From my experience with batteries on submarines....
It's rather standard to rate batteries (a configuration of
electrochemical cells) by a few different quantities:
- no-load voltage (open circuit) at full charge
- operating voltage (at a specified current) near full charge
- operating voltage (at a specified current) near end of useful charge
- maximum design current draw
- capacity, in ampere hours
- useful energy content (at a specified current)
- etc.
The useful energy content is the integral of voltage with respect to
time and then multiplied by the standard current. The bounds of this
integral are the voltages at full-charge and at minimum recommended
useful voltage. Drawing a current from a battery below minimum
recommended voltage can cause damage to the battery. In lead-acid
batteries one of the cells could reverse voltage, start outgassing
copiously, and cause an explosion. Another mode of failure is the
development and growth of dendrites that bridge the electrolyte to short
out the two poles in a cell.
A current of 10 A drawn from a nominal 6 V battery for 1 h delivers less
energy than a current of 10 A drawn from a nominal 12 V battery for 1 h.
Jim
Bill Hooper wrote:
On 2008 May 10 , at 12:45 AM, LPS wrote:
I have a battery that is hooked up to a bicycle to assist in getting
me up hills and such. It is represented as a 42 volt 20 ampere hour
battery.
I figure that a·h is really not using SI properly. The hour is not the
unit of time -- it is the second that would be correct.
So to properly indicate the capacity of the battery should I be using
Coulombs?
LPS,
It is true that multiplying amperes by a time unit will tell the total
electric charge that has flowed though your system in that time (and if
the time is expressed in seconds the charge will be in coulombs).
However, in operating you bicycle, it would seldom be of any value or
any interest to know the total amount of charge that has flowed
(essentially, the number of electrons).
The reason that the ampere-hour (A.h) is used at all is that the time of
use is of interest. If you know "the ampere-hours"* and how much current
(in amperes) is flowing through your system, then you know how long it
can run. That amount of time would conventionally be measured in hours.
So, just because ampere-hours could be measured in coulombs does not
mean it is necessary to do so. It is a bit like the measure
of gasoline usage in litres per 100 kilometres. It can be shown that
litres per kilometre can be expressed as an area (in square metres).
It's fun to figure out what the hell area that might represent, but is
certainly neither important nor useful. Thus, no one suggests that we
should measure gasoline consumption in square metres.
So go ahead and use your battery capacity in ampere-hours (or
ampere-seconds if you want to be pure SI) but I see no value in
insisting that it be expressed in coulombs just because it could be.
==================
*That thing that is measured in ampere-hours should have a name but I
don't know what it is. I find it objectionable to call something by its
unit of measure. It could perhaps be called "the capacity of the
battery", or something.
Regards,
Bill Hooper
Fernandina Beach, Florida, USA
==========================
Make It Simple; Make It Metric!
==========================
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James R. Frysinger
632 Stony Point Mountain Road
Doyle, TN 38559-3030
(H) 931.657.3107
(C) 931.212.0267
