Ask A Scientist© Environmental Earth Science Archive

status       other
age          60s

Question -   Is the rate at which batteries in a portable radio run
down affected by the volume at which the radio is played?
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In my opinion, ABSOLUTELY.  I haven't designed any radio's / demodulators
etc... But my guess is that the speaker and final amplifier are the most
"power hoggish" elements of the entire unit.

While camping, I use a power inverter to convert 12.6 Vdc to 120Vac (wall
current) so we can listen to the stereo and play CD's etc...  Well, the
inverter has a LED bar graph meter on it that shows power consumption (via
an LED bar graph ammeter).  The bar graph clearly moves up 1 or sometimes 2
"bar graph notches" when the stereo volume is turned up.

Great question.
Darin Wagner
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Difficult to say. A battery has a certain amount of chemical energy stored
in it. This energy is dissipated by the volume, but also by other losses
(Ohm's law), the type of receiver, etc. It is difficult to say how the
various pathways of energy loss will play out.

Vince Calder
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Since energy is conserved, the energy in the sound waves must come from
somewhere and the only source of energy is the batteries.  However, the
electronics could change the dependence of the rate of discharge on the
volume by converting more electrical energy to heat at low volume than at
high volume.  This is a function of the detailed design of the electronics.
Nonetheless, I would think that in general high volume runs the batteries
down faster than low volume.

It would be interesting to know.  If you have access to a battery operated
radio and have rechargeable batteries and a charger, you could measure the
lifetime of the battery charge at minimum and at maximum volume.  You might
want to repeat the experiment several times to see if the discharge times
are consistent.  It would be too expensive to do this experiment with normal
batteries, I would think.  Furthermore, recharging the same batteries in the
same charger would, I believe, generate more reproducible results than using
different batteries.

If you do this experiment, I would dearly love to see the results.  Thank
you in advance.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University
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Battery drain does depend on the volume.   Some of the power from the
batteries is used to tune in the radio station, demodulate the signal, and
other things.  The rest of the power is used to run the loudspeaker.
Playing the radio loudly will run the batteries down faster.  I measured the
power use of my Walkman.  With the volume turned down, power use was 30
milliwatts.  At modest listening volume, power was 30 milliwatts.  At fairly
loud listening volume, power was 38 milliwatts.  At full volume, which would
be too loud for listening, power was 60 milliwatts.

Bob Erck
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Yes.  High sound volumes usually use up batteries faster.
How much faster, depends on the design of the radio's electronic circuitry.
The "background" parts of the circuitry draw the same battery current no 
matter what sound is played.
But the final audio amplifier stage converts DC battery current to AC 
current in the speakers, which can be up to a few watts, very substantial 
for a battery.
Sometimes the effect is only noticeable for the highest 6dB of the radio's 
sound volume range.
Other times the background circuitry could be so efficient, so miserly, 
that the top 20-30dB matters,
and your battery life is determined primarily by how much sound you choose 
to hear.
And that refers to sound actually produced, not to the setting of the 
volume control knob.
Almost all final amplifier circuits these days are good enough to only 
draw large battery currents when actually driving the speaker with large 
sound currents.
With switching circuits they could do about a factor of 2 better in 
battery-to-speaker-current conversion efficiency,
and in a few years I suspect they will start doing so.

It's also true that a typical alkaline battery run at maximum output power 
will give you about 2x fewer amp-hours than the same battery run at 16x 
lower currents.
High currents do not allow "fuels" and "waste products" to redistribute as 
fast as they are used or generated, so the electric current must to go 
farther through "battery gel" to reach the electrode, so extra series 
resistance is experienced.  Rechargeable
Nicads, NiMH's, and lead-acids do not experience this.

Jim Swenson
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