Battery Ratings

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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