Battery Power Management and Control
The management of DC power is one of the basic challenges in designing an autonomous ocean lander. We can go pretty far down the rabbit hole with this topic. This article is intended to share a few thoughts for the readers’ consideration.We start with a premise. A lever without a fulcrum is just a stick. A fulcrum without a lever is just a door stop. Bring both together and you have a simple machine. Similarly, a sensor or microcontroller without an amplifier is just a novelty. Both must work together to effectively manage power in an ocean lander.BatteriesBatteries power circuits in autonomous vehicles. Different chemistries have their own issues with self-discharge and loss of capacity driven by low temperature. (See Lander Lab #10, Marine Technology Reporter, March/April 2024). Onboard circuits may require different voltages. Critical function circuits, such as the release, should be powered by a “sacred” battery pack, not shared with anything that might inadvertently deplete it.Charging batteries can be done in one of several ways, including: 1) opening the housing and charging, 2) charging through the end cap, or 3) external batteries that are swapped out using an underwater connector or inductive link.Charging through the endcap can be easily done with a 4-socket connector. I usually assign pin 1 as battery GND since the industry color code on the #1 wire is black. Other pin assignments flow from that. Pin 1: Battery GND, Pin 2: Battery POS, Pin 3: System GND, PIN 4: System POS. Using only PIN 1 and PIN 2, a mating connector connects a battery charger directly to the battery. During charging, the battery is disconnected from the circuit it powers. Charge as normal. After charging is complete, the battery is connected to the system using a shorting plug. A shorting plug looks like a dummy plug, but has pins connected inside the overmold: Pin1 to Pin 3, Pin 2 to Pin 4. Some chargers have a thermal sensor that dials back the charging if the battery temperature gets too high due to increasing internal resistance as the battery gets older. This can still be done through the endcap but requires two additional pins (Pin 5, and Pin 6) to connect the thermistor inside the battery to the charger control circuit. I’ve charged 4 different battery packs with separate chargers through an 8-pin connector. Like a wall plug, I used socket contacts to directly connect to the batteries. A jumper cable brought the power back into the sphere through another 8-pin connector, this one with pins. The jumper cable was essentially an extension cord, pins on one end, sockets on the other.If battery outgassing is a warranted concern, a PRV (Pressure Relief Valve such as made by Prevco or Deepsea) could be installed, or the purge port opened during charging (be sure to replace the pressure proof cap!), or endcap restraints, such as bolts, removed.If the system requires a fast topside turnaround, external battery packs are a practical solution. Recover the system, pull the spent batteries, install a fully charged second set, and redeploy. The first set then goes back on the charger.Voltage ConversionInternal to the unmanned ocean lander, a variety of DC voltages may be used, such as 3.3vdc, 5vdc, 10vdc, 12vdc, 21vdc, or higher. Battery discharge curves are never flat, though some are better than others. Multiple voltage regulators may be used to provide different voltages and constant power levels to separate circuits.Multiple switching regulators can be wired in parallel to a single battery, as long as the battery is large enough to supply the total demand current. Optocouplers can be used to isolate each circuit from the microcontroller.Voltage regulators (DC-DC Converters)There are two types of voltage regulators: 1) Linear or Analog, and 2) Switching.With power limited to batteries, efficiency is a priority. Linear or Analog regulators have conversion efficiencies of around 40%, so they’re out. With a switching regulator, efficiencies between 85-95% are common, while also providing a step up in current output.There are three types of switching voltage regulators: boost, buck, and a combination boost-buck. A boost regulator can step up a voltage, a buck regulator can step down a voltage, and a Boost-buck will do both.The simplicity of the buck design makes it more efficient than a boost, so when trying to put every electron to work, stepping down a higher voltage to lower makes sense.Figure 2. The Addicore LM2596 Step-down Adjustable DC-DC Switching Buck