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electrical:12v:directcharginglfp [2022/11/05 12:29]
frater_secessus [Isolator charging LiFePO4 banks]
electrical:12v:directcharginglfp [2024/03/20 12:07] (current)
frater_secessus [assessing your own setup for direct alternator charging]
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-====== Isolator 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 pull always 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 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. 
  
-Short digression:  there is a related **claim that LiFePO4 cannot be charged fully by isolator**.  Maybe, but... 
  
-  - LFP does not //need// to be fully charged [[electrical:12v:psoc|the way lead does]].   +This article will focus on current demand by LiFePO4 house banks.  Other objections will be addressed [[electrical:12v:directcharginglfp#objections|at the bottom]].  
-  - LFP will charge quickly to 100% at voltages as low as 13.8v, and most alternators put out more than that.((for smart alternators, see [[electrical:12v:alternator#smart_alternators|this article]]))  + 
-  - if solar is present it will also "top off" the lithium if that is something the owner wants to do +===== contraindications ===== 
-  - **overcharging** (holding Vabs after reaching 100% SoC) is actually a bigger concern, particularly if one drives for hours.  A manual switch on the isolator (D+ for solenoids, on the ground wire for VSR) will allow the owner to disable alternator charging at will+ 
 +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
  
-This article will focus on current demand by LiFePO4 house banks. 
  
 ===== the formula in 60 seconds ===== ===== the formula in 60 seconds =====
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     * The **voltage difference between alternator and battery bank**((If alternator voltage is 14.1v and the resting voltage of the battery is 13.0v, then V=1.1v.)) -- so **the lower the battery's voltage the higher the current demanded**.      * The **voltage difference between alternator and battery bank**((If alternator voltage is 14.1v and the resting voltage of the battery is 13.0v, then V=1.1v.)) -- so **the lower the battery's voltage the higher the current demanded**. 
-    * the **total resistance** of the entire circuit((including the batteries themselves)) - wire gauge, length of wiring run, components installed in the circuit, battery chemistry and capacity, etc will make a difference+    * the **total resistance** of the entire circuit((including the batteries themselves)) - wire gauge, length of wiring run, method of "grounding"((negative return)) components installed in the circuit, battery chemistry and capacity, etc will make a difference
  
 You don't have to fully understand it or get a I=V/R tattoo, but do keep the basic idea in mind as we plunge ahead.  You don't have to fully understand it or get a I=V/R tattoo, but do keep the basic idea in mind as we plunge ahead. 
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 ===== analysis of van-relevant installs ===== ===== analysis of van-relevant installs =====
 +
 +**TL;DR**:  in general we see 0.33C charge acceptance rates at low states of charge, tapering as bank voltage rises. Much like AGM. Across the broad middle of the LFP state of charge acceptance is closer to 0.2C. 
 +
  
 This section includes installs that one might find in a typical van: This section includes installs that one might find in a typical van:
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   * standard regulators (no external regulators, internal regulator hacks, etc)   * standard regulators (no external regulators, internal regulator hacks, etc)
   * with rated capacities ≥120A((where stated))   * with rated capacities ≥120A((where stated))
-  * plain isolators, not voltage-boosting [[electrical:12v:b2b|DC-DC chargers]]+  * plain relays, not voltage-boosting [[electrical:12v:b2b|DC-DC chargers]] or [[electrical:12v:alternator#proper_isolators|voltage-reducing isolators]].  [[electrical:12v:alternator#lithium-specific_vsr|BiM]] are included because they are fancy relays. 
 +  * NEG return typically through chassis rather than separate NEG wire to battery. 
  
