**
Portable Radio Ops Using Batteries**

**
With the Sun as an Energy Source!**

**
K5PA Discusses Using the Excel Spread Sheet Model ***BattOpTime.xls*

One of the neat aspects of ham radio
operations is going out into the field and operating using different energy
sources. I have been interested in battery operations from the field using QRP (5 W or less) RF
output power. At these power levels, a small battery, such as 7 Amp-Hours,
can supply enough energy to make many, many contacts in the field. What
happens if you want to work longer than the energy source allows? Well,
this is where alternate energy sources can be applied.

Photo Voltaic, or PV for short, are solar
cells normally mounted in an array to develop enough voltage and current to add
charge into a battery power source. The larger the PV array, the higher
the charging capability.

There are trade-offs between cost of energy
from different sources. Question, is it cheaper to add additional
batteries or to use larger solar cell arrays?

Another problem arises when additional RF
output power is desirable to establish communications. In other words,
when QRP ops is just not enough, how much more battery power and/or PV arrays
are needed to offset the increase in RF power?

I have performed an analysis of what
parameters need to be assessed to make decisions on battery capacity (in
Amp-Hours), solar cell array sizing, RF power output, and many more factors.
What started out to be a simple Excel sheet to perform these calculations (like
back of the envelope estimates) turned into a pretty nice spread sheet to help
manage all the factors. The filename is ' *BattOpTime.xls* .' A link
is given below for downloading from this web site.
(Please read and agree to the license agreement
below and included in the spread sheet prior to use of this model)

In the screen capture below, I show the panel
that is used for data entry and the resulting calculations. I used a
horizontal dotted red-line to indicate a 24 hour operating period. Other
time periods can also be used just by using the vertical axis to find you
operating time. Based on all the parameters in the spread sheet, a graph
is produced giving operating time versus battery capacity (Amp-Hours).

**To download Excel spreadsheet,
click here
BattOpTime.xls
(approximately 43 kB). **

**
With your mouse, **__right click__
for Windows to '*Save
Target As***' to
disk. **__Left click__ if you want to load and run the program from your
browser.

**
Definitions of Entries (manually enter data only in white
cells please)**

**
Battery A-Hr Rating Start Point. **The graph above has an x-axis that starts at a
certain A-Hr battery rating. Input the starting point for your battery in
this cell. For example, if you have a 7 A-Hr battery then you would enter
anything less than or equal to 7 in order for the graph to include your battery's capacity.
The graph is then incremented automatically in 25 A-Hr steps.

**
Solar Charge Rate A-Hr. ** If you are using a solar cell array to charge
your battery, then enter the maximum number of Amps that is available for your
array. If you do not have a solar battery charging capability, enter zero
(0). The value of zero will change the text in the bright yellow square to
indicate if solar power is used or not used.

**
Hours of Sun Availability. ** The amount of energy available from a solar
array depends on the amount of daylight. Put in your estimate here.
For example, if you expect full sunlight from 8 AM to 6 PM, then enter 10 hours. You
can also estimate that if your skies are partly cloudy, then a proportionally lower
number of sunlight hours are available. The current model is pretty simple in this area.
It is an area that will be enhanced if enough interest if given.

**Rx
Current. ** Radios require a certain amount of DC current during operations.
You can measure you nominal receive current or take it from your equipment's
specifications listed in the manual. Note that at high audio output, the
receiver current increases. If you use a fairly medium to low volume, then
the current is less. In this model, the receiver current is expected to be continuous during
radio operation.

**
Transmit Minutes Per Hour. ** The amount of transmit time in a one hour
period is entered in this cell. If the radio is not transmitting, it is
assumed to be receiving. Thus, this transmit time is used to calculate
transmit duty cycle. During contests, the duty cycle is likely higher than
during regular tuning, listening, and transmitting times. An entry of 20 minutes per
hour is calculated as a transmit duty cycle of 33.3% (20 minutes divided by 60
minutes).

**
Discharge Allowable on Battery.** Batteries have a certain capacity rated in
A-Hrs. For example, a 100 A-Hr (at 100% capacity) battery will completely
discharge to 0% capacity in 5 hours if 20 Amps per hour is being drawn from the
battery (simply 5 hrs times 20 A = 100 A-Hrs).

