Differences

This shows you the differences between two versions of the page.

--- electrical:12v:drop-in_lifepo4 [2024/10/12 10:07]
frater_secessus [BMS features and specs] rearranged low temp cutoff
+++ electrical:12v:drop-in_lifepo4 [2025/03/18 13:27] (current)
frater_secessus [myth: you must use DC-DC for alternator charging Li]
@@ Line 234: / 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 301: / Line 300: @@
 Also see:  Will Prowse's  [[https://www.youtube.com/watch?v=Yf9N9zBgyB8|Lithium Battery Longevity: Double or Quadruple the Life of Your Lithium Battery]]
+===== waking lithium batteries =====
+Lithium banks can go dormant at low voltages in order to protect themselves:
+  - the BMS turns off the DISCHARGE channel to keep cell voltage from going any lower
+  - 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.
+If you want to use the starter battery there are several possibilities:
+  * 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.))
+  * 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.
@@ Line 326: / 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 334: / 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 440: / 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 586: / 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.

RV and vandwellers wiki

User Tools

Site Tools

Differences

Page Tools