[Elecraft] Lead-Acid Battery Capacity (WAS: K3 : standby current)

[Ron D'Eau Claire]

... more like 50 AH. 

The biggest car battery I ever dealt with was for a VW diesel, and it was 
only 63 AH. Had to turn a 2 horsepower starter against the 23:1 compression
ratio 
of the engine, *and* feed the glow plugs which required more than 30 amps.

73 de Jim, N2EY

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In the USA at least, AH ratings for car batteries are hard to find. Most of
them are rated for the actual service in ways that make them difficult to
assess for running a radio. Three common ratings are the "Cranking Amperes"
(CA), "Cold Cranking Amperes" (CCA) and "Hot Cranking Amperes" and (HCA)
indicating how many amperes the battery can deliver at some given
temperatures (usually ranging from -18C to about +20C). They are seldom very
useful in determining how long you can run a radio like the K2 or the K3
from it, especially at 100 watts. 

Perhaps the closest indicator  is the "Reserve Capacity Minutes" (RCM). It's
the number of minutes a battery can deliver a significant current -
something like 25 amperes. Using that number one can make an educated guess
about how long it can deliver current to support transmitting with a
100-watt rig, but it's not a perfectly linear curve. You can't count on
multiplying the time by some factor for lower receive currents or lower
transmit powers. 

Most car batteries are designed to deliver short bursts of energy to operate
the starter and then "float" on the line while most of the electronics run
off of the alternator. These batteries have a larger number of relatively
thin plates to provide the greatest current flow through the electrolyte
during peak demands, but the thin plates limit the time the battery can
deliver the current. 

Deep discharge batteries designed for continuous service have fewer, thicker
plates to provide more ions for more current over time, but they have a much
smaller capacity to deliver high currents for a short time. 

Getting optimum performance is a matter of carefully matching the actual
demands of the rig and how it will be used with the right battery but,
typically, the "Golf Cart" or "Electric Chair" types designed to deliver
nominal currents over a longer time are the most suitable for radio use. 

Ron AC7AC