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electrical:12v:directcharginglfp [2023/12/15 11:29]
frater_secessus [BMS disconnect will damage the alternator] 		electrical:12v:directcharginglfp [2024/06/10 16:37] (current)
frater_secessus [reasons NOT to attempt direct charging]
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 ====== Direct charging LiFePO4 banks ====== ====== Direct charging LiFePO4 banks ======
  
-There is an [[electrical:12v:drop-in_lifepo4#mythyou_must_use_dc-dc_for_alternator_charging_li|often-repeated claim]] that LiFePO4 house banks will always pull monstrous levels of  current and destroy alternators if a plain [[electrical:12v:alternator|isolator]] is used instead of a [[electrical:12v:b2b|DC-DC charger]]. This claim is not supported by either theory or empirical testing. The data below provide **actual measurements** to help people make informed decisions.  We will see that **current demand by LFP banks follows a known formula** rather than the opinions of people on the internet. +There is an [[electrical:12v:drop-in_lifepo4#mythyou_must_use_dc-dc_for_alternator_charging_li|often-repeated claim]] that LiFePO4 house banks will always pull monstrous levels of  current and destroy alternators if a plain [[electrical:12v:alternator|combiner]] is used instead of a [[electrical:12v:b2b|DC-DC charger]]. This claim is not supported by either theory or empirical testing. The data below provide **actual measurements** to help people make informed decisions.  We will see that **current demand by LFP banks follows a known formula** rather than the opinions of people on the internet. 
  
  
 This article will focus on current demand by LiFePO4 house banks.  Other objections will be addressed [[electrical:12v:directcharginglfp#objections|at the bottom]].  This article will focus on current demand by LiFePO4 house banks.  Other objections will be addressed [[electrical:12v:directcharginglfp#objections|at the bottom]]. 
 +
 +===== contraindications =====
 +
 +Direct charging is not a good fit for all scenarios.  See [[electrical:12v:directcharginglfp#assessing_your_own_setup_for_direct_alternator_charging|section below]] for use cases where other solutions may be required or preferable. 
 +
  
 ===== the formula in 60 seconds ===== ===== the formula in 60 seconds =====
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 There is an outlier that pulls 0.67C at ~10% state of charge.((spreadsheet says 25%, but a later post clarified it was in the bottom knee))  That setup has **1awg cables** from the starter battery to house battery bank, resulting in extremely low total resistance (9mR).  The practical effect of resistance on current will be addressed below, using that owner's experiments for illustration.  There is an outlier that pulls 0.67C at ~10% state of charge.((spreadsheet says 25%, but a later post clarified it was in the bottom knee))  That setup has **1awg cables** from the starter battery to house battery bank, resulting in extremely low total resistance (9mR).  The practical effect of resistance on current will be addressed below, using that owner's experiments for illustration. 
  
-=== why does acceptance decrease as bank size increases? ===+=== why does C appear to decrease as bank size increases? === 
 + 
 +Current acceptance does increase with larger bank capacities but this increase can be surprisingly small:  
  
 >> IF you have other things in the network with a much higher resistance than the batteries (such as using the frame as a ground return path), changing the resistance of the battery bank [ie, increasing capacity] can have only a small effect. --MechEngrSGH((https://www.rvforum.net/threads/charging-lithium-batteries-with-alternator.135673/post-1255460)) >> IF you have other things in the network with a much higher resistance than the batteries (such as using the frame as a ground return path), changing the resistance of the battery bank [ie, increasing capacity] can have only a small effect. --MechEngrSGH((https://www.rvforum.net/threads/charging-lithium-batteries-with-alternator.135673/post-1255460))
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-Let's assume battery resting voltage is 13.0 and alternator voltage is 14.0 and circuit resistance is 20mR.+Let's assume battery resting voltage is 13.0 and alternator voltage is 14.0 and circuit resistance is 20mR (.020R).
  
