XPost: uk.rec.driving, alt.home.repair, uk.d-i-y   
   From: CFKinsey@military.org.jp   
      
   On Sun, 23 Jun 2019 17:27:09 +0100, TMS320  wrote:   
      
   > On 22/06/2019 22:58, Commander Kinsey wrote:   
   >> On Sat, 22 Jun 2019 17:03:08 +0100, TMS320  wrote:   
   >>> On 21/06/2019 21:19, Commander Kinsey wrote:   
   >   
   >>>> How does a lead acid battery charger (or car alternator) know when to   
   >>>> switch to trickle charge?   
   >>>   
   >>> It doesn't.   
   >>   
   >> Mine does, if I start my car when the battery is say 80% full, the   
   >> voltage will be 14.4V.  After a while, something causes that voltage to   
   >> drop to 13.8, because something knows the battery is full and should no   
   >> longer be charged at a high rate.   
   >  >   
   >>>> 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?   
   >>>   
   >>> Ohm's law.   
   >>   
   >> Explain how an alternator or charger can use ohm's law to distinguish   
   >> between:   
   >   
   > To determine current, measure the voltage across a series resistor, duh.   
      
   Why didn't you wait until I'd finished?   
      
   >> 1) A car battery which is full, with a load of 10 amps connected to it,   
   >> like two headlights.   
   >> 2) A car battery with no load, which is not full yet and draws 10 amps   
   >> for the charge.   
   >   
   > Two resistors?   
      
   Yes that would work, but not if it has no external sensors (as in external to   
   the alternator).   
      
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