 Since it includes screenshots the data will be for installs documented before Sept 15, 2022.   Since it includes screenshots the data will be for installs documented before Sept 15, 2022.  
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 {{https://img.mousetrap.net/misc/isolatorMaxCRate.jpg}} {{https://img.mousetrap.net/misc/isolatorMaxCRate.jpg}}
  
-The average max current drawn from the alternator is **0.33C**, or 1/3rd of what the warnings predict.   **This the same C-rate that AGM pulls when cycled to 50% DoD**.+The average **max** current drawn from the alternator is **0.33C**, or 1/3rd of what the warnings predict.   **This the same C-rate that AGM pulls when cycled to 50% DoD**.  It might seem counterintuitive but it is a combination of: 
 + 
 +  - higher voltage of LiFePO4 compared to lead; and (V in the forumula) 
 +  - the resistance introduced by chassis grounding (R in the formula) 
 + 
 +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 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)) 
  
  
-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 chassis to house battery bank, resulting in extremely low resistance (9mR).  The practical effect of resistance on current will be addressed below, using that owner's experiments for illustration.  
  
  
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 ==== failures ==== ==== failures ====
  
-There are two failures, and neither of them come from the van-relevant section.+There are three failures to date, and none comes from the van-relevant section.  
  
  
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 **500Ah** of LFP direct charged with a **225A** alternator.  This falls outside the van-relevant window since bank size >300Ah. **500Ah** of LFP direct charged with a **225A** alternator.  This falls outside the van-relevant window since bank size >300Ah.
  
-Mortons on the Move were able to overheat (not damage) the  225A primary alternator with 500Ah pulling 180A between the tow vehicle and travel trailer.  This setup might have worked with the theoretical ~400A capacity of the dual-alternator system, but the second alt was not cutting in the way they expected They ended up installing a DC-DC charger ([[https://amzn.to/3eTjIOa|30A Orion-TR]]) to limit current. +Mortons on the Move were able to overheat (not damage) the  225A primary alternator with 500Ah pulling 180A on 1/0 cables between the tow vehicle and travel trailer.  This setup might have worked with the theoretical ~400A capacity of the dual-alternator system, but the second alt was not cutting in the way they expected due to [[electrical:12v:alternator_details#ford|Ford's implementation]];  see [[https://youtu.be/uwrG3gUdDT8?t=500|the 8 minute mark in this video]]. 
 + 
 +They ended up installing a DC-DC charger ([[https://amzn.to/3eTjIOa|30A Orion-TR]]) to limit current. 
  
  
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 **300Ah** of LFP (no BMS) direct charged with a **90A** alternator.  This falls outside the van-relevant window since alternator rating <120A.   **300Ah** of LFP (no BMS) direct charged with a **90A** alternator.  This falls outside the van-relevant window since alternator rating <120A.  
  
-Victron damaged a 90A car alternator with 300A of lithium on a bench (no airflow as when driving).  They were able to charge the same bank from a 100A-120A((not clearly stated)) externally-regulated Balmar alternator.  The Balmar observes alternator temperature and regulates current to maintain alternator-safe conditions.+[[electrical:12v:drop-in_lifepo4#but_that_victron_video|Victron damaged a 90A car alternator with 300A of lithium on a bench]] (no airflow as when driving).  They were able to charge the same bank from a 100A-120A((not clearly stated)) externally-regulated Balmar alternator.  The Balmar observes alternator temperature and regulates current to maintain alternator-safe conditions.
  
 +=== 1977 RV ===
  
-=== lessons ===+The 60A alternator in RJS's 1977 RV died after approximately 1 year of direct-charging 200Ah of Lithium at rates up to 41A through a manual switch.  He indicates the alt was ≥15 years old at the time. 
  
-Think twice about  direct-charging banks that have 2-3x the capacity of your alternator's rating.   Use [[electrical:12v:b2b|a DC-DC charger]], external regulation, or current reduction with resistance as described elsewhere in this article. 
  
-Do not idle to charge. + 
 +=== 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. 
 +  - do not idle to charge. 
  