__Lead
acid__, 12 VDC batteries have an open circuit voltage equal to the following:
100% capacity > 12.68 Volts; 75% capacity = 12.44 volts; 50% capacity = 12.23
volts; 25% capacity = 12.02 volts; and 0% capacity equals 11.8 volts. To
lengthen the number of charge cycles for a battery, the discharge should be kept
to around 50%. You can increase this, say to 75% as long as it is on an
irregular basis. Otherwise, the battery's useful life time will suffer.

__Gel
Electrolyte__, 12 VDC batteries have a different open circuit voltage from
lead acid. Their voltage and capacity are: 100% capacity > 12.95 Volts;
75% capacity = 12.71 volts; 50% capacity = 12.50 volts; 25% capacity = 12.29
volts; and 0% capacity equals 12.07 volts.

**Tx
Current at Max. Output Power.** The transmitter has a maximum current rating
when the maximum output RF power is generated. The radio's specification
sheet (see manual) normally gives this number. Typically, a 100W RF output
transmitter requires about 20 A of DC current.

**
Max. RF Power Capable.** Enter the maximum RF power capability for the radio
that coincides with the maximum DC amperage above.

**
Desired RF Output Power.** Most radio provide a means to __decrease__ the RF
output power. This is useful for decreasing DC current requirements which
then lengthens the time a battery can be used. This model assumes a linear
relationship between maximum RF output power at maximum DC current with a lower
bounds at the transmitter's quiescent current (transmit mode stand by current
when no modulation is present). The value of the quiescent current is
entered as listed below.

**Tx
Mode Peak-to-Average Ratio.** Different modes of operation have
different peak power to average power ratings. This is important since
operating with voice SSB requires a different average DC current than a digital
mode such as PSK31. A good assumption with SSB operations without speech
compression, the peak to average is about 3:1, entered as 3. For Morse
code, CW
operations, the peak to average can be entered as 2. For PSK31, enter a
value of 1. These are not rigorous values and certainly open to
interpretation. If speech compression is being used, the peak to average
ratio is increased. I suggest a value of 1.3.

**
Quiescent Tx Current.** This is the nominal current the transmitter draws from
the battery when __no__ modulation is applied. This becomes the lower bounds
for the transmit current consumption. The value may be found in the
radio's specification sheet in the manual. The quiescent current is likely
higher than the receiver current.

The
figure below shows the effect of using a solar cell array charging of the battery.
In this example, the solar cell array is listed as having 2.44 A-Hr of capacity.
This means that in a one hour time period, the solar cell array will provide
2.44 Amps into the battery. The solar cells operating amperage can be found in the specification sheet. Notice the bright yellow
cell indicates the system is being charged from the solar cell source.

To interpret this graph, remember the dotted
red-line is 24 hours. So to operate under the conditions given in the
white data entry cells, you can operate for about 24 hours if you are using a 65
A-Hr battery capacity. Compare this to the first graph above. With
__no__ solar cell array battery charging, the same conditions requires a
battery capacity of 89 A-Hrs.

So to address an earlier question, is it
cheaper to add the solar assisted battery charging or just purchase a larger
battery? These types of trade-offs can be found by using this model
One
important concept to remember however is that, *at some point,* all batteries require charging
So you have to have a charging source from somewhere.
**Why not solar?**

**
***BattOpTime.xls* Excel Spread Sheet License Agreement

1) This
MS Excel spread sheet software comes without warranty of any kind. The user
agrees to bear all risk associated with using the spread sheet software. The
author shall not be liable for damages of any kind which result from use of this
software in any manner.

2) This
Excel spread sheet software may be freely distributed, but only in its original
form (that is, only as the file *BattOpTime.xls*, as originally published
and unmodified). No fee (other than a nominal fee for the cost of media) may be
charged for this software. This software may not be distributed as part of a
package of commercial hardware and/or software (for which an other-than-nominal
fee is charged) without first obtaining permission from the author.

If you do
not agree with these terms and conditions, you are not authorized to use this
software and must delete it from your computer.

Please
contact the author with any questions or comments regarding this software:

Gene
Hinkle, K5PA

e-mail:
k5pa@arrl.net or fhinkle@austin.rr.com

To read
instructions on the use of the spreadsheet and to download the latest copy,
visit the author's website at

http://www.k5pa.com/

This
Excel spread sheet software is protected by the copyright laws of the United
States of America, *F. E. Hinkle K5PA Copyright 2003*

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