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--- electrical:12v:drop-in_lifepo4 [2023/12/17 15:23]
frater_secessus [and that Sterling video]
+++ electrical:12v:drop-in_lifepo4 [2024/04/12 23:02] (current)
frater_secessus [bluetooth]
@@ Line 42: / Line 42: @@
   * Li is relatively expensive
   * 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 full discharge.
+  * Li can be **damaged** by long duration at full charge or high voltage, or high ambient temperatures
   * 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 226: / Line 232: @@
 Charging Li at very high rates may also strain the [[electrical:12v:alternator|alternator]].
+==== myth:  you can't use a combiner to charge batteries of different chemistries ====
+There are two different challenges here:
+  - **different resting voltages** - if the lead rests at 12.8v and LiFePO4 at 13.6v then when charging stops the lead batt could put a drain on the li batt.  The [[electrical:12v:alternator#combiners|charging relay]]((or [[electrical:12v:b2b|DC-DC]])) is primarily there to stop house loads from draining the starter battery, but this separation also means **the starter battery cannot drain the house battery**.((unless self-[[electrical:12v:self-jumpstarting|jumpstarting]])) ((note, however, [[electrical:12v:alternator#gotchas|a gotcha with VSRs]]))
+  - **acceptable charging voltages** - the alternator voltage needs to be acceptable (not necessarily //optimal//) to both batteries.  Read on.
+=== acceptable charging voltage ranges ===
+We can assume **the alternator voltage is acceptable to the starter battery** because the manufacturer designed that system.((and we can observe that the vehicle starts on demand)).  So we only have think about whether or not the alternator voltage is acceptable to the house bank.
+For the following thought experiment we will use some a typical alternator output voltage of 14.2v and house bank charging voltage [[electrical:solar:charge_controller_setpoints|setpoints]] ("Absorption" or "Boost" voltage, Vabs); check your vehicle's alternator voltage and  battery manufacturer charging specs to make your actual decision.
+^ Chemistry  ^ Acceptable Vabs  ^ Optimal Vabs                                                                                                        ^
+| Gel        | 14.0v - 14.3v        | 14.2v                                                                                                               |
+| AGM        | 14.2v - 14.5v        | 14.4v                                                                                                               |
+| Flooded    | 14.4v - 14.8v        | 14.6v                                                                                                               |
+| LiFePO4    | 13.6v - 14.4v        | 14.0v((this is a matter of some debate.  LFP mfg charging recommendations are often [[opinion:frater_secessus:lifepo4_charging_voltage|quite high]] - secessus))  |
+Let's think about some combinations.  In all these cases [[electrical:12v:alt_and_solar#how_alternator_charging_helps|alternator charging is extremely useful]] for [[electrical:12v:charging#bulk_stage|Bulk stage charging]] but may not be sufficient on its own:
+  * **easy to meet charging requirements by alternator alone**
+    * starter battery + LiFePO4. Charging LFP at 14.2v is a good balance between moderate voltage and charging time. Lithium is not affected by [[electrical:12v:psoc|partial states of charge]] so you can charge as little or much as you want.  Caveat:  alternator charging should be [[electrical:12v:alternator#disabling_alternator_charging|disabled]] if you drive long enough to reach your desired state of charge. Specifically, LFP should not be held at high voltage after reaching full charge.
+  * **unlikely to meet charging requirements by alternator alone** - could theoretically meet charging by alternator alone //if// given sufficient time.  Unfortunately most people don't drive enough hours ((typically 5-6 hours from 50% SoC)) to complete Absorption;  [[electrical:12v:psoc|incomplete charging damages lead batteries]].  less one is driving for many hours each day it is unlikely that there will be sufficient time to complete Absorption. For this reason solar or other long-duration charging source is often [[electrical:12v:alt_and_solar#how_solar_helps|added to handle Float and late Absorption]].
+    * starter battery + AGM.  14.2v from the alternator is in the acceptable range, but just barely.  Extremely long Absorption would be required and adding solar is highly recommended.
+    * starter battery + Gel.  The alternator is putting out the exact Vabs spec'd by the manufacturer.  Caveat:  gel can be damaged (electrolyte cavitated) by excess current. Ensure your setup charges withing the mfg current specs.
+  * **Useful but impossible((practically)) to meet charging requirements by alternator alone ** - cannot reach Vabs in any case.  Solar is nearly mandatory, although DC-DC charging will do it if one is driving many hours a day.
+    * starter battery + flooded.  Alternator charging is useful for Bulk stage (shoving Ah into the bank) but it cannot meet the minimum Vabs.
 ==== myth:  you have to charge Li to 100% ====
@@ Line 285: / 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 328: / 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 476: / 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 538: / 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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