 ===== tweaking current with resistance  ===== ===== tweaking current with resistance  =====
  
 +We don't have a lot of control over alternator voltage (diode-based isolators nothwithstanding) but we can affect resistance. 
 +
 +
 +>> [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 ====
 +
 +>> using a copper cable ground between the starter and the auxiliary battery is actually part of the overcurrent problem. Using a frame ground tends to add a lot of resistance to the network... -- MechEngrSGH((https://www.irv2.com/forums/f87/alternator-burn-out-from-lithium-batteries-558284-11.html#post6434367))
 +
 +
 +Direct-charging setups typically only run the POS wire from the battery((or alternator)) to the relay;  NEG return (aka "ground") is made from the house bank to the vehicle chassis or frame.((there will also be a lightweight NEG on the combiner to allow it to run its own electronics))
 +
 +Steel has much more resistance than copper:
 +
 +>> copper (considered 100%), Aluminum is 71%, Brass is 25%, **steel is 12-14%** (depending on the alloy), lead (solder) is around 12% and 304 stainless steel (what stainless fasteners are made from) is 2.5%((YT comment by WorkingOnExploring))
 +
 +...so a chassis NEG return will increase resistance on that leg by something like 8x.  The mass of the chassis material is so great no heating will be observed. 
 +
 +Corollary:  it may be possible to tweak overall resistance by trying different NEG return points on the chassis.  
 +
 +
 +
 +==== case study ====
 + 
 The owner of the van in the higher-current outlier mentioned above had very low resistance in his wiring.  He added resistance to see the practical effect on current drawn by the house bank:((https://diysolarforum.com/threads/experiences-charging-lfp-from-alternator-without-a-dc-dc-charger.14884/post-234435)) The owner of the van in the higher-current outlier mentioned above had very low resistance in his wiring.  He added resistance to see the practical effect on current drawn by the house bank:((https://diysolarforum.com/threads/experiences-charging-lfp-from-alternator-without-a-dc-dc-charger.14884/post-234435))
  
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-Let's assume battery resting voltage is 13.0 and alternator voltage is 14.0 and circuit resistance is 20m Ohm.+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)/**0.020m Ohm** = 1v/0.020m Ohm = **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)/**0.025m Ohm** = 1v/0.025m Ohm = **40A**+  * I=V/R = (14v-13v)/**0.025R** = 1v/0.025R = **40A**
    
 ===== tweaking current with voltage ===== ===== tweaking current with voltage =====
  
-[[electrical:12v:alternator#solid_state_isolatordiode-based|Diode-based isolators]] incur voltage losses, averaging around 0.5v.  This will decrease the difference between battery and apparent alternator voltages, thereby reducing charging current.   
  
-It would also reduce final charging voltages, so this might allow vehicles with high chassis voltages like to safely charge LFP.  Example:  14.6v -> diode-based isolator -> 14.1v.  +Anything that reduces the voltage difference ("delta") between the bank and alternator will reduce current.  This delta is the ''V'' in ''I=V/R''.
  
 +==== diode-based isolators ====
  
-==== application ====+[[electrical:12v:alternator#solid_state_isolatordiode-based|Diode-based isolators]] incur voltage losses, typically around 0.7v.((germanium diodes are -0.3))  This will decrease the difference between battery and apparent alternator voltages, thereby reducing charging current.     
  
 +It would also reduce final charging voltages, so this might allow vehicles with high chassis voltages like to safely charge LFP.  Example:  14.6v -> diode-based isolator -> 13.9v.  
  
-Let's assume battery resting voltage is 13.0 and alternator voltage is 14.0 and circuit resistance is 20m Ohm. Diode isolator cause 0.4v loss.  We'll do it with a relay first: 
  
-  * I=V/R = (**14v**-13v)/0.020m Ohm = 1v/0.020m Ohm = **50A**+**Example**:  let's assume battery resting voltage is 13.0 and alternator voltage is 14.0 and circuit resistance is 20mR. Diode isolator cause 0.7v loss.  We'll do it with a relay first: 
 + 
 +  * I=V/R = (**14v**-13v)/0.020mR = 1v/0.020mR = **50A**
      
