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electrical:12v:directcharginglfp

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electrical:12v:directcharginglfp [2024/04/04 11:51]
frater_secessus [reasons NOT to attempt direct charging] temp 		electrical:12v:directcharginglfp [2025/03/28 16:27] (current)
frater_secessus [failures]
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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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 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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 [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.] [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 ====
  
- 
-If you are still interested, here is one approach to assessing your setup for direct charging lithium.   
  
   - 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. 
-  - celebrate 
  
-**Caveats**:  +==== testing phase ==== 
 + 
 +[**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)) 
 +  - 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).  
 + 
 + 
 + 
 + 
 +==== 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).  
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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.1712245908.txt.gz · Last modified: 2024/04/04 11:51 by frater_secessus

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