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Text and pictures for first 36 issues of Homepower mags can be
found on the internet by searching for homepower archive.
From Homepower Mag issue 26 1992 pages 43 to 45
Build an Ampere-HourMeter by Hollister McNeal
On any alternative energy installation, knowing the amount of energy
produced or consumed is very useful. When storage batteries are
used, Amp-hour meters on the input and output provide information on
the energy production or consumption (since Amp-hours x battery
voltage = energy in watt-hours). Although such Amp-hour meters can
be purchased commercially, I chose to build one.
A block diagram of the Amp-hour meter is shown in Figure 1. The
SHUNT is used to measure the current. It must be inserted in the
negative leg of the battery input or battery output. A shunt is
actually a very low resistance precision resistor that produces a
voltage across it proportional to the current flowing through it.
The voltage that develops across the shunt is then amplified by 100
in the AMPLIFIER section. The output of the AMPLIFIER is then summed
(integrated) in the ADDER section. When the ADDER output reaches a
certain voltage level, the comparator changes its output state. This
causes two things to happen: The LCD COUNTER is incremented by 1 and
the ADDER is reset. The 5-digit LCD counter actually indicates
hundredths of Amp-hours.
Therefore the maximum count is 999.99 Amp-hours. The POWER CONVERTER
section is needed to provide the appropriate regulated voltages to
the electronics. It receives power from the 12 Volt battery bank and
has +5, -5, and +1.5 Volt outputs. One power converter can easily
support two separate amp-hour meters with a current draw of
approximately 0.01 Amp.
Amplifier Section
Figure 2 shows the AMPLIFIER section in detail. It uses an LM324
op-amp configured as a differential amplifier with a voltage gain of
100. The resistors should be 1% tolerance for best accuracy. As
there are 4 separate opÄamps in a single LM324 package, the other 3
opÄamps are available for the ADDER section or a 2nd Amp-hour meter.
Adder Section
Figure 3 shows the ADDER section in detail. It uses an LM324 op-amp
configured as an integrator. The adder is actually a subtracter, as
its output goes from 0 to -2.5 Volts as it accumulates the current
input. Note that the resistors R1, R2, R3, and capacitor C1 do not
have values specified. R1 and C1 are selected to obtain the correct
counting rate. Their values are also dependent on the resistance of
the shunt (RS). The values can be approximately determined from the
equation R1 = 1.44/ (RS x C1), with R1 in kiloOhms, C1 in æF
(microFarads), and RS in Ohms . I would recommend choosing C1
from available values, then calculating R1. R1 should be between 1k
and 200k Ohms.
For example, my input shunt is 0.001 Ohm. I chose a 15æF capacitor.
R1 is then calculated to be 1.44/(.001 x 15) = 96 kiloOhms. My
output shunt is 0.0001 Ohm. I chose a 4.7 æF capacitor. R1 is then
calculated to be 1.44/(.0001 x 4.7) = 31 kiloOhms. To compensate
for component tolerances, I would recommend using a potentiometer for
R1 and adjusting it after the circuit is built for proper
calibration (see CALIBRATION). Also, R2 should be chosen
approximately equal to R1 (within 10%). Capacitor C1 should be
tantalum for best accuracy. Make sure the capacitor is installed to
match the indicated polarity.
Resistor R3 is selected to optimize offset adjustment using the 100k
Ohm potentiometer. Offset adjustment is necessary to assure that the
Amp-hour meter is accurate for low or zero input currents. If R3 is
too low a value, it is very difficult to set the pot to the optimum
value, since a slight change in the pot will cause a large output
variation. If R3 is too high a value, no pot setting will compensate
for the offset. I used two 10 megaOhm resistors in parallel (5
megaOhm effective) on my two meters.
I used an IRFZ40 MOSFET to reset the adder by shorting out the
capacitor C1. Other MOSFETs such as the IRF511 (Radio Shack
#276-2072) will probably also work. Note that the MOSFET has an
integral diode to keep the tantalum capacitor from going more than a
few tenths volt reverse polarity.
Comparator Section
Figure 4 shows the COMPARATOR section in detail. It uses an LM339
comparator. As there are 4 comparators in a single LM339 package,
the other 3 comparators are available for a 2nd Amp-hour meter or
other circuitry (I used 2 comparators to implement a high battery
voltage/low battery voltage detection circuit). The zener diode in
the schematic is a precision reference diode LM385Z. When the adder
output reaches -2.5 Volts, the comparator output will rise to
approximately 3.7 Volts, which will both increment the counter and
reset the adder. The comparator output will remain high until the
adder output approaches 0 Volts. The comparator output will then go
back to approximately -5 Volts.
LCD Counter Section
Figure 5 shows the LCD COUNTER section in detail. The LCD counter is
Radio Shack #277-302. As 1.5 Volts is generated by the power
converter circuitry, no 1.5V AA battery is needed. The zener diode
in the schematic is a precision reference diode LM385Z-1.2. It is
used along with the 22 kOhm resistor to provide the correct logic
level to the counter. The reset switch is used to reset the counter
display to 00000. The display will read up to 999.99 Amp-hours.
