Battery cable inductance

mcgivor · August 2019

Found this interesting information and thought it may be of interest to others. The table didn't copy paste but taped together is 3.3 micro Henries, 12" 6.0 and 48" 8-9 respectively.

BATTERY CABLE 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 a voltage in the

original conductor. The voltage induced into 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.

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. If you aren’t

convinced that taping battery cables together helps reduce inductance, consider the following table of

information collected by Xantrex. We tested two sixteen foot long #4/0 AWG battery cables connected

together at one end and parallel to one another.

Table 8, Battery Cable Inductance

Distance Between

Battery Cables Taped Together 12” Separation 48”+ Separation

Inductance

in micro-Henries 3.3 6.0 8-9

The above 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 the 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. This 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.

To avoid this problem, route your positive and negative DC cables in parallel, as close together as

possible. Secure the cables against movement with clamps or straps every 18 inches. Avoid routing

conductors near heat sources such as dry exhaust or furnace piping. Avoid chafing sources such as

steering cables, engine shafts, and throttle connections.

Hopefully this information gives a much more realistic and clear idea of why battery cables should be kept

short and close together. Maximum performance is the goal of any well designed power system and the

details such as this will help achieve the goal.

jonr · August 2019

While it certainly won't hurt to follow their advice, 3.3 vs 9 uH will probably make no practical difference.

A simulation (LTSpice) indicates that there could be an oscillation/capacitor over-voltage problem either way - depends on the design of the inverter (specifically capacitor size/voltage and protection devices) and most importantly, the internal resistance of the battery (more is significantly better - it creates a damped circuit). Too bad they didn't measure the ripple/transient voltage rise seen with typical inverters/batteries and specify a limit. Anyone with an oscilloscope can easily check their system.

> which directly causes a loss of inverter performance as greatly reduced efficiency.

The inverter input capacitor smooths this out - it should be a non-issue. But again, they provided no actual measurements to indicate when it becomes how much of a problem.

Dave Angelini · August 2019

mcgivors advice is from the days of Trace Engineering. It was as good then as it is now in my opinion.

mcgivor · August 2019

The passage is an excerpt from a user manual for informational purposes, the findings would no doubt be discovered through research, they probably did complete the research arriving at the conclusion they did. Publishing all the research details is generally not covered in user manual, the engineers at Trace at the time, were at the forefront of the inverter development and some went on to develop some of the most iconic names in the solar industry.

Reverse EMF or Flyback is a serious problem often overlooked by those lacking experience in DC circuitry, many fire alarm systems installed by electricans who are not familiar in DC circuitry, fail to install snubbing diodes across slave contactor coils, resulting in premature master contact failure, same goes for DDC installs.

Marc Kurth · August 2019

mcgivor , yep I was in the low voltage DDC business for years and learned that lesson the hard way in the early 80's.. It was hard for me to wrap my thick head around burning contacts because of a coil! (Did projects using mostly Solidyne but some Staefa, and the original CSI) Some of the systems that I did in the 80's/90's are still running today.

Dave Angelini · August 2019

I remember a stacked pair of 5548's driving a deep well. Moved the battery cables close together and the load started pumping water. They hired me for over 20 years on that one.

jonr · August 2019

A useful user manual or spec sheet would quantify the acceptable inductance. For example, say I tape the cables per the recommendation. So is 20' then OK? Or maybe even 10' taped is causing significant loss of life/efficiency? A recent Xantrex manual says 10' max and nothing about taping. Others (Outback and Magnum) recommend taping but don't specify a maximum length. The result is "engineering by guess".

Dave Angelini · August 2019

So what is your point? You think the 5548 manual does not discuss cables?

Battery cable inductance

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