Battery Ratings

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From: John_Foote_at_HDN-BCSE@ccgate.ml.nec.com
Date: Thu Nov 30 1995 - 16:38:28 EST


     Can anyone help me with a battery question?
     
     I thought, and have read, that (lead acid, "stationary") batteries
     deliver current to a load as follows:
     
     First, take the amp hr. rating, which is AMPS X HOURS
     
     Next, divide by the hours to get the amps delivered to the load during
     that time. But by what time do you use here (to divide)?
     
     So, for example, a "200 amp-hour" battery should deliver 25 amps for
     8 hours -- OR -- 50 amps for only 4 hours.
     
     But here's where the "stationary battery" manufacturers put in their
     footnotes.
     
     (The faster the battery is discharged, the lower its EFFECTIVE amp-
     hour capacity.) Standard ratings ASSUME that the time for discharge
     is 8 hours, when establishing the published amp-hour rating for any
     particular battery.
     
     The conclusion is that the 200 amp-hour battery will not deliver 50
     amps for 4 hours; it delivers somewhat fewer than 50 amps.
     Stationary batteries usually come with a chart that essentially
     "de-rates" the "8 hr. AMP-HR" rating so you can find out how much
     current to expect in a shorter (or longer) time period.
     
     BUT. I read that gell cell batteries used by hams typically have
     ratings of 4.5 to 7 amp-hour. If I use the 8 hour rule to get the
     current delivered to my rig the answer makes little sense. A 7.5
     amp-hour gell cell would give me about 930 ma. Now I know that guys
     (and gals) take 7.5 ah batteries into the field and support QRP rigs
     (that need about 3 amps on transmit). With only 900 ma expected,
     the gell cell will be dead in no time.
     
     Question: Is the assumed discharge time for establishing ratings of
     small batteries standard? Or does it vary with the battery and the
     manufacturer? If so, what is it? If not, what so I use?
     
     Thanks for the info. Next we can discuss the assumed "end voltage"
     for a gell cell when establishing the discharge time under a given
     load. (But not now.) Whew!
     
     72 de KR4GL
     J. Foote
     
     
     
      


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