converter can drive a 3A load with 90% efficiency, with excellent line and load regulation, thermal shutdown and current limit features. The LN2596 has an addressable minimum low power standby mode of 80 μA. (Cost: $2.48) Photo: AddicoreQuiescent current during light-load or standby modes is important to consider when efficiency is a priority. Power to a voltage regulator may also be cycled on a schedule using a microcontroller controlled MOSFET.MicrocontrollerA microcontroller is a programmable computer on a chip that offers intelligent control of a system. They are small and power efficient. Examples include Arduino, Raspberry Pi, ESP32, and others. Power output is limited. An Arduino I/O pin can output 5v @ 20 mA max. Make Magazine publishes an annual Boards Guide that describes dozens of new microcontrollers and single board computers.MOSFETMOSFET stands for metal-oxide-semiconductor field-effect transistor. The MOSFET has three terminals: gate, drain, and source. MOSFETs are valued for their ability to control large currents using small gate voltages, their efficiency, and small package size. Some MOSFETs can be switched fully on by 5v (Vgs) logic levels, such as the IRL540 (Cost: $0.77). Amazon sells a sampler kit of logic level MOSFETs for $19. (Search term in Amazon “EEEEE 70 Pcs Logic Level MOSFET”.)A small electrical signal can turn on LED lights for imaging or a motor in a pump. Great devices to know about. Google “How to Choose a MOSFET” for many good links, including videos. A helpful component selection worksheet may be found at https://www.addohms.com/mosfet-guide/.A MOSFET schematic and 3-lead part in a TO-220 package. (Photo: Hong Kong Olukey Industry)SCRSCR stands for Silicon Controlled Rectifier. Another three lead component, (Anode, Cathode, Gate), normally used to rectify an AC signal, has the interesting characteristic in a DC circuit of behaving like a latching relay. Consider the NTE5455 SCR (Cost $0.80). When a 1.5v pulse is applied to the gate, current begins to flow from the anode to the cathode, and continues to flow even when the gate voltage is removed. Current will flow until the current drops below a certain level, called the holding current, at which point it turns off. A simple, yet reliable, time release is a small lab countdown timer that outputs a 1.5v signal to a piezoelectric beeper. Bring the signal for the beeper to the gate of the NTE5455 instead to start a 10vdc source corroding a burnwire.SequencerPowering up multiple parallel circuits might be challenging due to initial in-rush currents. The TI LM3880 Simple Power Supply Sequencer is a device that controls power up and power down sequencing of three independent voltage rails. By staggering the startup sequence, the instantaneous battery loads are moderated. The robust part is qualified for automotive applications and features a low quiescent current of 25 μA.Figure 4. Schematic of the 6-pin TI LM3880 Simple Power Supply Sequencer. (photo: Texas Instruments). (Cost: $1.02)Transducers and SensorsThe terms “Sensors” and “Transducers” are sometimes used interchangeably, though there are subtle differences.Sensors may be required to initiate action by the microcontroller or dedicated circuit.There are only two types of transducers. Active transducers produce a voltage in response to a change in a parameter. These include thermocouples, photovoltaic, and piezoelectric. Passive transducers produce a change in resistance (potentiometer, strain gauge, thermistors, reed switch), capacitance (gauges), or inductance (differential transformer) as the response to a change in a parameter.A sensor detects a specific physical, chemical or biological quantity and converts the value it receives into an electrical signal. Sensors require an amplifier as they are limited to signal level power, often less than 1W. They cannot pass a great deal of power themselves. For power management, use a sensor to control an amplifier circuit: a relay, transistor, optocoupler, or MOSFET. Reed switchOne of the originals: a reed switch is a magnetically activated switch. It opens or closes depending on the presence or absence of a magnetic field. Because they can’t handle much current, it’s important to think of a reed switch more as a magnetic sensor than a switch. They are commonly available as SPST and SPDT. They come in a variety of sizes. Smaller units are more sensitive to magnetic fields, but carry the least power. Reed switches have small leakage currents compared to solid state devices. They have low resistance. The reeds are hermetically sealed within a tubular glass envelope, which will implode with direct exposure to increasing depth. Thus, a reed switch must be placed inside a nonferrous housing, such as plastic, aluminum, or titanium. They can be potted in a hard epoxy for medium depth applications. A circular ring of reed switches can be simultaneously triggered by a single magnet located in the center. Hot switching, switching with maximum power on, can damage the part. As the switch opens or closes, an electrical arc can burn or weld the contacts. As the contact plating is damaged, the resistance will eventually rise until the reed switch no longer works.Figure 5. Reed Switches come in a variety of sizes and packages, small to large, with different power, switching voltage and current ratings. A better design uses these to control a MOSFET to handle the real power. Photo: LittelfuseHall Effect Sensor A Hall Effect sensor is another magnetically activated switch. They produce a low signal level and require amplification. Its output is controlled by the presence or absence of a magnetic field. Like a reed switch, a Hall Effect sensor can be operated inside a nonferrous housing, such as plastic, aluminum, or titanium.Because the Hall Effect sensor is a solid-state device, it is not prone to breakage, mechanical wear, and is pressure tolerant. A Hall Effect sensor can be potted and operated in a high-pressure water environment.Hall Effect sensors are sensitive to higher temperatures, but generally not within the range most ocean landers will see. Higher temperature variants are available.Hall Effect sensors come in two flavors: Unipolar and Bipolar. Each have uniquely useful characteristics.Unipolar Hall Effect sensors act like a SPST switch. The Unipolar Hall Effect switch is normally closed. The part can be selected to be sensitive to either a north or south pole magnetic field. Exposing the part to the opposite magnetic polarity does not affect the output state. (ref: Melexis US5881, Cost: $0.60))Bipolar Hall Effect sensors act like a latching relay. They can be selected to latch open with a north or south pole magnetic field. The opposite magnetic field will latch the bipolar Hall Effect switch in the closed state. (ref: Melexis US2882, Cost: $0.63)Figure 6. A bipolar Hall Effect Switch acts like a latching relay. (Photo courtesy Melexis)Microswitch, Momentary On-OffA microswitch is a miniaturized mechanical device. The momentary on-off push button type can be used to determine position limits of components, such a piston in a bore. The smaller the switch, the smaller load it can handle.Other sensors include Light, temperature, salinity, and vibration.Experimenters WorkshopFor those interested in tinkering with the parts discussed here, consider some of the components and kits offered by SparkFun.com, Adafruit.com, Makershed.com, and Addicore.com, among others. Some are under a dollar or two. Practice makes perfect, or at least gives room for thought.Future developmentsNew pressure tolerant and pressure protected battery technologies are being developed for marine applications. Likewise, globally, sensor engineers are investigating, characterizing, and developing new sensors to translate the marine environment into a digital equivalent for scientific investigation, machine control, and government oversight.Invitation to ReadersWould you like to share your insights on this topic? We’ll publish some of the best replies we’ll get to the questions below.Can an Op Amp be used as an amplifier for a sensor? Are there advantages? Explain.Can an Optocoupler be used as an amplifier for a sensor? Are there advantages? Explain.Can an Accelerometer be used to indicate, internal to the command/control sphere, when an ocean lander has reached the seafloor? Explain.Citations“Fundamentals of Transducers,” R.H. Warring and Stan Gibilisco, ( ISBN 0-8306-1693-4)“Practical Electronics for Inventors,” Paul Scherz, Simon Monk, (ISBN 978-0-07-177133-7)“The Art of Electronics,” Horowitz and Hill, (ISBN 978-0-52-137095-0)“Lander Lab” is a hands-on column of Ocean Lander technologies and strategies, a unique class of unmanned undersea vehicles, and the people who make them. It is meant to serve the global ocean lander community in the manner of Make Magazine and other DIY communities.Comments on this article, or suggestions for stories of interest to other Landereans are welcome. Ocean lander teams are encouraged to write in about their work. Please feel free to contact Kevin Hardy