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The 'Hog' Tamer

They say that necessity is the mother of invention. Having to add 3 new batteries to my battery bank has caused me to go to work on a solution to Tame these Charge Hogs.

The drawing above shows what I affectionately call the Hog Tamer. I designed this circuit to tame those new batteries that you may have the misfortune of having to add to your battery string to replace prematurely deceased batteries.

The Problem: Adding a new battery to an older string works to starve the older batteries during charge. The new battery charges easily and quickly while the older batteries are far from full. When the new battery is fully charged, it causes the charge current to drop down so the others do not fully charge in a reasonable time period. What's more, the new battery's terminal voltage will rise to 8 V or more, robbing the other batteries of charge voltage.

The Hog Tamer monitors the new battery's terminal voltage and does not allow it to rise to a level that will rob the older batteries of charge. During the charge process, the lamp will light when a battery terminal voltage threshold is reached. The lamp will draw the battery down a little and turn off. When the battery returns to the trip voltage, the light will once again come on to draw the battery down. This process keeps current flowing to the other batteries that have not finished their charge. At the end of the charge process, the lamp will flash on and off very slowly or simply remain off.

How it works: The LM311 integrated circuit is a basic voltage comparator. The 5.1 V Zener diode voltage is used as a reference voltage at pin 3 to which a sample of the battery terminal voltage at pin 2 is compared. If the voltage at pin 2 exceeds the voltage at pin 3, the output of the comparator goes high, pulling the gate of the MOSFET high and turning it on. The H3 lamp comes on to drain off charge from the Hog (new battery). The Variable Resistor (VR) is set so the trip point is around 7.2 V.

Why the lamp? A high-power lamp is used because it is much cheaper than a high-power resistor and because it converts some of the energy to light instead of heat and you can watch it work. It operates much cooler than if a resistor were used. Also, this lamp will last indefinitely because it is operated at only 7 V instead of 12+V.

Junk-box Heatsink Works Fine
Mounted on the ABS plastic sheathing in the battery compartment, it is isolated from the chassis.

Note: If you would prefer purchasing a Hog Tamer fully assembled, calibrated and ready to connect, contact Mark: mail@evhelp.com The unit will not have the same appearance as the prototype shown above. $69.95

Construction: Use a heatsink to mount the lamp and the MOSFET. A 4" by 5" finned heatsink will do. If you can't find a decent heatsink, use a piece of 1/8" thick aluminum plate roughly 6" by 6" or an aluminum project box in the 4" by 2" by 2" range. Whatever you use for the heatsink, make sure you isolate it so it does not come in contact with the chassis of the vehicle or any wiring. Note that when you mount the MOSFET and the lamp on the same heatsink, the Drain (D) of the MOSFET will be electrically connected to the metal side of the lamp base to automatically make that connection. Note that the metal tab and back of the MOSFET is the Drain connection along with the center lead. The wire on the lamp will connect to the positive battery wire. The wires leading to the battery posts should be as short as is practical of no smaller than AWG #14 (use #14 or #12 flexible stranded wire). The circuit is built on a protoboard (project board) from Radio Shack. It has a lot of holes for component leads and copper foils on one side. Be careful when handling the MOSFET. Static electricity can destroy the Gate junction.

Components: The H3 lamp can be purchased from any automotive store - make sure it says 100 W on the package. All resistors are rated 1/4 Watt - values shown are in Ohms. All capacitors are in microfarads. The Zener diode is either 1/8 or 1/4 Watt. All components should be available at Radio Shack or from DigiKey. The MOSFET is an N-channel type in a TO-220 or TO-247 case style. The Gate threshold voltage must be low - a TTL level MOSFET will do. The MOSFET should be rated for a current of 15 A or higher and the Drain-to-Source voltage rating should be in the range of 12 to 60 V. The on resistance of the MOSFET should be less than 0.25 Ohm. The Fairchild NDP4060L is a good choice. The heart of the device is the LM311 comparator integrated circuit. The LM111, LM211 and LM311 are all the same for this application. Radio Shack sells an LM339 quad comparator, meaning that there are 4 comparators in one package. The package is a little larger, 14 pins instead of 8. It's OK to use only one of the comparators in the LM339 package. The diagram that comes with the LM339 will help you associate which pins should be used for the diagram below.

Alternative Circuit Using a Relay: Hog Tamer - 2, shown above, uses a relay instead of a MOSFET. It may be easier for some folks to obtain the relay. Note diode D1 across the relay. This diode is essential. It protects the LM311 from damage due to coil voltage kickback when it is turned off. The diode is any generic 1A, 50V diode - these are very common.

Adjusting VR:

Method 1 - Battery NOT Connected

If you have a variable DC power supply, the adjustment is easy. Adjustment is made with the unit disconnected from the battery. Temporarily disconnect the lamp from the relay. Connect the variable DC voltage supply to the two battery wires leading to the board. Connect a digital multimeter, set to measure DC volts, to the same battery wires in parallel with the power supply and board. Increase the DC supply voltage to 7.2 V. Adjust VR to trip. You should hear the relay click on. Carefully adjust the variable DC supply up and down slightly to observe the trip action at 7.2 V.

Method 2 - Unit Connected to Battery

In this method, it is assumed that you do not have a variable DC supply to set VR. Connect the unit to the 6-V battery as in normal use. Start a charge cycle using your bank charger as you normally would. Use your digital multimeter to observe the charge voltage on that battery. When the charge voltage at the terminals reaches 7.2 V, adjust VR to trip and turn the lamp on. Adjust VR carefully to the point at which it just trips. If it trips on too soon, adjust VR to turn it off.

- Mark

Disclaimer: Mark E. Hazen, the author of this Electric-vehicle Conversion Web site, is not responsible for injury or losses of any kind resulting from use or misuse of information contained herein.