XPost: uk.rec.driving, alt.home.repair, uk.d-i-y   
   From: CFKinsey@military.org.jp   
      
   On Fri, 21 Jun 2019 21:54:38 +0100, Rod Speed  wrote:   
      
   > Commander Kinsey  wrote   
   >   
   >> How does a lead acid battery charger (or car alternator) know when to   
   >> switch to trickle charge?   
   >   
   > From the current the battery takes.   
   >   
   >> I can understand it noticing a drop in charging current if the battery is   
   >> on its own, but what if a random changing load is connected, as there is   
   >> in a running car?   
   >   
   > You just look at the current going to the battery. The variably   
   > loads like with lights isnt supplied by the battery when the   
   > engine is running, its supplied by the alternator.   
      
   But how can the regulator on the alternator possibly know the current it's   
   passing to the battery is going into the battery and not going straight across   
   to the lights?  If you look at the battery in your car, there are two or three   
   thick wires coming    
   off each terminal.  One will go to the alternator, another to the fusebox for   
   all the lights etc.  Unless there's some clever circuitry monitoring each   
   battery wire individually and subtracting the currents, the alternator can't   
   tell the difference    
   between a battery taking 12 amps, and a battery taking 2 amps plus lights   
   taking 10 amps.  The second one requires switching to trickle charge, the   
   first doesn't.   
      
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