-  * I=V/R = (14v-13v)/**0.020mR** = 1v/0.020mR = **50A**+  * I=V/R = (14v-13v)/**0.020R** = 1v/0.020R = **50A**
      
 Let's add 5m Ohm of resistance: Let's add 5m Ohm of resistance:
  
-  * I=V/R = (14v-13v)/**0.025mR** = 1v/0.025mR = **40A**+  * I=V/R = (14v-13v)/**0.025R** = 1v/0.025R = **40A**
    
 ===== tweaking current with voltage ===== ===== tweaking current with voltage =====
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 If the SoC was very low (near cutoff) the combiner would slightly faster, ~7.5 minutes.  If the SoC was very low (near cutoff) the combiner would slightly faster, ~7.5 minutes. 
-===== assessing your own setup for direct alternator charging =====+===== reasons NOT to direct charge LFP =====
  
-There are good reasons //not// to attempt direct charging: 
  
 +  * you have [[electrical:12v:alternator#smart_alternators|a smart alternator]] or the alternator voltage is otherwise out of charging spec
 +  * you have a **small or already-overloaded alternator**
   * you **already own a DC-DC**    * you **already own a DC-DC** 
-  * you **need consistent, predictable charging**, like 40A over the next hour.  DC-DC will provide this((barring interference from big chassis loads, BMS protections, high ambients, etc));  isolator charging starts out high and drops as battery voltage rises.  You might learn its behavior over time, but it will never be as easy as "40A for 1.5 hours = 60Ah".  +  * your BMS has no low temp charging disconnect and you have no desire to work around that((to be fair, this also rules out DC-DC that lack low-temp disconnect)) 
-  * you **don't want to think about the battery**.  DC-DC will either stop charging (Renogy) or drop to float (all others) when charging is complete.  Isolators need to be switched off after charging, if you are driving long enough to fully charge the bank.+  * you **want predictable charging rates**, like 40A over the next hour.  DC-DC will provide this((barring interference from big chassis loads, BMS protections, high ambients, etc));  isolator charging starts out high and drops as battery voltage rises.  You might learn its behavior over time, but it will never be as easy as "40A for 1.5 hours = 60Ah".  
 +  * you want **stable charging rates** regardless of state of charge((acceptance will taper in Absorption if present))   
 +  * if **alternator is your sole charging source**.  Direct-charging's taper makes it difficult to charge aggressively when the bank is already at high SoC.  A large DC-DC or dedicated secondary, externally-regulated alternator might be a better solution for this use case.   
 +  * you **don't want to think about the battery**.  DC-DC will either stop charging (Renogy) or drop to float (all others) when charging is complete.  Combiners need to be [[electrical:12v:alternator#disabling_alternator_charging|switched off after charging]] if you are driving long enough to fully charge the bank.
   * **choosing wiring** can require some research   * **choosing wiring** can require some research
     * [[electrical:12v:alternator#wiring_for_the_isolator|with DC-DC chargers wire sizing is straightforward]]:  charger output rating + 20% to account for DC-DC conversion overhead     * [[electrical:12v:alternator#wiring_for_the_isolator|with DC-DC chargers wire sizing is straightforward]]:  charger output rating + 20% to account for DC-DC conversion overhead
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       * a set of heavy jumper cables and a clamp-on monitor might help assess max current draw before buying any wiring.          * a set of heavy jumper cables and a clamp-on monitor might help assess max current draw before buying any wiring.   
  
-If you are still interested, here is one approach to assessing your setup for direct charging lithium.  + 
 + 
 +===== assessing your own setup for direct alternator charging ===== 
 + 
 +[Note:  this section errs on the side of methodical plodding in the spirit of "don't eat this mattress" or "do not juggle chainsaws on the top step of this ladder".  For existing setups it can be as simple as starting the engine briefly to see how much current is drawn and going from there.  The existing fuse would intervene if the current is harmfully large.] 
 + 
 +==== assessment phase ==== 
  
   - pay attention to the **chassis' normal voltage** while idling and cruising - this is a benchmark for how the vehicle behaves without aux charging.    - pay attention to the **chassis' normal voltage** while idling and cruising - this is a benchmark for how the vehicle behaves without aux charging. 
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   - now make a **sanity check** about the ability of alternator, your isolator, and your Li battery to cooperate.  Refer to the actual reports in the spreadsheet to see if any approximate your setup.  If you can measure the total resistance do so and work the formula.     - now make a **sanity check** about the ability of alternator, your isolator, and your Li battery to cooperate.  Refer to the actual reports in the spreadsheet to see if any approximate your setup.  If you can measure the total resistance do so and work the formula.  
   - ensure the **fuse** between your chassis and battery bank is sized so //you cannot draw more than the alternator and wiring can handle//   - ensure the **fuse** between your chassis and battery bank is sized so //you cannot draw more than the alternator and wiring can handle//
-  - make the first test run a brief one and with the Li fairly well charged.((higher states of charge will typically lessen current demands to some degree))((if you have paralleled batteries you might want to do this step with just one in place to get a feel for the draw.))  Start the engine and see if the charging current and voltage is acceptable.  See if chassis voltage remains stable.  Turn off the engine. 
-  - test it with a drive. 
-  - repeat the last two steps with the Li bank at lower and lower states of charge, down to the lowest state of charge you expect to recharge from alternator. 
-  - disconnect the isolator with **a switch** when you come to the desired SoC, when charge rate exceeds your preferences, when stuck in traffic, etc 
  
