Deep-discharge Protection for Lead-Acid Batteries on Mobile Robots

Description

This circuit and its corresponding software were developed in response to a problem that the researchers at LPR were having with their mobile robot: users were forgetting to attach the charger to the battery after using the robot, and the battery was deep-discharged several times. Deep-discharges are destructive to lead-acid batteries; only two or three deep-discharge cycles are needed to reduce the effective capacity of the battery to almost zero.

The purpose of the circuit is the following: monitor the state of charge of the battery, measuring the battery voltage and current delivered to the load. When the charge level falls to a level at which only about 10 minutes of energy is left in the battery at the present current level, send a signal to the computer advising that the battery charge is low. If this condition persists for 5 minutes, send a second signal to the computer that shutdown is imminent. After 10 minutes of a low battery indication, shut off the power to the robot and its computers. An additional turn-off condition is an overload current: the load is consuming more current than desired, but no so much that the fuse is blown.

Two turn-on conditions are also defined: The first is the charging voltage that indicates that the charger is attached and that the power to the robot can be turned on without discharging the battery. The second is the minimum open-circuit voltage of the battery that indicates more than 10 minutes of energy is available to power the load.

Source Files

The circuit and associated programs are available as a compressed tar file. This circuit was designed for two 12V lead-acid batteries connected in series. The maximum rated voltage for the voltage sampling side of the circuit is 40V. Included in the tar file are the following (the names of some of the files contain "mspm", which stands for Marvin (the robot name) Smart Power Management):

• Schematics for the battery monitor circuit and the opto-isolator to RS232 converter circuit. These are Microsim^® DesignLab^TM schematic and printed-circuit board layout files (PostScript^TM files of the schematics are included). The battery monitor circuit is based on the PIC16C71 microcontroller with A/D converter from Microchip.
• Assembler program for the PIC microcontroller. This program implements a serial interface and the battery charge monitor state diagram that is shown below. The maximum baud rate of the circuit is 4800, a limit imposed by the opto-isolators. The sampling rate for the voltage and current measurements is the baud rate. The serial interface includes commands for monitoring the state of the battery (voltage, current levels) as well as the following three control commands:
  • Turn Off: Shut off the power now.
  • Turn On: Bypass the "Wait Off" state. The purpose of the "Wait Off" state is to provide a minimum off time for the batteries.
  • Delay Turn Off: Delay the shut down of the computers (a maximum of 8 of these is permitted for each low battery condition). This command is provided in case just a little more time is needed to shut down the computers properly.
• A "Suicide" program. The intended use of this program is that it be the last command of the shut-down process of the computers on the robot: when the respective operating systems have halted, the power is no longer needed.
• A daemon program for monitoring the parallel port control signals sent by the battery monitor circuit. This program, called mspmd.c, is written for Linux running on a PC (it uses the ioperm() command on the appropriate addresses for the parallel ports on a PC). When the status of the battery changes, it calls a script (/etc/mspm) with one of four parameters:
  • low: The batteries are now low. The script should do the appropriate actions (i.e., notify the users to plug robot into the charger).
  • dead: The batteries have been low too long. The script should perform the appropriate actions to shut down the computers.
  • ok: The batteries are ok. This occurs if there was a transient low-battery condition. The script should notify users that it is OK to procede again.
  • lost: The connection to the battery monitor circuit was lost (this is detected by connecting an output bit of the parallel port to one of the status signals). For this condition, the script should complain about the lost connection, and demand that it be re-connected.
• A skeleton mspm file (called by the above program). The actions appropriate for your system should be put in the corresponding places in this script.
• A shell script for starting the daemon program on linux. This will require some changes for other operating systems.

For more details, see the relavent source files (particularly mspm_cir.asm, which describes which parameters change because of variations in the circuit and its componenets).

Connection and State Diagrams

Larger views of the figures below are available in a format suitable for printing (i.e., black text on a white background); just click on the image. The following figure shows the connections between the batteries, the battery monitor circuit, the computer, and the load (refered to as "Power Supply" in the figure):

The MOSFET used on Marvin is the IRFP064 N-channel MOSFET from International Rectifier, is capable of handling 70A, and has an R_DS(on) of about 6.4 milli-Ohms at V_GS of 15V. Any other MOSFET with a sufficiently large current rating can also be used (the MOSFET chosen as well as the maximum current expected for the load affects the gain required from the op-amp stage that measures the current by measuring the voltage across the MOSFET). The choice for the catch diode is not critical; its purpose is to provide a path for the current from inductive loads when the power is turned off.

The assembler code for the PIC microcontroller implements the following state diagram for monitoring the status of the battery:

Note that the outputs refered to in the figure are the two digital signals shown in the previous figure, and that the sub-nodes next to the "Wait Off 1" and "Wait Off 2" have the same outputs.