Table of Contents
Advertisement
2.1.3 Determining Life
Expectancy of Charge
2.1.4 Charged and
Discharged Cell
Voltages
In this example, a flow meter has an average current draw of 26
mA. We want to power it with an Isco Nickel-Cadmium Battery.
The battery has a capacity of 4.0 Ah. First, convert 4.0 Ah into
4,000 mA-h (milliampere-hours). Then divide 4,000 mA-h by 26
mA. The result is 153.8 hours. Dividing 153.8 by 24 (hours/day),
we get 6.41, or almost 6
battery ampere-hour rating to milliampere-hours to make the
rating match the equipment current rating, which is in milliam-
peres. If you prefer, you could leave the battery value in
ampere-hours and convert the flow meter current rating from 26
mA to 0.026 amperes.
If you want to use the Isco Lead-Acid Battery, you would convert
the ampere-hour rating from 6.5 A-h to 6,500 mA-h, and divide
that by 26 mA to get 250 hours. Dividing 250 by 24, we get 10.4
days.
Note that the figures given for current consumption for various
Isco products are average figures based on very specific oper-
ating conditions (for flow meters), such as a bubble rate of one
bubble per second and a chart advance rate of one inch per hour.
If you set the chart advance faster, or increase the bubble rate,
the current consumption will increase.
In the same way, current consumption for a sampler depends on
how often the unit takes a sample. That will vary widely from
one sampling program to another. Note that the current figures
supplied are average currents, meaning that when the plotter
or the pump run, peak current consumption will be consid-
erably greater.
The important issue here is to be aware that current con-
sumption for your equipment will depend largely on your pro-
gramming choices. Teledyne Isco cannot guarantee a specific
current consumption for a product, because programming flexi-
bility has so great an effect on it. You may have to experiment for
some time to match the battery life expectancy with your pro-
gramming choices.
When fully charged, a nickel-cadmium battery tests 1.2 to 1.3
volts per cell. A lead-acid battery shows 2.2 to 2.3 volts per cell.
A nickel-cadmium cell is considered fully discharged below
1.0 volt. A lead-acid battery is considered fully discharged at
1.75 volts per cell. Both values are at normal (room) tempera-
tures. You can see from these figures that a "dead" battery will
not really be dead, with both batteries showing at least 10 volts
left in them. Why not discharge this capacity to zero?
The answer lies in the fact that batteries are chemical in nature.
Chemical reactions inside the cells provide the source of elec-
tricity. Discharging a battery to zero volts risks having either of
two serious things occur. One is damage to the cell's plates from
depletion of the active elements; the other is cell reversal.
Because no two cells in a battery are identical, discharge occurs
at different rates inside each cell. One cell usually reaches
depletion before the others. If you continue to try to draw power
from the battery, the cells with remaining capacity will force
Power Products Guide
Section 2 Batteries
1
/
days. Note that you convert the
2
2-3
Hide quick links:
Advertisement
Table of Contents
