Hi Roy,

On 22-Oct-03, Roy J. Tellason wrote to Jason Vierik:


 RJT> Jason Vierik wrote in a message to Jasen Betts:

  >> GM> But a voltmeter that reads 12 volts... It's a 12 volt battery,
  >> so GM> everything must be alright, mustn't it?  When the
  >> altenator's running it should read closer to 14...

 JV>> Yeah, doesn't an altenator run at or around 13.8?

 RJT> 13.6-13.8 is considered "float" voltage for a charged battery,
 RJT> you could consider that a reasonable minimum.  When I was selling
 RJT> batteries we looked for 13.5-14.5,  with the engine running.
 RJT> Some vehicles went even higher,  though 16 volts was considered
 RJT> about the maximum acceptable figure.  Any higher than that and
 RJT> there were regulator problems,  especially if the battery was
 RJT> already charged at that point

2.3 V/cell is considered float charge voltage. 2.4 V/cell is cyclic
charge voltage, and an automotive application (apart from long haul
trucks/busses) is a cyclic application. With modern sealed batteries I
would be concerned at any regulator that let the battery charge
voltage rise above 2.45 V/cell (thats 14.8 Volts for a car), the
chances of gassing off and thus drying out the cell are too high.
Fork lift trucks and the like tend to go to slightly higher voltages
to get better capacity, but 2.5 V/cell is as far as I would normally
consider safe.

 RJT> And regulators _were_ a problem all right.  I saw some,
 RJT> especially those found in Ford products,  that would leak current
 RJT> to ground when the car was turned off.  Some would go dead short.
 RJT>  One guy with a brand new battery in the car was irate the next
 RJT> day and had his car towed in to the garage because that new
 RJT> battery was stone cold dead.  The garage brought it over to me,
 RJT> and the terminal voltage was darn near zero!  I did charge it
 RJT> back up,  and it recovered okay,  the culprit in his case turning
 RJT> out to be the regulator..

There are some aweful designs out there. A magazine published one
once and one of my collegues sent them a letter pointing out all the
problems with the design. They wrote back saying the one they built
worked OK for them so he (in fact it was we) were in error, there were
no problems...

In fact there were very significant problems, but it's too long ago
for me to remember the details. Things like insufficient gain if the
transistors were at the low end of their gain range, poor choice of
zener voltage, and more beside...

George

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