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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 [2025/11/22 00:16] (current)
frater_secessus [BMS features and specs]
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
Line 55: Line 55:
 ===== 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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-Capacitive balancing wastes less power since most of the excess is transferred to the low cell[s].  There are efficiency losses involved (~50%((https://www.ti.com/download/trng/docs/seminar/Topic%202%20-%20Battery%20Cell%20Balancing%20-%20What%20to%20Balance%20and%20How.pdf))) but it's far less than the 100% loss of passive balancing.  The main drawbak of capacitive balancers is the current spec (like 5A) only occur at huge deltas;  the rest of the time the balancing current is much lower. +**Capacitive balancing** wastes less power since most of the excess is transferred to the low cell[s].  There are efficiency losses involved (~50%((https://www.ti.com/download/trng/docs/seminar/Topic%202%20-%20Battery%20Cell%20Balancing%20-%20What%20to%20Balance%20and%20How.pdf))) but it's far less than the 100% loss of passive balancing.  The main drawbak of capacitive balancers is the current spec (like 5A) only occur at huge deltas;  the rest of the time the balancing current is much lower.  
 + 
 +Heads up:  because active balancers are just shuffling voltage between cells they can overdischarge the cells if left alone without adult supervision.  To avoid this: 
 + 
 +  * use [[electrical:12v:lvd|an LVD]] to cut the circuit to the balancer's single NEG lead when pack voltage is <13.0v or some other value of your choosing. Or, 
 +  * choose an active balancer that has a built-in LVD.((The cutoff tends to be something like 3.0Vpc, so 12.0v for a 4S 12v pack.))  Or, 
 +  * only run the active balancer manually when required
  
-In theory inductive active balancers would get around this delta/current relationship.  [as of this writing in 2024 I know of no drop-ins that use inductive active balancers - secessus]+In theory **inductive active balancers** would get around this delta/current relationship.  [as of this writing in 2024 I know of no drop-ins that use inductive active balancers - secessus]
  
  
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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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 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 ====
  
electrical/12v/drop-in_lifepo4.1745081442.txt.gz · Last modified: 2025/04/19 16:50 by frater_secessus

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