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electrical:12v:drop-in_lifepo4 [2025/03/18 17:27]
frater_secessus [myth: you must use DC-DC for alternator charging Li] 		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 ===== ===== 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 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   * Li can be **damaged** by long duration at full charge or high voltage, or high ambient temperatures
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 ===== choosing a drop-in LFP battery ===== ===== 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 ==== ==== voltage ====
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 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) 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 === === 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]] 
  
  
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   * going into a period where you will need max capacity   * going into a period where you will need max capacity
   * to perform a capacity test   * to perform a capacity test
-  * to reset the BMS amp/SoC counter+  * to [[electrical:12v:battery_monitor#drift_and_reset|reset the BMS amp/SoC counter]]
   * to top-balance cells((to the degree this works))   * to top-balance cells((to the degree this works))
  
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 Further reading:  [[https://www.youtube.com/results?search_query=offgrid+garage+absorption|Off-grid Garage videos]] testing various absorption approaches 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 ==== ==== 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. 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: Here is the order of operations:
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   * **Absorption voltage** (Vabs) - whatever charging voltage your battery manufacturer recommends.((see the section on longevity in this article))   * **Absorption voltage** (Vabs) - whatever charging voltage your battery manufacturer recommends.((see the section on longevity in this article))
   * **Absorption duration** - whatever the battery manufacturer recommends, typically 0 to 20 minutes.((charging voltages ≥14.0v typically require no absorption duration at all))   * **Absorption duration** - whatever the battery manufacturer recommends, typically 0 to 20 minutes.((charging voltages ≥14.0v typically require no absorption duration at all))
-  * **Float voltage** (Vfloat) - Something like 13.4v((3.35vpc)) is a good compromise. See the discussion on float below.+  * **Float voltage** (Vfloat) - Something like 13.3v-13.4v((3.35vpc)) is a good compromise. See the discussion on float below.
   * **Absorption reconnect** - this voltage is the setpoint below which Absorption(("boost" in Renogy/EpEver nomenclature)) is restarted. Normally in a solar configuration Vfloat is held until sun goes down, solar conditions otherwise deteriorate, or a load is applied that is more than the solar can support.  Start with a value like 13.2v and see how your system behaves.  Adjust as needed.    * **Absorption reconnect** - this voltage is the setpoint below which Absorption(("boost" in Renogy/EpEver nomenclature)) is restarted. Normally in a solar configuration Vfloat is held until sun goes down, solar conditions otherwise deteriorate, or a load is applied that is more than the solar can support.  Start with a value like 13.2v and see how your system behaves.  Adjust as needed. 
   * **Equalize voltage** (Veq) - Li does not require equalization.  If it cannot be disabled in the controller it is common to set Veq the same as Vabs so it becomes a non-issue.((some folks who charge to lower voltages like 13.6v may use Veq to raise bank voltage into the 14s for various purposes.  See the section on longevity.))     * **Equalize voltage** (Veq) - Li does not require equalization.  If it cannot be disabled in the controller it is common to set Veq the same as Vabs so it becomes a non-issue.((some folks who charge to lower voltages like 13.6v may use Veq to raise bank voltage into the 14s for various purposes.  See the section on longevity.))  
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 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.   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 ==== ==== myth:  you must use DC-DC for alternator charging Li ====
  
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 >> Yes, you can.  Under most circumstances you don't even need to modify your system. >> Yes, you can.  Under most circumstances you don't even need to modify your system.
  
-They do recommend [[electrical:12v:alternator#lithium-specific|a BIM]] or [[electrical:12v:b2b|DC-DC charger]] //for banks >300Ah//.+They do recommend [[electrical:12v:alternator#lithium-specific|a BIM]] or [[electrical:12v:b2b|DC-DC charger]] //for banks >300Ah//.   
 + 
 +If not already present, a small switch to [[electrical:12v:alternator#disabling_alternator_charging|disable the combiner]]((same goes for [[electrical:12v:b2b|DC-DC chargers]])) at will is a good idea. 
  
  
electrical/12v/drop-in_lifepo4.1742318863.txt.gz · Last modified: 2025/03/18 17:27 by frater_secessus

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