Parallel batteries
PARTS AND MATERIALS
High-wattage 12-volt lamps may be purchased
from recreational vehicle (RV) and boating supply stores.
Common sizes are 25 watt and 50 watt. This lamp will be used
as a "heavy" load for your batteries (heavy load =
one that draws substantial current).
A regular household (120 volt) lamp socket
will work just fine for these low-voltage "RV" lamps.
CROSS-REFERENCES
Lessons In Electric Circuits, Volume
1, chapter 5: "Series and Parallel Circuits"
Lessons In Electric Circuits, Volume
1, chapter 11: "Batteries and Power Systems"
LEARNING OBJECTIVES
SCHEMATIC DIAGRAM
ILLUSTRATION
INSTRUCTIONS
Begin this experiment by connecting one
6-volt battery to the lamp. The lamp, designed to operate on
12 volts, should glow dimly when powered by the 6-volt
battery. Use your voltmeter to read voltage across the lamp
like this:
The voltmeter should register a voltage
lower than the usual voltage of the battery. If you use your
voltmeter to read the voltage directly at the battery
terminals, you will measure a low voltage there as well. Why
is this? The large current drawn by the high-power lamp
causes the voltage at the battery terminals to "sag" or
"droop," due to voltage dropped across resistance internal
to the battery.
We may overcome this problem by connecting
batteries in parallel with each other, so that each
battery only has to supply a fraction of the total current
demanded by the lamp. Parallel connections involve making
all the positive (+) battery terminals electrically common
to each other by connection through jumper wires, and all
negative (-) terminals common to each other as well. Add one
battery at a time in parallel, noting the lamp voltage with
the addition of each new, parallel-connected battery:
There should also be a noticeable difference
in light intensity as the voltage "sag" is improved.
Try measuring the current of one battery and
comparing it to the total current (light bulb current).
Shown here is the easiest way to measure single-battery
current:
By breaking the circuit for just one
battery, and inserting our ammeter within that break, we
intercept the current of that one battery and are therefore
able to measure it. Measuring total current involves a
similar procedure: make a break somewhere in the path that
total current must take, then insert the ammeter within than
break:
Note the difference in current between the
single-battery and total measurements.
To obtain maximum brightness from the light
bulb, a series-parallel connection is required. Two
6-volt batteries connected series-aiding will provide 12
volts. Connecting two of these series-connected battery
pairs in parallel improves their current-sourcing ability
for minimum voltage sag:
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