RV and vandwellers wiki

User Tools

Site Tools

Trace: • output • charging • mandatory_dcdc • voltage_sensing
electrical:12v:directcharginglfp

Differences

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

Link to this comparison view

Both sides previous revision Previous revision
Next revision 		Previous revision
electrical:12v:directcharginglfp [2024/07/17 15:25]
frater_secessus [testing phase] 		electrical:12v:directcharginglfp [2025/03/28 16:27] (current)
frater_secessus [failures]
Line 68: Line 68:
 === 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))
Line 119: Line 119:
 === 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. 
  
Line 313: Line 313:
  
 ==== testing phase ==== ==== 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.    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.   
Line 320: Line 323:
   - look over the sanity checks in the Assessment section above   - 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))   - 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]] so you can see how the chassis electrical behaves without the extra load.  +  - 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.  Charging current from the alternator to house bank should be zero. +  - 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.  If either value 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**.  Observe the charging current.  +  - enable the relay.  Observe the engine RPM, chassis voltage, and charging current 
-  - increase RPM slightly (2000rpm?) and hold it.  Observe chassis voltage and charging current.  If either value one rises and stabilizes this suggests **the alternator can handle the load only while driving**.((ie, the alternator spinning faster)).  +    - 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**.   
-  - hold the RPM and disable the relay.  Charging current from the alternator should drop to zero.  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.+  - 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.   - 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))
Line 375: Line 382:
  
 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.  
 +
 +
  
  
electrical/12v/directcharginglfp.1721244307.txt.gz · Last modified: 2024/07/17 15:25 by frater_secessus

Page Tools

affilliate disclosure | the RVwiki runs on a virtual server from Phanes

Except where otherwise noted, content on this wiki is licensed under the following license: CC Attribution-Noncommercial-Share Alike 4.0 International
CC Attribution-Noncommercial-Share Alike 4.0 International Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki