[This article appeared in the August/September 1997 issue of Home Power Magazine] -> Note: some of the part numbers have changed from when the article was -> published to when the circuit board was manufactured. Low Voltage Battery Disconnect Circuit automatic battery shutoff for medium power DC loads (C) G. Forrest Cook 1997 email: cook@solorb.com INTRO Lead Acid and NiCd batteries will last a long time if they are operated within the proper charge and discharge voltages. A charge controller circuit is a necessity for preventing battery over-charge, conversely, a low voltage disconnect circuit (LVD) prevents excessive battery discharge. By using a combination of both circuits, it is possible to keep the battery running within the proper operating range. This article describes a low to medium power LVD circuit which operates like a common on-off toggle switch. The circuit is very efficient, consuming a mere 8 milliamperes of current while running and essentially no power when shut off. The LVD circuit was designed to use parts that are commonly available from many sources. The prototype was built entirely from parts out of my junk-box. THEORY The heart of the LVD circuit is the power MOSFET transistor, Q1. Transistor Q1 operates as a switch in the positive line of the external circuitry. Switching of the positive side of the circuit allows for a common negative ground between the battery and the load which aids in many applications, especially automotive ones. To achieve this "high side" SWITCHIng, it is necessary to generate a gate drive voltage that is higher than the supply voltage, this is accomplished by a voltage tripler circuit. Op-Amp U1b generates a 5 Khz square wave, this is fed into the diode/capacitor ladder circuit which successively boosts the voltage to about 3 times the peak voltage of the square wave. This signal is then used to gate on the power MOSFET. Resistor R5 is used to discharge the gate circuit when the lvd is shut off, allowing the MOSFET to turn off. The voltage comparator circuit consists of U2, a standard 5 volt regulator which is used as a voltage reference, U1a, which is wired as a comparator, and VR1 which is a voltage divider which provides a set point for the low voltage shutoff. When the battery voltage is above the threshold, U1a provides a positive output which is used to create a bias level via the R2/R3 voltage divider, that allows the U1b oscillator to run. When the battery voltage drops below the set point, the output of U1a goes to zero and the U1b oscillator shuts off, causing the voltage tripler and MOSFET to shut down. Resistor R7 gives the comparator circuit some hysteresis to prevent comparator oscillation near the shutoff voltage. The circuit is analogous to a solid-state latching relay in that it shuts its own power off when the MOSFET turns off. This is achieved with diode D6 and the on-off switch. When the circuit is switched on, capacitor C11 acts like a momentary short-circuit, pulling the op-amp Vcc line up to the battery voltage. The whole circuit fires up long enough to turn the MOSFET on, after which operating current flows through the MOSFET and diode D6. When the switch is shut off, the comparator is forced off by shorting pin 3 to ground, this turns off the MOSFET. Capacitor C11 is discharged through the other half of the switch and current limiting resistor R8, which is required for circuit start-up the next time the switch is turned on. Diode D7 is used to protect the MOSFET from negative spikes generated by motors or other inductive loads. Capacitors C7, C8, C9, and C10 provide filtering in various parts of the circuit. Fuse F1 protects the circuit from overload and should be a fast blow fuse that is rated at about 80 percent of the maximum current that Q1 can handle. Numerous MOSFET transistors can be used for Q1, parts selection is based on cost and maximum current. The IRFZ34 MOSFET is rated at 30 amps continuous current and should be used with a heat sink and thermally conductive grease. A lower power MOSFET such as an IRF520 may be used for up to 8 Amp loads. ALIGNMENT Alignment is straightforward, the equipment required consists of a variable voltage power supply and a load such as a small 12V light bulb. Connect the power supply to the battery input terminals and the light bulb to the load output terminals. Set potentiometer VR1 to the midpoint, set the variable voltage supply to 13 Volts. Turn the circuit on, the light should go on, if it does not, adjust VR1 towards ground and switch the LVD circuit off and back on, repeat that until the light stays on. At this point, slowly turn the variable power supply voltage down until the light goes out, this is the LVD set point. Adjust VR1 until the shutoff voltage is where you want it to be, I usually set it to 11 Volts for gel-cell batteries. Remember to switch the circuit off and back on while adjusting, it will not turn on by itself. CONSTRUCTION I built the prototype circuit on perforated circuit board using point to point wiring. Teflon insulation over tinned bare wire is my personal favorite method for making prototypes since the teflon can withstand a lot of abuse without melting. Wire-wrap or other methods could also be used for construction. Be sure to use thick wires for the current carrying part of the circuit. In the prototype (see Photo 1) I built the circuit into an aluminum box and used computer DB25 connectors for the input and output Connectors. USE Simply connect the circuit between the battery and the load and use it like a switch. If the battery sags below the set value, the circuit will shut off. After the battery is charged up again, the circuit can be switched off and back on. The circuit works well with a 12V car tail light and a gell-cell battery. BATTERY CAPACITY METER An interesting application for this circuit is to use it as a component in a battery capacity meter. For the load, simply wire a high wattage resistor in parallel with a 12 Volt mechanical clock, set the clock to 12:00 and charge the battery up. Select a load resistor that gives the desired discharge current. To make a clock that runs on 12V I used a 1.5V travel alarm clock with the voltage dropping circuit shown in figure 2. Discharge the battery via the LVD circuit and measure the hours that it ran after the LVD shuts off. This circuit is very useful for sorting through a set of marginal batteries and gives an indication of the useful power that the battery can provide. Figure 2: ------- 1K,1/2w 2X 1N4001 -------|+ +|----------/\/\/\------->|--->|----- | | LVD | | | | --- |b l| / | + | - |a o| \Load 1.5VDC | --- |t a| /Resistor CLOCK | - | d| \ | - | | | | / | | -------|- -|-----|----------------------------- -------