What is this thing on the negative battery cable?

[mikels]

Originally Posted by absinthe

It monitors the current through the neg cable and sends the feedeback to the ecm. If there are any problems that it detects, it throws a voltage code and will typically put a "svc charging system" message on the dic.

Actually it's the current sensor for RVC (regulated voltage control). Many newer vehicles don't just 'charge' the battery when running, they calculate battery state of charge and monitor current in and out of battery to optimize energy usage and charging of battery. A little more complex than that, but that's the general idea.

This sensor signal is actually read by the BCM (body control module) where RVC algorythm resides. BCM then directs ECM to control generator to desired output. This is why on many new vehicles, you can see system voltages around 12.4 volts with engine running (rather than ~14 like older cars). System has determined that battery cannot absorb more energy, so directs generator output to maintain voltage - not charge.

Bottom line is another means of improving fuel economy by intelligent energy management.