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Fire! (and other power loss
problems)
A terrifying occasion in
any event but especially frightening in your electric vehicle. That is what may happen
when a terminal post melts off a battery. It not only leaves you stranded, but it could
cause a battery to explode or burn your car to the road. Not good options, and a problem
that is so easy to prevent. This isn't about cleaning the terminal not tightening the
cables, but how to test the connection.
Many automotive electrical problems can be traced to the resistance
between the battery terminals and cable clamps including low speed. (Note: This technique
can be used on the motor terminals also.) The battery vent caps, which release small
amounts of acid, are in close proximity to the terminals. Corrosion is formed as a result
of a chemical reaction between the acid, terminal and clamp material. When the corrosion
occurs on the mating surfaces of the clamp and terminal, a small amount of resistance is
created. This interferes with the normal flow of electrical current. To see how much even
a small resistance affects the operation of the vehicle, consider the following:
Assume the resistance between the clamp and the terminal is
.05 ohms (5/100 th of an ohm) Way too small to read on a standard analog volt/ohm meter or
even most digital meters. Further, assume your motor (load) draws 100 Amperes. Using Ohm's
Law (E=IR), we find the voltage drop to be 5 volts! (0.05 ohm X 100A). That means you
don't have that 5 volts for your motor. That is what happens at each terminal that does
not have a clean tight connection! Want to change that to power? P=IE: so take that 5
volts multiple by the 100 amperes and you find 500 watts of power! How many high power
connections does your electric car have?
How to test the connection. Place the positive lead of the
voltmeter on the positive battery post and the negative lead on the cable. Reverse the
leads on the negative posts. With the drive wheels off the ground, apply power, being
careful not to overspeed the motor. If the voltmeter indicates more than .2 volts, the
terminal resistance is excessive. The clamp must be removed, the clamp and terminal post
cleaned and reinstalled. If you don't do it, physics will; leaving you with a puddle of
lead where the post use to be - or worse.
Note in this example that the voltmeter
indicates a full .5 volt drop in battery voltage across only one terminal!
NOTE: Exercise caution! Use your safety equipment. You don't
smoke around gasoline so don't smoke around batteries. A watchband (or other jewelry) from
a positive point to ground will certainly ruin your day.
A terrifying occasion in
any event but especially frightening in your electric vehicle. That is what may happen
when a terminal post melts off a battery. It not only leaves you stranded, but it could
cause a battery to explode or burn your car to the road. Not good options, and a problem
that is so easy to prevent. This isn't about cleaning the terminal not tightening the
cables, but how to test the connection.
Many automotive electrical problems can be traced to the resistance
between the battery terminals and cable clamps including low speed. (Note: This technique
can be used on the motor terminals also.) The battery vent caps, which release small
amounts of acid, are in close proximity to the terminals. Corrosion is formed as a result
of a chemical reaction between the acid, terminal and clamp material. When the corrosion
occurs on the mating surfaces of the clamp and terminal, a small amount of resistance is
created. This interferes with the normal flow of electrical current. To see how much even
a small resistance affects the operation of the vehicle, consider the following: