RESEARCH SERIES:
Batteries: The Good, The Bad, The Ugly!
BY
Arnold Roquerre
All Material Copyrighted 2006
Secrets Of ElectronicThat Can Hurt You
Most manufacturers of electronics requiring batteries assume the user has adequate understanding of battery power. This is not usually the case, especially electronics used in rocketry. The assumption is that the user is fully aware of the technical issues of providing power to one or more mission critical electronic components. Current drain, voltage, temperatrue and duration of battery drain all impact whether an electronics package functions properly or not. Space Warp Technology (SWT) will attempt to provide some useful insights on batteries used to power the electronics used onboard rockets. In most cases batteries used supply too little voltage or current ofthen end in failed deployment, loss video, and loss telemetry. It is critical that the user understand just what can be expected from a given power supply selected to deliver power at the voltage and amperage needed for safe, dependable operation within the expected temperature range the electronics will be exposed to. The issues that are pertinent to anyone using battery power to operate electronics used in rockets are:
This paper focuses on primary batteries and not rechargeable batteries. The power and voltage provided by primary batteries outweighs any potential cost saving that would be gained from using rechargeable batteries in amateur rocketry. Rechargeable batteries come with their own set of problems, e.g., aging, over/under charging, safety regulators, charging specificity, etc. One has to remember to charge the pack before use. Not a big issue for a project with defined roles for the participants (think NASA). For a small group of volunteers charging is a task that can easily be forgotten or an issue if the charger(s) not brought to the launch site or the launch day is moved to a later date and the pack begins to discharge. The basic principals temperature, voltage, amperage draw and battery capacity apply equally to primary, as well as rechargeable batteries.
Arnold Roquerre
Batteries (Primary & Rechargeable):
Digital Thermometer
Digital MultiMeter
"Watt's Up" RC
Battery Watt Meter & Power Analyzer
The simplest way to provide constant power for a given voltage and amperage draw over a given time interval is to purchase or assemple a custom battery pack. A battery pack should be as small, light, and contain the minimum number of connectors as possible. The packs I use go against conventional wisdom. The pack conisists of batteries that have different voltage ratings, but the use the same chemistry. This allowed for a very light weight battery pack capable of delivering power that stays within the required voltage range for several hours works. The small differences in amperage and voltage between the batteries used did not cause problems. This does not mean it will not! Do not try this and not expect serious problems to occur, possibly, even death to your are someone else! In this example, this pack provided 9 volts at 300 mAh for several hours. The pack weighed in at 3.2 ounces. It consisted of three lithium CR123A 3 volt 1200 mAh batteries and one lithium AAA 1.5 volt with a 1500 mA rating, connected in series. The AAA battery was placed at the end of the series with the negative terminal wire from the power connector attached to it. The positive terminal of the battery connector was attached to the positive end of the last 1/2N 3 volt battery in the series.
Why this configuration? Size was a serious contraint. I needed a power supply that would deliver a voltage between 9 and 10.5 volts for several hours in a very small package that weighed less than 3 ounces. The battery pack consisted of just 4 batteries (three CR123A Lithium Batteries and one AAA Lithium battery) which simplified construction of the battery pack and kept the weight down too. The above configuration provided a voltage range between 9 to 10.5 at a 300 mA current draw for over 2 hours. The configuration starts out at 10.5 volts, drops to around 9.7 volts and stays in the range of 9.7 to 9.4 for a solid hour. After an hour the voltage begins to drop to 9.1 to 9.0 for another hour. More than enough time needed for the project it was designed for. If size and weight had not been an issue, 8 AAA or AA lithium batteries could have been used. This is an example of why understanding the characteristics of batteries is useful if situations arise requiring unusual configurations.
Below is the battery pack discussed above and used in 2006 to power an RDAS unit with telemetry and GPS. After two hours of recovery effort, the pack was at 8.9 volts. Note: do not solder or weld over the lithium battery vents. You should read up on how to make your own battery packs for more detailed information. This site is not a how to make battery packs site.
Currently, this author has moved to 12 and 15 volt battery packs that, due to amperage load, drop down to a steady voltage of 11.2 and 12.2 volts, respectively for several hours. The current electronic package has a higher tolerance for high voltage. The current pack provide power for over three hours at 350 mAh.