There is no decimal point in the display. Perhaps one could be
"taped" onto the display if desired.
Power Converter Section
Figure 6 shows the POWER CONVERTER circuitry in detail. I used a 1/2
Amp fuse although a smaller fuse (1/4 Amp) should also work fine.
The fuse was placed in a Radio Shack #270-1211 inline fuse holder.
An On-Off switch after the fuse could be added also. Four op-amps
are used, with 3 being in one LM324 package (U3) and the 4th in
another LM324 package (U4). Do not attempt to combine these op-amps
any other way as their power pins are hookedÄup differently (U3 uses
+12 and GND, U4 uses +5 and -8). Be careful to observe the
polarities on the 10 æF and 47 æF capacitors. The +5 and -5 voltages
are not exact and will vary with the tolerances of the resistors
used. The -8 voltage is not regulated and varies with the 12 Volt
battery voltage and with the current draw on the -5 voltage output.
Pin 8 of U3 oscillates at approximately 1300 Hertz.
Construction
I placed the electronic circuitry for two Amp-hour meters on a
single Radio Shack #276-162 printed circuit board mounted inside a
Radio Shack #270-232 box. The circuit board was quite crowded. I
would recommend using either two of them or else using a larger
board. I mounted the switches and LCD counters on the surface of the
box. Terminals mounted on the surface of the box were used to wire
the shunt inputs and battery inputs. The inline fuse was placed
close to the batteries. I drilled holes for access to the pots
mounted on the printed circuit board inside. I also brought out the
adder output terminals for ease in offset adjustment.
Calibration
The offset potentiometers are best adjusted with zero current in the
shunt. Adjust the pot until the output of the ADDER section is a
stable (unchanging) negative voltage between 0 and -2.5 Volts.
Particularly with the 0.0001 Ohm shunt this is difficult to do. Use
a digital voltmeter if possible. A rate of change of 0.01 Volt per
second will give a total error of 3.6 Ampere-hours per day.
The counting rate is adjusted by changing R1 in the ADDER section.
The shunt should now be drawing a steady current (preferably on the
high side, such as with the solar panels in full sun for an input
shunt, or a water pump or other heavy load for an output shunt).
Measure the voltage across the shunt with an accurate digital
voltmeter and divide by the shunt resistance to determine the
current. Use a watch or preferably a stop-watch to time a set number
of counts on the LCD counter (the more counts the more accurate the
result). The number of seconds should then be SECONDS = COUNTS X
36/CURRENT. For example, with a current input of 10 Amps, 10 counts
should take 36 seconds. For a current output of 50 Amps, 100 counts
should take 72 seconds.
Operation
In my setup in Northwestern Pennsylvania, one AmpÄhour meter
monitors battery input from six Solarex MSX-60 solar panels. The
other monitors the battery output going to a Trace 2012 inverter.
Each day before sunrise both meter readings are recorded and then
the meters are reset to 00000. The meters have been in operation for
about eight weeks. So far I have been averaging about 100 Amp-hours
per day for both battery input and battery output. I expect these
midÄsummer readings will drop considerably by late fall and winter.
Limitations
If the voltage drop across the shunt is more than 0.03 Volts
(corresponding to 30 Amps on an 0.001 Ohm shunt or 300 Amps on a
0.0001 Ohm shunt) the 100X amplifier may saturate. This will not
damage anything but it will cause the AmpÄhour meter not to record
at more than the amp rate corresponding to 0.03 volts. Reducing the
gain from 100x will solve the problem. Replace both 110 kOhm
resistors with the same smaller value. However, R1 and R2 or C1 in
the ADDER section will also need to be proportionately reduced.
The LCD counter has a maximum counting frequency of 7 counts per
second which corresponds to approximately 250 amps. Again, no damage
will occur if this frequency is exceeded but the counter may not
increment correctly. The solution would be to have the counter
increment for tenths of amp-hours instead of hundredths of ampÄhours.
The easiest way to do this would be to replace both the 100 kOhm
resistors in the amplifier section with 10 kOhm resistors, thereby
making a 10X amplifier.
It is not possible for this Amp-hour meter to count backwards.
Therefore the shunts should be placed so that the current flow is
always in one direction. The shunts have to be installed in the
negative leg of the battery.
As the temperature of the circuitry changes the offset adjustment
will change. I would recommend setting the offset at least once a
month, preferably at a temperature that is "normal" for that month.
The circuitry should be indoors and not exposed to excessive heat,
cold, or humidity. Probably the best improvement in this Amp-hour
meter would be to use opÄamps for the AMPLIFIER and ADDER that have
a lower offset voltage and less drift with temperature. The OP177
op-amp is much better than the LM324, although it is not at all pin
for pin compatible.
Amphour.zip Text as 7+ 1 part 5k insize
Amphour.png Circuit diagram as 7+ 5 parts 9k insize
to be sent out later.
73's Brian
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