[Elecraft] OT - Battery ratings

[N2EY at aol.com]

In a message dated 2/9/08 3:24:59 PM Eastern Standard Time, k0pp at acninc.net 
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


**************
Biggest Grammy Award surprises of all time on AOL 
Music.
     
(http://music.aol.com/grammys/pictures/never-won-a-grammy?NCID=aolcmp00300000002548)