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electrical:12v:directcharginglfp [2024/06/04 18:36]
frater_secessus [testing phase] 		electrical:12v:directcharginglfp [2025/10/29 17:57] (current)
frater_secessus [tweaking current with resistance] emphasis
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 === why does C appear to 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:  +All other things being equal, 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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 === lessons === === lessons ===
  
-  -  Think twice about  direct-charging banks that have 2-3x the capacity of your alternator's rating.   If you need to do this use [[electrical:12v:b2b|a DC-DC charger]], external regulation, or current reduction with resistance as described elsewhere in this article.+  -  Think twice about  direct-charging banks that have 2-3x the capacity of your alternator's rating.   If you need to do this use a **properly-sized** [[electrical:12v:b2b|DC-DC charger]], external regulation, [[electrical:12v:alternator_details#thermal_switches|thermal switches]], current reduction with resistance as described elsewhere in this article, etc
   - do not idle to charge.    - do not idle to charge. 
  
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->> [with lead chemistries] the battery resistance [is] the large and controlling factor, **in the case of LFP, its the wiring** and the battery resistance is inconsequential. -- MechEngrSGH((https://www.irv2.com/forums/f87/alternator-burn-out-from-lithium-batteries-558284-10.html#post6433111)) (**emphasis** added)+>> [with lead chemistries] the battery resistance [is] the large and controlling factor, **in the case of LFP**, its the wiring and the **battery resistance is inconsequential**. -- MechEngrSGH((https://www.irv2.com/forums/f87/alternator-burn-out-from-lithium-batteries-558284-10.html#post6433111)) (**emphasis** added)
  
 ==== NEG return through chassis ==== ==== NEG return through chassis ====
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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. 
-===== reasons NOT to attempt direct charging =====+===== reasons NOT to direct charge LFP =====
  
  
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 ==== testing phase ==== ==== 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.+[**Note**:  this section might seem like overkill.  It is intended to walk first-timers through the process in a methodical way.  Experienced DIYers will already have internalized this kind of process.] 
 + 
 + 
 +For measuring charging current you can use the battery's BMS, [[electrical:12v:battery_monitor|battery monitor]], or [[https://amzn.to/3Vt8Jhh|clamp meter]].  If your vehicle has gauges for RPM and voltage you can use those.  If not, you might pick up an inexpensive bluetooth OBDii dongle [[https://amzn.to/3LuvXxw|like this]] to see the values.    
 + 
 + 
 + 
 +  - look over the sanity checks in the Assessment section above 
 +  - make the first test run a brief one and with the Li well-charged.((higher states of charge will reduce current demand)) 
 +  - begin with the relay [[electrical:12v:alternator#disabling_alternator_charging|disabled]] 
 +  - start the engine and let it idle for a minute.  Observe both the engine RPM and the chassis voltage; this is your baseline for how the vehicle acts when not charging the house bank.  Charging current from the alternator to house bank should be zero. 
 +  - enable the relay.  Observe the engine RPM, chassis voltage, and charging current 
 +    - If either the RPM or chassis voltage drops and stays there((dropping and recovering quickly is fine;  it may take a moment for the system to adjust to the increased load)) then **the alternator is overloaded at idle**.   
 +  - increase RPM slightly (2000rpm?) and hold it.  Observe chassis voltage and charging current.   
 +    - If either value rises and stays higher this suggests **the alternator may be able to handle the load only while driving**.((ie, the alternator spinning faster)).  
 +  - hold the RPM and disable the relay.  Charging current from the alternator should drop to zero.   
 +    - If the chassis voltage stays stable then the alternator can handle this level of current at this RPM. 
 +    - If chassis voltage rises and stays there this would suggest **the alternator is overloaded no matter the engine/alternator RPM** and direct charging is not recommended. 
 +  - 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))   - 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 with the Li bank at lower and lower states of charge, down to the lowest state of charge you expect to recharge from 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. 
 + 
 + 
 +Note:  this same process also works for testing the size of a DC-DC charger.  The difference is SoC doesn't matter since DC-DC output will remain stable for the most part regardless of SoC.  If the DC-DC can be derated we would adjust the output current until the alternator was happy at idle (or at higher RPM, whichever suits your use case).  
 + 
  
  
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 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.  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: advantages of direct charging =====
 +
 +**Cost**.  If you already own a combiner then it's already paid for.  If you are installing one new it would be much cheaper for the average current than DC-DC.
 +
 +Direct charging **tapers during Bulk** and DC-DC does not.  In effect the combiner provides the **most current when you need it most** (batteries are low) and the least when you need it least (batteries nearly charged).  This happens because when the batteries are more charged there is a smaller difference ("delta") between the alternator output and bank resting voltage (I=**V**/R again). There are some related effects:
 +
 +  * **direct charging backs off when other charging sources are present**.  This happens because the other charging sources affect the "resting" voltage of the bank and so the voltage delta.  This can help prevent excessive charge rates.  Example:  100Ah battery, 30A DC-DC and 30A solar.  If both systems were running full blast the combined charging would be 60A, or [[electrical:12v:battery_capacity|0.6C]].  That is above the recommend charging current (0.5C) of most drop-ins.  A combiner in the same situation might((based on my informal observations -- secessus)) drop to 15A contribution for a total of 45A.  That would be 0.45C. Of course if you want to hammer at full speed all the time then DC-DC is better for that. 
 +  * **direct charging backs off at higher states of charge**((in the upper knee)).  Cell manufacturer datasheets typically derate charging limits when the battery is >80% SoC. Engineering Explained has [[https://youtu.be/qYJk1Qljwgg?si=XPUVa3N9opEQIR0Z&t=288|a relatively-accessible video]] that explains how SoC and charge rate affect LiFePO4 degradation.  
 +
 +
  
  
 ===== 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]]  It includes setups that lack incomplete info, or interesting related setups. 
electrical/12v/directcharginglfp.1717526212.txt.gz · Last modified: 2024/06/04 18:36 by frater_secessus

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