 +==== testing phase ====
 +
 +  - make the first test run a brief one and with the Li well-charged.((higher states of charge will reduce current demand))  Start the engine and see if the charging current((per the BMS, [[electrical:12v:battery_monitor|battery monitor]], or [[https://amzn.to/3Vt8Jhh|clamp meter]])) and voltage is acceptable.  Check to see chassis voltage and engine RPM remain stable((do not drop precipitously anything longer than a moment while alternator output adjusts)) and belts do not squeal.  Turn off the engine.
 +  - if solar or other charging source is present, disconnect those chargers and repeat #1.((other chargers will push up apparent bank voltage, which would decrease current demanded of the alternator))
 +  - repeat steps above with the Li bank at lower and lower states of charge, down to the lowest state of charge you expect to recharge from alternator.
 +
 +
 +==== caveats ====
  
-**Caveats**:   
  
   * Only alternator charge while driving ([[rv:idling|no idling]] except for brief preliminary test).     * Only alternator charge while driving ([[rv:idling|no idling]] except for brief preliminary test).  
-  * Pay attention while charging from alternator to keep from overcharging the Li or holding for long periods at high [[electrical:depth_of_discharge|states of charge]]. You may want a manual disconnect or [[electrical:12v:alternator_charging_hvd|HVD]] to shut off alternator charging due to excess voltage or if the bank is already charged.  These can be added on the D+ wire (ignition triggered) or electronics ground wire (VSR, solenoid, or voltage-triggered DC-DC) +  * Pay attention while charging from alternator to keep from overcharging the Li or holding for long periods at high [[electrical:depth_of_discharge|states of charge]]. You may want [[electrical:12v:alternator#disabling_alternator_charging|a manual disconnect]] or [[electrical:12v:alternator_charging_hvd|HVD]] to shut off alternator charging due to excess voltage or if the bank is already charged.  These can be added on the D+ wire (ignition triggered) or electronics ground wire (VSR, solenoid, or voltage-triggered DC-DC) 
-  * Fuse the wiring from the chassis so the bank cannot pull more than you specify +  * if current is excessive but you want to direct-charge, you can tweak acceptance with resistance or voltage as seen elsewhere in this article
-  * if current is higher than you want consider adding resistance to the circuit+
  
 ===== the parts list ===== ===== the parts list =====
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 ===== objections ===== ===== objections =====
  
-No one is insisting you must to direct-charge your LFP.  This article intends to show direct-charging is practical and effective far more often than people think.+No one is insisting you must to direct-charge your LFP or claiming it is a good fit for all use cases.  This article intends to show direct-charging is practical and effective in more use cases than people think.
  
 Let's address some other common objections.  Let's address some other common objections. 
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   - why are you overcharging your bank so badly that the BMS is shutting down to keep you from damaging it?     - why are you overcharging your bank so badly that the BMS is shutting down to keep you from damaging it?  
   - the starter battery is in the circuit and can absorb the transients.  What do you think happens when you shut off the headlights,  or heater blower, or those heated seats?   - the starter battery is in the circuit and can absorb the transients.  What do you think happens when you shut off the headlights,  or heater blower, or those heated seats?
 +
 +==== you will overcharge your battery ====
 +
 +If you are driving long enough to reach the desired state of charge (80%, 100%, whatever), you can [[electrical:12v:alternator#disabling_alternator_charging|disable alternator charging]] if desired. 
 +
  
 ===== addendum:  other charging reports ===== ===== addendum:  other charging reports =====
  
 This info has been moved to the Other Reports tab on [[https://docs.google.com/spreadsheets/d/145Mx0DQkUbznaiGdTcYmKEc3AqOL3Eu79hqQuzh83oM/edit?usp=sharing|the main spreadsheet]] This info has been moved to the Other Reports tab on [[https://docs.google.com/spreadsheets/d/145Mx0DQkUbznaiGdTcYmKEc3AqOL3Eu79hqQuzh83oM/edit?usp=sharing|the main spreadsheet]]
electrical/12v/directcharginglfp.1702657756.txt.gz · Last modified: 2023/12/15 11:29 by frater_secessus

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