 Let's add the diode isolator((assumes the diode isolator has the same internal resistance as the relay)) to reduce apparent alternator voltage: Let's add the diode isolator((assumes the diode isolator has the same internal resistance as the relay)) to reduce apparent alternator voltage:
  
-  * I=V/R = (**13.6v**-13v)/0.020m Ohm = 0.6v/0.005m Ohm = **30A** +  * I=V/R = (**13.3v**-13v)/0.020mR = 0.3v/0.020mR = **15A**
-  +
-===== current demands of DC-DC chargers vs isolators =====+
  
-[For this section we will stipulate that alternator voltage is 14.0v and 60A DC-DC's charging setpoint is 14.4v and float is 13.4v.  The bank will accept 85A if directly connected to the alternator at low state of charge.] 
  
-Charging a 100Ah bank from 20% 100% SoC would take 80Ah.  Direct-charging and DC-DC both deliver the same 80Ah over time but in different ways:+==== state of charge ====
  
-{{https://img.mousetrap.net/misc/isolatorOverTime.jpg}}+When the bank State of Charge (and therefore voltage) is higher the delta and current will be reduced 
  
-This crude and fictional graph illustrates the //general patterns of current demand// on the alternator from both isolators (purple) and DC-DC chargers (green).+**Example**:  we would expect a bank at 12.5v accept more current than one at 13.5.  Using the 14.0v alternator output above, 
  
-The isolator starts out high at 85A and current starts falling off immediately.  It continues dropping gradually as battery bank voltage comes up to 14.0v at about 30 minutes.((made up timeline))  Current continues to drop but **voltage is held at 14.0v unless the isolator is disabled**.((the green bump at the end is a typo didn't catch until later -secessus))+  *  I=V/R = (14v-**12.5v**)/0.020mR = 1.5v/0.020mR = **75A** 
 +  *  I=V/R = (14v-**13.5v**)/0.020mR = 0.5v/0.020mR = **25A**
  
-The DC-DC holds a steady 60A until the 14.4v setpoint is reached around 50 minutes.  Current drops off steadily until Absorption durations times out at 100 minutes.((or however configured)).  DC-DC drops to float 13.4v and current tapers away.  
  
-==== breakeven ====+==== concurrent charging sources ====
  
-Note:  the crossover point between green and purple at ~20mins marks the time DC-DC charging passes more current than the isolator By about 35mins the approaches are at breakeven, both having returned about the same Ah to the bank.  DC-DC charging voltage might or might not have climbed higher. If one drives for longer periods the advantage may be with DC-DC.  If one makes short drives <35mins the isolator might win.+Even if SoC has not increased meaningfully yet other charging sources can drive up apparent bank voltage, thereby reducing the delta and current. This voltage "surge" during charging is the flipside of [[electrical:12v:voltage_sag|voltage sag]] during discharging.
  
 +In practice the most common [[electrical:12v:multipoint_charging|concurrent charging source]] for an alternator is solar.  
  
-==== isolator charging outside the knees ====+ 
 +**Example**:  Let's assume the same 14.0v alternator output and solar contribution that pushes up Vbatt by 0.3V 
 + 
 +  * I=V/R (14v-**13.0v**)/0.020mR 1.0v/0.020mR 50A without solar 
 +  * I=V/R = (14v-**13.3v**)/0.020mR = 0.7v/0.020mR = 35A with solar 
 + 
 +Note:  this occurs because alternators are voltage regulated while solar charge controllers can output whatever voltage they want internally as long as voltage setpoints at the terminals are exceeded.  So two solar controllers will each contribute their max no matter what the other does until the [[electrical:solar:charge_controller_setpoints|Absorption setpoint]] is reached. OTOH, current from the voltage-regulated alternator will decrease as other forms of charging push up Vbank and SoC.((if this effect is undesirable a DC-DC is also not voltage-regulated and will charge at full power under given circumstances)) 
 + 
 +===== current demands of DC-DC chargers vs isolators ===== 
 + 
 +For these examples we will assume alternator voltage of 14.4v and circuit resistance of 20mR. 
 + 
 +==== combiner charging current acceptance patterns ====
 {{ https://i.stack.imgur.com/U46cp.jpg?150}} {{ https://i.stack.imgur.com/U46cp.jpg?150}}
-Lead batteries have linear voltage curves.  Lithium has a relatively flat curve in the middle 80% and dramatic "knees" at either end (see graph at right).+Lead batteries have roughly-linear voltage curves.  Lithium has a relatively flat curve in the middle 80% and dramatic "knees" at either end (see graph at right).  In practice the **current acceptance pattern is //shaped// like the discharge voltage pattern seen to the right**
  
