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--- electrical:12v:alternator_charging_rates [2023/09/10 12:45]
frater_secessus [TLDR]
+++ electrical:12v:alternator_charging_rates [2023/09/10 13:25] (current)
frater_secessus [estimating combiner charging]
@@ Line 1: / Line 1: @@
-====== How much will by battery charge by alternator? ======
+DRAFT
+====== How much will my battery charge by alternator? ======
 This should be easy, right?
@@ Line 5: / Line 8: @@
 ''time charging x charge rate in A = Ah replaced in the bank''
-We know the time part.  But the charge rate part can be hard to predict with certainty.
+We know the time part because we're the ones driving.  But the charge rate part can be hard to predict with any certainty;  this is especially true for combiner ("split charger") charging.
 ===== TLDR =====
-  * [[electrical:12v:b2b|DC-DC]] charge rate is usually the rated output, so roughly speaking hours driving x rated output Ah of charging
+  * [[electrical:12v:b2b|DC-DC]] charge rate is usually the rated output, so roughly speaking //hours driving x rated output = Ah of charging//
-  * charging with a relay is less predictable because current and voltage are not actively managed
+  * charging with [[electrical:12v:alternator#combiners|a relay]] is less predictable because current will vary over time
-    * The charge rate will likely start off ≤0.33[[electrical:12v:battery_capacity|C]] (33A per 100Ah of capacity) and taper off as the battery charges.
+    * The charge rate will likely start off ≤0.33[[electrical:12v:battery_capacity|C]] (33A per 100Ah of capacity) and //taper// off as the battery charges.
     * the shape of the taper hinges chiefly on battery chemistry
+    * this unpredictablity is less important when [[electrical:12v:alt_and_solar|solar is added]].
 ===== what limits the current? =====
@@ Line 32: / Line 32: @@
 Since current is a function of C, **bigger banks will pull more current than small ones**.  At 0.2C a 100Ah bank will pull 20A and a 200Ah bank 40A.
-Banks will pull more current at lower states of charge and less current at higher states of charge.  This affects combiners more than DC-DC chargers (see below).
+Banks will **pull more current at lower states of charge** and less current at higher states of charge.  This affects combiners more than DC-DC chargers (see below).
@@ Line 46: / Line 46: @@
-==== charging method ====
+===== estimating DC-DC charging =====
+DC-DC charging rates are more predictable because of how they work.  A 20A DC-DC will likely pump 20A into the bank most of the time.    So 1/2 hour driving x 20A = 10Ah returned to the battery bank.  The price of this predictability (and other features) is... price.
+If you need predictable charging then DC-DC is likely the answer.  There are [[electrical:12v:alternator#limitations|other scenarios]] where DC-DC is effectively required.
+===== estimating combiner charging =====
+This is the tough one.  We know roughly where the charge rate will start out for discharged batteries (≤0.33C) but the taper complicates things.  All the batteries would eventually charge to full((or nearly so)) given enough time, so shorter charging periods are what interest us.  Let's assume a short drive = 30 minutes.
+**NOTE**:  this section contains guesswork from observations and theory.  You will learn how your particular setup behaves in actual use.
-There are two main ways to charge from the alternator:  [combiners](https://rvwiki.mousetrap.net/doku.php?id=electrical:12v:alternator#combiners) (relays, split chargers) and [DC-DC chargers](https://rvwiki.mousetrap.net/doku.php?id=electrical:12v:b2b).
+  * **FLA** taper is steep;  charging current starts at ~0.2C and quickly falls off within seconds or minutes.  Current will likely stabilize around 0.1C by the end of the drive.  So we might use an average of 0.12C for our charge rate when estimating.
+      * 100Ah x .12C = 12A.
+      * 0.5hours of driving x 12A = 6Ah replaced.
+  * **AGM** taper is more gradual, so charging currrent stays higher on short drives. It might only taper to 0.15C by the end of the drive.  We might use 0.175C as our average rate. 8.75Ah replaced.
+  * **Lithium** has an odd taper because the charging voltage curve is not linear.
+      * in the broad "flat" portion of the voltage curve (20%-80 state of charge) current will likely stabilize around 0.2C.  We could use 0.2C as our average rate.  10Ah replaced.
-Combiners are simple connections, and so the charge rate will vary based on alternator voltage, bank voltage, and resistance in the circuit (including the battery itself).  We can safely say **the bank will draw more current at low states of charge** and less current at higher states of charge.
+==== in the real world ====
-T
+In most installs this variability isn't a major issue:
-DC-DC charging rates are more predictable because of how they work.  A 20A DC-DC will likely pump 20A into the bank most of the time.    So 3 hours driving x 20A = 60Ah returned to the battery bank.  The price of this predictability (and other features) is... price.
+  * combiner charging is commonly paired with solar charging (also highly variable!)
+  * alternator charging is often a backup or "nice when you can get it" charging source.  Exception:  there are [[electrical:12v:mandatory_solar|scenarios that depend on it]].

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