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electrical:12v:directcharginglfp [2023/12/17 17:02]
frater_secessus [assessing your own setup for direct alternator charging] 		electrical:12v:directcharginglfp [2024/04/04 11:51] (current)
frater_secessus [reasons NOT to attempt direct charging] temp
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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 attempt direct charging =====
- +
-There are good reasons //not// to attempt direct charging:+
  
-For example, if 
  
   * you have [[electrical:12v:alternator#smart_alternators|a smart alternator]] or the alternator voltage is otherwise out of charging spec   * 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 have a **small or already-overloaded alternator**
   * you **already own a DC-DC**    * you **already own a DC-DC** 
 +  * 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 **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 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))     * you want **stable charging rates** regardless of state of charge((acceptance will taper in Absorption if present))  
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       * In the van-relevant examples above LFP banks typically pull 0.33C, or 33A per 100Ah of capacity, so it might be acceptable to size them to 0.5C (noncritical).       * In the van-relevant examples above LFP banks typically pull 0.33C, or 33A per 100Ah of capacity, so it might be acceptable to size them to 0.5C (noncritical).
       * 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.   
 +
 +
 +
 +===== 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.]
 +
 +
  
 If you are still interested, here is one approach to assessing your setup for direct charging lithium.   If you are still interested, here is one approach to assessing your setup for direct charging lithium.  
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   - test it with a drive.   - 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.   - 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 +  - celebrate
  
 **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 =====
electrical/12v/directcharginglfp.1702850528.txt.gz · Last modified: 2023/12/17 17:02 by frater_secessus

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