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--- electrical:12v:drop-in_lifepo4 [2024/09/08 16:39]
frater_secessus [myth: lithium doesn't need absorption]
+++ electrical:12v:drop-in_lifepo4 [2025/03/18 13:27] (current)
frater_secessus [myth: you must use DC-DC for alternator charging Li]
@@ Line 81: / Line 81: @@
 LFP cells can be damaged by undervoltage.  Discharge is typically disabled when one or more cells drops to ~2.625v (~10.5v for the pack)
+If the BMS has shut down due to low voltage/SoC it usually requires external voltage to "wake" it.  Connect another battery, start the vehicle if [[electrical:12v:alternator|alternator charging]] is present, [[electrical:12v:self-jumpstarting|self-jumpstart]], etc.  Read the battery manual for specifics.
@@ Line 103: / Line 105: @@
 Note that **just because you can charge/discharge at higher rates doesn't mean you have to or that you should**.  Generally speaking LFP prefers current to stay around 0.2C (40A for our 200Ah example) for longevity and cell balance. Also if you will only ever need to discharge at 75A then a 200A BMS is not required.
+=== 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]]
 === cell balancing ===
@@ Line 132: / Line 147: @@
 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]
-=== 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.
@@ Line 153: / Line 160: @@
 === heater control ===
-[rare]
+[less common]
 BMS in batteries with [[electrical:12v:drop-in_lifepo4#self-heating_batteries|a self-heating function]] trigger the heating when they sense that cell temps are at some defined setpoint.
@@ Line 227: / Line 234: @@
   - verify that charging completes as expected.  If cell voltages are visible verify their balance is improving.
   - optional:  start moving back up by 0.05v or 0.1v increments if desired, watching as in step 2 above.  Example:  13.8v, then 13.85v, then 13.9v, etc.  **There is little reason to charge >14.0v (3.5Vpc)**.
@@ Line 233: / Line 239: @@
 ==== an approach to greater longevity ====
-> Bottom line, stay within the manufacturer recommended specs, and you should be fine, go beyond that (more conservative) and you should be extra fine. -- [[https://diysolarforum.com/threads/sok-206ah-battery-concerns.32368/post-397575|Dzl]]
+>> ... the reactions that cause [LiFePO4] aging are strongly correlated with voltage - David Howey, Professor of Engineering Science at the University of Oxford((https://youtu.be/1LygMkJpN6Q?si=MiLB6m-QUjPSzcs8&t=2372))
+>> Bottom line, stay within the manufacturer recommended specs, and you should be fine, go beyond that (more conservative) and you should be extra fine. -- [[https://diysolarforum.com/threads/sok-206ah-battery-concerns.32368/post-397575|Dzl]]
 [//Fine// vs //Extra Fine// is like //normal driving// vs //hypermiling//;  getting big MPG numbers is possible but requires forethought and a willingness to alter one's own driving style.  -- secessus]
@@ Line 294: / Line 303: @@
-=====self-heating batteries=====
+===== waking lithium batteries =====
-Lithium cannot be charged in freezing temps.  We can either:
+Lithium banks can go dormant at low voltages in order to protect themselves:
-  -  cut off charging in the chargers; and/or
+  - the BMS turns off the DISCHARGE channel to keep cell voltage from going any lower
-  - warm the batteries, either internally (self-heating) or externally (typically with warming mats).
+  - so the battery internally has ~11.0v or whatever but does not pass that voltage to the terminals on the outside of the battery case
+  - which means smart chargers don't see battery voltage
+  - and think there's a problem so they won't start(("too smart for their own good"))
+What we need is a //dumb// charging source to get the party restarted.  Another battery.  Power supply.  Charger with a "wake LiFePO4" mode, vehicle starter battery, etc.
-Self-heating is convenient, and does not require lithium- or temperature-aware chargers. The downsides are:
+If you want to use the starter battery there are several possibilities:
-  *  typically substantially more $$$ than warming pads
+  * Rigs with IGN-triggered relays can briefly turn the key to ACC then back off.((don't leave it on ACC long or the dead Li will suck power from the starter battery.  Either turn ACC off again or actually start the vehicle.))
-  *  can miss out on charging opportunities. Not a big deal with smaller solar-only setups, but can really hamper alternator or big solar.
+  * Rigs with voltage-sensing relays will have to actually start the engine or press a manual override switch to activate the VSR and wake the lithium bank.
+  * Rigs with diode- or FET-based isolators would start the engine to spin the alternator and get power flowing through the isolator to the sleeping lithium
+  * If you have no other options you can remove either the starter battery or house battery and locate them so a pair of jumper cables can connect them.
-==== how self-heating batteries work ====
-The last issue is a function of how they work.
-  - When the BMS detects dangerously-low temps it deactivates charging to the battery cells
-  - any charging power is sent to the internal warming pads, typically ~50w
-  - when the BMS detects the temps are ok it turns the charging back on.  The warming may be switched off, or may continue to warm the battery further to a given temp setpoint.
-=== how this could cause a missed charging opportunity ===
-Imagine a half-hour drive on a freezing morning with a 50A [[electrical:12v:b2b|DC-DC charger]].((Doesn't have to be DC-DC but it makes the math easier because charging current is more stable.)) You could pump 25Ah((minus the energy it took to hold temp overnight)) into an externally-warmed battery, or you could use the alternator to run the 50W internal heater and get 0Ah replaced.
-With only small solar the Wh consumed overnight and Wh not produced in the morning might be a breakeven.
-==== external warming ====
-If your preferred battery has everything but heating you might want to [[https://diysolarforum.com/threads/lifepo4-heating-pad-for-cold-temperatures.5/page-26|DIY a heater]].  It is typically much cheaper than built-in heat and offers more control.
-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.
+=====self-heating batteries=====
+[see [[opinion:frater_secessus:self-heated_lifepo4|self-heating vs DIY warming]]]
@@ Line 349: / Line 352: @@
 === 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.
+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.  And remember that lithium chemistries don't need to be fully charged [[electrical:12v:psoc|the way lead batteries do]].
 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.
@@ Line 357: / Line 360: @@
 | 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))  |
+| LiFePO4    | 13.6v - 14.4v        | 14.2v((this is a matter of some debate.  LFP mfg charging recommendations are often [[opinion:frater_secessus:lifepo4_charging_voltage|quite high]] - secessus))  |
@@ Line 463: / Line 466: @@
 ==== myth:  you must use DC-DC for alternator charging Li ====
-Depends on the battery, the alternator, the use case, and even the [[electrical:12v:alternator|combiner]].  For example, [[https://www.youtube.com/watch?v=VY2b71zoyvg|Battle Born says]] this about direct-charging lithium:
+If you have been successfully charging an AGM bank through a relay then an LFP bank of similar rated capacity will likely charge similarly.  The actual results depend on the battery, the alternator, the use case, and even the [[electrical:12v:alternator|combiner]].
+[[https://www.youtube.com/watch?v=VY2b71zoyvg|Battle Born says]] this about direct-charging lithium:
 >> Yes, you can.  Under most circumstances you don't even need to modify your system.
@@ Line 609: / Line 614: @@
   - Li doesn't like to be held at 100% State of Charge for long periods
-  - In practice a solar-charged bank doesn't bounce between Float and Absorption during the course of a day
+  - In practice a solar-charged bank doesn't bounce between Float and Absorption during the course of a day((unless a heavy enough load pulls Vbatt down to the re-Bulk setpoint)).
 The 2nd point takes a bit of explaining.   A [[electrical:solar:charge_controller|solar charge controller]] completes Absorption then falls to into Float where it will remain as long as the sun((and system capacity)) cooperates.  Absorption //can// be re-triggered if voltage falls below the Absorption Reconnect setpoint, but that setpoint is even lower than Vfloat.  If that happens the solar charging has already been overtasked and we will get a "microcycle" during that day in any case if the sun comes back.

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