What Vfloat setpoint should actually be is a matter of some discussion and experimentation.  Each setup (and use case) is different, but we can start with some ballpark assumptions:((and using nominal 12v math))
+  * <13.4v will allow the bank to settle below 100%
+  * ~13.4v will hold the bank near whatever SoC it was charged to.  If in doubt, this is a good default for solar charging.((When charging from shore power 13.4v will eventually charge and hold at 100% SoC, which may be undesirable))
+  * >13.4v will continue to charge the bank beyond the SoC it was charged to during Absorption.  This may be useful if the Vabs value is set intentionally low.
+If you cannot set a Float within the confines of the Li profile then leverage the USER or GEL profile, modifying as described in the previous section.
+==== myth:  you can't equalize LFP ====
+LFP does not require [[electrical:12v:charging#equalization|Equalization]] (controlled overcharge) the way lead does.((in a sense [[opinion:frater_secessus:lifepo4_charging_voltage|charging LFP at higher-than-necessary voltages]] is akin to EQ)).
+Having said that, folks who regularly charge to less than 100% SoC might use the EQ setpoint to schedule occasional forays to 100% in order to [[electrical:12v:battery_monitor#drift_and_reset|reset the Ah counters]].   A user who regularly charges to 13.5v might use EQ to drive the bank to 14.4v every few weeks.
 ==== myth:  you can't charge Li with a lead battery charger ====
 Depends on the charger and how your Li wants to be charged.  Most **fully-configurable** chargers can be used to charge Li.((Some simpler controllers that only have selectable presets like AGM or gel //may// have a preset that overlaps with the correct charging specs for your battery.  Read the specs carefully.))    Note that some so-called "lithium compatible" chargers may have presets that do not match the requirements of your particular battery, so read the specs.
+This info can also be used to make a custom (USER) profile for LiFePO4 banks.
 Here is the order of operations:
@@ Line 444: / Line 485: @@
 Note:  if you are willing to pay minimal attention, even a single-voltage power supply or [[electrical:12v:alternator#combiners|relay]] would work.  Stop charging if/when the voltage hits your desired setpoint.
-==== myth:  you shouldn't Float lithium ====
-Li certainly doesn't need Float voltage (Vfloat) in the sense lead-chemistry batteries do, but the Float setpoint is still useful for Li battery banks.((Li batts with active balancers or other parasitic loads may be drawn down by them))
-Reminder:  lead requires Float because
-  - lead banks need to be held at 100% SoC whenever possible for their long-term health
-  - the self-discharge rate is so high that they lose capacity just sitting there
-Neither of these is true for Li, which dislikes sitting at 100% SoC and has vanishingly-low self-discharge rates.((but see [[https://www.technomadia.com/2020/06/what-killed-our-rv-lithium-batteries-8-5-years-of-lifepo4/|this cautionary tale]] about add-on balancers depleting/killing a $4,000 bank)) So **with lithium Float is used for a different purpose**, as a **voltage floor**.  It is a voltage below which the charger shouldn't let the bank fall while charging is present.  Without Vfloat (or a very low one) the bank would charge then fall until reaching the "re-bulk" setpoint.((when a fresh charge cycle begins)).  After initial charging loads would run off the battery instead of the charging source.  Having a sane Vfloat allows Li to "relax" after charging while retaining the desired amount of Ah/Wh capacity.
-What Vfloat setpoint should actually be is a matter of some discussion and experimentation.  Each setup (and use case) is different, but we can start with some ballpark assumptions:((and using nominal 12v math))
-  * <13.4v will allow the bank to settle below 100%
-  * ~13.4v will hold the bank near whatever SoC it was charged to.  If in doubt, this is a good default for solar charging.((When charging from shore power 13.4v will eventually charge and hold at 100% SoC, which may be undesirable))
-  * >13.4v will continue to charge the bank beyond the SoC it was charged to during Absorption.  This may be useful if the Vabs value is set intentionally low.
-If you cannot set a Float within the confines of the Li profile then leverage the USER or GEL profile, modifying as described in the previous section.
 ==== myth:  you must use DC-DC for alternator charging Li ====

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