OT - Battery ratings

[Ken Kopp]

I apologize if this ends up as a duplicate posting

----- Original Message -----
From: Ken Kopp
To: [hidden email]
Sent: Saturday, 09 February, 2008 19:52
Subject: OT - Battery ratings


The "real" AH ratings for batteries ... the kind we're referring to
in this topic ... are difficult to learn.  Most automotive-related
ratings are specified in"cranking amperes", and this is -not- the
rating that of interest here.  It's consumer hype.

I'm retired from an electric utility's communications department
where we used various typed of LARGE battery banks.  Our
microwave stations all had "large" battery banks.  Few realize
that substation's relays and/or switching is done with 120 VDC.  
(They gotta function when the power's off.)  Ditto for telephone
systems at 48 VDC.  A source of battery info ... in general ... can
be found on the Trojan Battery site.

I have an RV with 440 AH of Trojan batteries, a wind generator and
12A of solar panels on the roof, BTW.

73! Ken Kopp - K0PP
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[N2EY]

In a message dated 2/9/08 3:24:59 PM Eastern Standard Time, [hidden email]
writes:


> The "real" AH ratings for batteries ... the kind we're referring to
> in this topic ... are difficult to learn.  Most automotive-related
> ratings are specified in"cranking amperes", and this is -not- the
> rating that of interest here.

Agreed. That's because amp-hour rating doesn't mean much by itself in auto
applications.


 It's consumer hype.

Not at all.

A modern car battery really has only two jobs:

1) To deliver the enormous currents required to start the engine, in all
temperatures and after having sat idle for long periods of time.

2) To power the electrical system for short periods when the engine isn't
running or the charging system fails.

1) is the main job, 2) is secondary. Modern alternator/charging systems are
designed to power everything on the car with the engine at idle.

What we hams want for backup (small currents for long periods of time) is the
opposite of what an engine-cranking battery is built to do.
>
> I'm retired from an electric utility's communications department
> where we used various typed of LARGE battery banks.  Our
> microwave stations all had "large" battery banks.  

I work in railroad signalling, with similar requirements. The battery banks
can get quite impressive, such as sixty 420 AH lead-acid cells for the switch
machines - and that's one bank in one medium-size location.

For standby service, some RRs use wet-cell NiCads, made by SAFT and other
companies. They have better regulation, longer life, and tolerate things like
being discharged down to almost nothing. They also stand extreme temperatures
better. But they are only 1.25 volts per cell and cost more than lead-acid.

Two other factors:

Car batteries have to tolerate not only heat and cold but shock and
vibration. Stationary batteries usually have much better conditions.

Amp-hour ratings are dependent on discharge rate and final voltage - the
higher the rate and/or final voltage, the lower the apparent amp-hours. A typical
rating is 8 hours and 1.75 volts per cell such as "40 AH at the 8 hour rate,
to 1.75 volts per cell", which means that a fully-charged new battery can
deliver 5 amps for 8 hours before its voltage falls to the specified 1.75 volts per
cell. (Note that 1.75 volts per cell in a six-cell battery is only 10.5
volts)

At higher rates, the same battery will appear to have fewer AH - at a
discharge rate of 10 A, it may only last 2 hours before reaching 1.75 volts per cell.
OTOH, at low rates the cell may deliver much more than the 8 hour rating.
Similar results from accepting higher or lower final volts-per-cell.

73 de Jim N2EY


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