-  * at very low states of charge (<10% state of charge) voltage will be very low, around 12.0v.  This means current demanded will be maximal((I=V/R)), then quickly drop off as voltage comes up into the flat middle.  +  * at very low states of charge (<10% state of charge) voltage will be very low, around 12.0v.  This means current demanded will be maximal((I=V/R)) briefly, then quickly level off as voltage comes off the bottom knee~120A.   
-  * in the middle 80% voltage will be stable, in the 13.2v - 13.6v range. Charging current will be moderate and stable. +  * in the middle 80% voltage will be stable, in the 13.2v - 13.6v range. Charging current will be moderate and stable.  40A-60A, average 50A 
-  * at high states of charge (>90% state of charge) voltage will climb above 14.0v.  Current demand will be very low and remaining charging will be slow.  This gives ample opportunity to shut off the isolator manually.  +  * at high states of charge (>90% state of charge) voltage will climb above 14.0v.  Current demand will be very low and remaining charging will be slow.  This gives ample opportunity to shut off the isolator manually.  ≤20A
  
 +Word to the wise:  if your SoC is ≤20% you may want to allow solar or other charging to bring up voltage a bit before activating alternator charging.  At least make sure the vehicle is at cruising speed so the alternator can handle the brief inrush.  
 +==== DC-DC charging current acceptance patterns ====
  
-===== assessing your own setup for direct alternator charging =====+For this section we will assume the DC-DC is sized for the moderate currents seen in the middle 80% above (~50A) 
 + 
 +  * at very low states of charge current will be limited to the DC-DC's rated output, 50A 
 +  * in the middle 80% voltage will roughly be the DC-DC's rated output, 50A 
 +  * at high states of charge DC-DC will hold rated output slightly longer then taper faster at the end, 50A, then falling to 0A. 
 + 
 + 
 +==== comparison ==== 
 + 
 +{{ :electrical:12v:multi-dcdc.png?400|}} 
 + 
 +As long as we stay away from the bottom knee (lowest 10% of SOC) our //patterns// are largely the same.  Only the currents change. 
 + 
 +**Current levels with DC-DC will remain the same regardless of bank size**.((other than very small banks)) 
 + 
 +**Current levels with combiners will vary with bank size**, about 0.33C and tapering (see above). 
 + 
 + 
 +---- 
 + 
 + 
 + 
 +==== breakeven ==== 
 + 
 +Breakeven points (the amount of time it takes for each to replace the same Ah/Wh) are easy to assess with DC-DC since their output is table and difficult with combiners since they taper with bank voltage. 
 + 
 +But we can try to estimate how much driving it would take to replace 10Ah in a 200Ah bank (going from 50% to 55% SoC, for example: 
 + 
 +  * 60A DC-DC - 10 minutes 
 +  * 50A DC-DC - 12 minutes 
 +  * combiner - ~12 minutes 
 +  * 40A DC-DC - 15 minutes 
 +  * 30A DC-DC- 20 minutes 
 +  * 20A DC-DC - 30 minutes 
 + 
 +If the SoC was very low (near cutoff) the combiner would slightly faster, ~7.5 minutes.  
 +===== reasons NOT to attempt direct charging =====
  
-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".  +  * 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 **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 chargingif you are driving long enough to fully charge the bank.+  * 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:  
  
