RESEARCH SERIES:
Batteries: The Good, The Bad, The Ugly!
BY
Arnold Roquerre
All Material Copyrighted 2006
Secrets Of ElectronicThat Can Hurt You
Most
manuals accompanying electronics used in rocketry do not
adequately address power. The assumption is that the user is fully aware of the
issues in providing power to one or more electronic components.
Space Warp Technology
(SWT) will attempt to provide some useful insights into batteries used to power the electronics used in rocket's mission critical
systems. If batteries
supply too much or too little volts or current, the consequence can
be a failed deployment, loss video, no telemetry. It is critical that the power supply deliver power at the
voltage and amperage needed for safe, dependable operation within the expected
temperature range the electronics will likely be in. The issues that are
pertinent to anyone using battery power to operate electronics used in rockets
are:
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 power level for x time is to purchase or put together a custom battery pack. The approach I am giving goes against tried and true methods of putting together battery packs. A battery pack should be as small, light, and contain the minimum number of connectors as possible. I have found that one can mix batteries that have different voltage ratings to produce a very light weight battery pack capable of delivering power that stays within an acceptable voltage range for several hours. Small differences in amperage between batteries based on the same chemistry used in a battery pack do not seem to cause problems. An example, I have designed a very light battery power pack that provides 9 volts at up to 300 mA for several hours. The pack weighs in at 3.2 ounces It consists of three lithium CR123A 3 volt 1200 mA batteries and one AAA 1.5 volt with 1500 mA rating connected in series. The AAA battery is at the end of the series with the negative terminal wire from the power connector attached. The positive terminal of the battery connector is attached to the positive end of the last 1/2N 3 volt battery in the series.
Why this configuration? It delivers voltage between 10.5 and 9 volts for several hours in a very small package weighing in at under 3 ounces. The battery pack consists of just 4 batteries (three CR123A Lithium Batteries and one AAA Lithium battery) which simplifies construction of the battery pack and keeps the weight down too. The above configuration provides the above voltage range 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.
One could use 8 AAA or AA lithium batteries except the weight is more, the size of the pack is larger and the number of tabs to weld or solder are more. The real problem is that the pack will drop down to 8+ volts very quickly. The voltage will stay in the high 8s which may or may not be alright for your application.
Below is a battery pack used in a recent launch 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.
Secret of Electronics That Can Hurt You
The reason voltage ranges are given in the specification accompanying electronics for rocketry is because most electronics need only a fraction of the voltage. Power is delivered to the electronic components through voltage regulators that need less volts than the given rating. The higher voltage is required for the voltage regulators to function properly. A 5 volt regulator could require 7 or 8 volts in order to provide 5 volts steady voltage. 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 and 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 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 run 5 volts form a 9 volt battery, will get much hotter if 12 or 15 volts are supplied. In all likely hood the regulator will get so hot that it shuts down. Electronics either shut 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.
One has to thoroughly understand the heat characteristics of the electronics going into a rocket for several reasons: one is time on pad before launch and another is high altitude flights where air pressure drops to a level that, if sustained too long ,results 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. Either of the above consequences from over heating lead to:
Alkaline batteries voltage drops quickly. A 9-volt alkaline-manganese dioxide battery will drop to 8 volts in a little over 12 minutes and 7.5 volts in about 15 minutes with a 100 mW load. With a 250 mW load a 9-volt alkaline will in a little over 9 minutes drop to 8 volts. With a 10 mW load, a 9-volt alkaline battery will drop to 7.5 volts in a little over 24 minutes. The above assume ambient temperature is at 70 degrees Fahrenheit (21 degrees C). At lower temperatures the voltage drop will be faster.
Lithium batteries:
Primary:
Light weight, high current output, long shelf life, easy to use, no charging
issues, more expensive than using rechargeable batteries, less affected by
temperature extremes than batteries based on other chemistry.
- Lithium AA amp hour rating come as high as 3 Ah and
provide up to 1.5 Ah.
- Lithium AAA amp hour rating come as high as 1.5 Ah.
Rechargeable: Cheaper than primary batteries, lithium batteries age from time manufactured which leads to ever diminishing capacity, battery starts losing power soon after charging, higher momentary current, less affected by temperature extremes than batteries based on other chemistry. Power packs can be provided to meet desired power needs. One needs to pay attention to the power specifications given for the batteries used and make sure the pack has been charged ahead of time.
Many of the technical
manuals accompanying electronics give a maximum and minimum voltage range and
then suggest a power supply that actually provides a much lower voltage after a
few minutes. This can be a problem if the user is not aware of this. A simple
solution would be for manuals
to include a power
chart that shows how long a given battery pack configuration
would be able to power up and run electronic device in different temperature
scenarios.
This is unlikely simply because building such a chart
would be a major undertaking in itself, take a lot of time and bring with it
liability issues and the inevitable law suite. The time proven method is to
simulate as close as possible the pre-launch and flight environment in duration,
temperature and current draw. At some point the electronic components to
be used should be inserted into the simulation. In the absence of more detail
information in documentation, failure to test in as realistic manner as possible
is basically a prescription for unexpected failure. Even after doing all the
above, something may fail.