Owner`s manual

35
What is Inductance?
When current passes through a conductor a magnetic field is set up around the conductor. As this
magnetic field builds, it induces voltage in any conductor that is close by, and it induces voltage in the
original conductor. The voltage induced in the original conductor is called self-inductance, and tends
to oppose the current that produced it.
The magnitude of the self-induced voltage is proportional to the size of the loop formed by a wire.
The larger the loop, the larger the self-induced voltage. The positive and negative battery cables in a
system are in reality only a single circuit (wire), and so the inductance of the battery circuit depends
on how the cables are physically positioned or arranged with respect to one another.
Tape Battery Cables Together to Reduce Inductance!
If battery cables are separated by a distance, they have much more inductance than if they are close
together. If the two battery cables were coaxial there would be virtually no induced current flow since
the magnetic fields would cancel one another. However, we don’t have coaxial battery cables, but we
can approximate them by taping the cables together every four to six inches. When the cables are
taped together the magnetic fields around each battery cable tend to cancel each other. When
cables are separated the magnetic fields add together and increase the inductance of the battery
cables.
The table shows that with only a foot of
distance between the battery cables the
inductance almost doubles, and at four feet
between cables the inductance is almost three
times the inductance of cables taped together.
Since induced voltage in a conductor varies as
the inductance times the rate of change of current in the inductor, the induced voltage may be three
times greater than it would be if cables were not taped together. For more advanced readers,
consider flyback effects and the induced voltage spikes can get into the thousands of volts range if
the battery were suddenly removed from the circuit (worst case).
These induced voltage changes cause ripple in the battery cables and must be absorbed or filtered
by the filter capacitors in the inverter. The ripple will lead to eventual premature breakdown of the
filter capacitors and performance loss in the inverter. In addition to the problems mentioned, the
induced current opposes the applied current (battery current), which directly causes a loss of inverter
performance as greatly reduced efficiency.
Hopefully, this discussion gives you a better understanding of why battery cables should be kept short
and close together. Maximum performance is the goal of any well-designed power system and the
detail items such as this will help achieve this goal.
6.2 BATTERY CABLE INDUCTANCE
Distance Between
Cables
Inductance in
micro-henries
Taped Together 3.3
12” Separation 6.0
48” Separation 8-9