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--- opinion:frater_secessus:lfprationality [2024/03/21 19:36]
frater_secessus ["what you need is a Yeti mug"]
+++ opinion:frater_secessus:lfprationality [2024/03/21 20:30] (current)
frater_secessus [limiting current from the alternator]
@@ Line 31: / Line 31: @@
 **Yeti aficionado:** If you don't use a Yeti the coffee will pour at up to 1000 degrees!!!!! \\
 **Me:**  Ok, that's not how the physics of water works.  At normal atmospheric presssures.....  \\
-**Yeti aficionado:** What is all this mumbo-jumbo about atmospheres?  Buy a Yeti and be done with it.
+**Yeti aficionado:** What is all this mumbo-jumbo about atmospheres?  Buy a Yeti and be done with it. \\
-**Me:**  You don't have to memorize the minutiae.   The larger point is water boils at about 212 degrees F under normal conditions.  And you can force that temp up or down, if desired, by adjusting the air pressure.
+**Me:**  The larger point is water boils at about 212 degrees F under normal conditions.  And you can force that temp up or down, if desired, by adjusting the air pressure. We can measure this temperature with a thermometer.
@@ Line 47: / Line 47: @@
 **Me:** <sigh>
+Ok, enough silliness and satire.  Let's get serious.
 ===== what DC-DC is designed to do =====
@@ Line 53: / Line 55: @@
 ==== limiting current from the alternator ====
-An alternator has an Amp rating and [[electrical:12v:alternator_details#current|only so much power it can donate to house battery charging]] under given conditions.  In the absence of better information we can assume:
+An alternator has an Amp rating and [[electrical:12v:alternator_details#current|only so much power it can donate to house battery charging]] under given conditions.  In the absence of official information we can ballpark:
   * the alternator can output about half of its rating continuously //while driving at normal speeds//.
   * both the vehicle's own power requirements (lights, engine computer, fans, etc) also have to come from available power
+  * very conservative use of the alternator might be 1/4th of its rated capacity((while driving)), moderate use might be 1/3rd of rated, and aggressive use might be 1/2 of rated.
+  * if the alternator is overtasked (especially when airflow is impaired) it can overheat and fail.
+  * DC-DC chargers have stable current outputs;  direct charging current tapers as the bank charges.
+=== 200Ah example ===
+Let's assume we have a 200Ah bank and a 150A alternator.  Let us further assume we have decided that 1/3rd of rated alternator output (50A) continuous is acceptable while driving.  We can handle brief excursions to 75A (1/2 of rated) so we have a 75A fuse on the POS wire.
+First things first - **how much current would the bank pull** if you connected a wire from the starter battery POS to the house bank POS?
+  * **theory** tells us that I=V/R.  This means that current will be the ([alternator voltage] - [resting voltage of the battery] / total resistance in the circuit.   We almost always know the voltage part but few know the resistance of their charging circuit.  After an observation we can back into that value.
+  * an **often-repeated (and wrong)** answer is 200A, because the BMS cuts off at 1C.
+  * **testing** tells us(([[electrical:12v:directcharginglfp|article]]))
+    * current at middling states of charge averages ~0.2C, or 40A.
+    * current at high states of charge((upper knee)) averages ~0.1C, or 20A.
+    * current at low states of charge((lower knee)) averages ~0.32C, or 64A. <- we need to think about this
+A is clearly above our 50A target for continuous current but below our 75A max.  There are two factors in play that make this Generally Not A Big deal:
+  - direct charging current starts tapering immediately after charging starts (the V in I=V/R is ever-decreasing); and
+  - LFP voltage pops out of the lower knee quite quickly after charging starts (the V in I=V/R is suddenly reduced).
+Related idea:   - **how much current would the bank pull** if you connected 50A DC-DC between the starter battery a wire from the starter battery POS to the house bank POS?
+The obvious answer is 50A, but there's an asterisk.  Any time the DC-DC is boosting voltage((starter battery 14.2v but charging house bank at 14.4v, for example)) //it will pull more current from the alternator than it sends to the house bank//.   So we would probably [[electrical:12v:b2b#sizing|size]] a 40-45A DC-DC to make sure we don't have >50A continuous.
+=== 100Ah example ===
+Let's do it again with the same setup but a 100Ah bank.
+  * the DC-DC will charge at 50A <- 0.5C is a little high, we should think about that
+  * direct charging current at middling states of charge averages ~0.2C, or 20A.
+  * direct charging current at high states of charge((upper knee)) averages ~0.1C, or 10A.
+  * direct charging current at low states of charge((lower knee)) averages ~0.32C, or 32A.
+Now which one is harder on the alternator?
+=== 300Ah example ===
+Once again, 300Ah bank
+  * the DC-DC will charge at 50A
+  * direct charging current at middling states of charge averages ~0.2C, or 60A. <-- not good, as this will be continuous
+  * direct charging current at high states of charge((upper knee)) averages ~0.1C, or 30A.
+  * direct charging current at low states of charge((lower knee)) averages ~0.32C, or 96A. <-- clearly unacceptable
+There is a good chance DC-DC will be required here to stay at our 50A continuous max.  But there is little harm in turning the key to take quick measurements since our wiring is fused at 75A.  If current is as high as predicted the fuse pops and we start thinking about a DC-DC.
+==== output specific charging voltage[s] ====

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