Back
Temperature is 900
pound gTemperature is the 900
pound gorilla in the room! Different chemistries behave differently in
high and low temperatures. The primary difference is the range of the effects of
temperature. Batteries lose voltage below freezing and at very high temperatrue
too. The chart by Tadiran Battaries provided a good example:
Li/System
Li/SOCl2/
w/Hybrid Layer
Capacitor
Li/SOCl2
Bobbin
Li/SOCl2
Jelly Roll
Li/SO2
Li/MnO2
Energy Density
(Wh/l)
1420
1420
800
410
650
Power
High
Low
High
High
Moderate
Voltage
3.6-3.9V
3.6V
3-3.6V
2-3V
2-3V
Pulse Amplitude
High
Small
Moderate
High
Moderate
Passivation
Low
High
Moderate
Fair
Moderate
Performance at
Elevated Temp.
Excellent
Fair
Moderate
Moderate
Fair
Performance at Low
Temp.
Excellent
Fair
Excellent
Excellent
Poor
Operating Life
Excellent
Excellent
Moderate
Moderate
Fair
Self Discharge Rate
Very Low
Very Low
Moderate
Moderate
Moderate
Operating Temp.
-55°C to 100°C
-55°C to 150°C
-55°C to 85°C
-55°C to 60°C
0°C to 60°C
0°C to 60°C
Other battery chemistries based on NiMH or alkaline behave differently. One should secure the temperature chart provided by the battery manufacturer for each battery to be used, and test the battery under expected conditions of temperature and runnng time to know how the battery pack will perform in the field. The chart below, which can be found in the paper Common Mistakes in Battery Pack Development (And how to avoid them) by Katherine Mack, Rose Electronics and further discussion on the does and don'ts of battery pack design, is an example of how temperature impacts battery performance.
The simple rule of thumb is cold and heat
decrease voltage and amperage. If the voltage and/or amperage drop too low the
electronics will fail and "down will come rocket, electronics and all!
Always test before using!
Back
Secret of Electronics That Can HSecret of Electronics That Can Hurt You TThe reason voltage ranges are given in the
specification accompanying electronics for rocketry is because most electronics
do not need the maximum voltage provided by a battery.
Power is almost always delivered to the electronic components through voltage regulators that
need fewer volts than the given rating. The higher voltage is required for the
voltage regulators to function properly. For example, a 5 volt regulator could
require 7 or 8 volts in order to provide a steady 5 volts. Specifications
that give a voltage range is good, except when the
stated lower voltage limit actually higher than what the recommended battery
can actually deliver. For example, a 9 volts battery is recommended for an
electronic unit. The recommended voltage range is 15 volts to 9 volts. 9 volt batteries drop to 8
volts plus very quickly depending on
current drain. If the stated limit is 9 volts, then one needs to provide a power
supply that will deliver a minimum of 9 volts if problems in performance are to be avoided. Upper voltage limits present a different
set of problems - heating. The greater the voltage difference and the larger the
drain, the more a voltage regulator will heat up. A regulator designed to drop a 9
volt battery to 5
volts will get much hotter if 12 or 15 volts are
supplied. In all likelihood the regulator will get so hot that it shuts down or burn out. Since voltage regulators can get hot and need to dissipate
heat they have heat sinks to dissipate the expected heat. Often the upper limit
given in an electronic component specification takes into account the electronic
and regulators
ability to get rid of heat. Always keep the above in mind when building electronic
bays that fit into airframes that get hot sitting on the pad. One has to thoroughly understand the heat
characteristics the electronics going into a rocket will likely be exposed to. The time
a rocket is sitting on a launch pad before launch and the time a rocket will be
at high low pressure altitudes need to be anticipated. Underestimating heat will result in hot
electronics that either shut down because of temperature over load, suffer a
component failure resulting in a shut down or malfunction, or burn out. Possible consequences
from over heating are: Another
consideration one must address in putting together a power supply is the
amperage the battery supply is to deliver. Batteries differ in how much amperage
and the length of time power can be continuously supplied. All batteries have
upper limits as to how much amperage can be supplied for a given length of time. The rated continuous amperage
a a battery can supply and the amperage requirements of the electronics to be
powered must
be known. This requires that the user spend a little time on reading the battery
specifications which in itself can require some effort.