  
-  - if the isolator is already in use with lead-chemistry banks before the upgrade, //pay attention to the voltage and current at which the existing bank is being charged//.  This is your baseline for alternator chargingAh-for-Ah lithium //might// draw more current than lead, but Li banks are typically lower in Ah than the lead banks they replace as DoD can be lower.((Lithium requires about 0.62 as much Ah as lead for the same amount of //usable// capacity.)+===== assessing your own setup for direct alternator charging ===== 
-  - Read and understand your Li battery manufacturer's charging specs - undercharging is fine((assuming one stays above 20% State of Charge)); overcharging is less so but is relatively rare in an isolator context. + 
-  - Read and understand your alternator's specs +[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.] 
-  - measure how much current from the alternator your vehicle requires to run its own loads.  Since alternators can, //while driving//, run continuous current ~1/2 of their rating this means the bank can use (ALTERNATOR RATING / 2) - VEHICLE LOADS.  If your bank wants to pull more than that an appropriately-sized DC-DC may be required.  + 
-  - Read and understand your isolator's specs and functionality + 
-  - observe your vehicle's chassis voltage during normal operation - note that the voltage //at the battery's location// will likely be lower due to long wiring and also lower when the wiring is carrying a hefty charging current.   + 
-  - make a first approximation 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.   +If you are still interested, here is one approach to assessing your setup for direct charging lithium.   
-  - install [[electrical:12v:battery_monitor|a battery monitor]] so you can observe current and voltage at the battery.  Or use the battery's own BT access. + 
-  - ensure the fuse between your chassis and battery bank is sized so //you cannot draw more than the alternator and wiring can handle// +  - pay attention to the **chassis' normal voltage** while idling and cruising - this is a benchmark for how the vehicle behaves without aux charging.  
-  - 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.  Turn off the engine.+  - if the isolator is already in use with lead-chemistry banks before the upgrade, pay attention to the **voltage and current at which the existing bank is being charged**.  A [[electrical:12v:battery_monitor|a battery monitor]] or clamp meter will be very helpful here  Specifically we are interested in: 
 +      **current acceptance at 50% State of Charge** this will be the highest level of current the battery sees 
 +      - **voltage when the bank is at 100% State of Charge** (likely close to chassis voltage- this will be the highest voltage the battery sees 
 +  - Read and understand your **Li battery manufacturer's charging specs** - undercharging is fine((assuming one stays above 20% State of Charge)); overcharging is less so but is relatively rare in an isolator context. 
 +  - Read and understand your **alternator's specs**.  If your vehicle is ok charging a similar-capacity AGM then it will probably be fine with Li.  If you want more info: 
 +    - measure how much current from the alternator your vehicle requires to run its own loads.  Since alternators can, //while driving//, run continuous current ~1/2 of their rating this means the bank can use (ALTERNATOR RATING / 2) - VEHICLE LOADS.  If your bank wants to pull more than that an appropriately-sized DC-DC may be required.  
 +  - Read and understand your **isolator's specs** and functionality 
 +  - 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// 
 +  - 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.   - 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 +  - 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 =====
Line 232: Line 335:
  
  
 +===== objections =====
 +
 +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. 
 +
 +==== the alternator cannot fully charge LFP ====
 +
 +Yes, it can.  LFP will charge to ~100% SoC quite smartly at ≥13.8v.  Does your alternator put out ≥13.8v?  LFP will actually charge to ~100% SoC at voltages as low as 13.6v (3.4Vpc), it just takes longer than most of us drive in one stretch.  
 +
 +Which brings us to the next point:  LFP does not //need// to be fully charged [[electrical:12v:psoc|the way lead does]].  Get as much charge as you need and stop if you want.  Or keep going.  
 +
 +==== BMS disconnect will damage the alternator ====
 +
 +  - 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?
 +
 +==== 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 =====
 +
 +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.1667665741.txt.gz · Last modified: 2022/11/05 12:29 by frater_secessus