Monday, January 31, 2011

Manage those electrons

I've talked at length about managing your onboard source of electricity.  To do this effectively, there is one key instrument you *need* to have - an amp-hr meter.  This device keeps track of the amount of electricity going into and coming out of your batteries - it is effectively a battery "fuel gauge".

The devices are not hard to install.  To do their work, they need to sense the battery voltage(s) and the current flow.

For current flow, all will come with a "current shunt" - a precision very low resistance resistor made up of copper plates stretched between two copper blocks.  Final calibration of these resistors is done at the factory by grinding away some of the plates, so don't be surprised to see this. The shunt is typically installed in the negative lead to the batteries, between the batteries and the first connection (multiple battery banks will require multiple shunts - often combined in a single unit).  That is, no load (including the engine starter) on the batteries can escape passing thru the shunt.  Two light gauge wires will be connected across the shunt and directed to the amp-hr meter.

Next, the amp-hr meter needs to know the battery voltage.  If you have multiple battery banks, it will need to know the voltage of each.  On the Link 2000, the negative connection at the shunt is used for the negative voltage connection; a separate light gauge wire is run to the batteries for the positive connection (yours may differ).  It is *extremely important* that the positive connection be made as directly to the batteries as possible.  Ideally, to the battery terminal itself.  If this is not possible, the connection should be made to a very heavy conductor (say at the "1,2,Both" switch).  Any current being carried in the wire to which the voltage sense lead is connected will distort the voltage reading, potentially to the point where the system becomes useless, if the current is high enough or the wire is small enough. 

But the good news is that all the sense leads need only be very light gauge.  Typically they should be twisted pairs to minimize interference.  If all your wires are going from the meter to a single area, a standard RJ45 ethernet cable is a great source of multiple twisted pair wires in a single jacket.

The Link 2000 shown above also controls our inverter/charger.  That connection is made with a standard telephone cable (if that still has any meaning in this era of cell phones...).

So, to install, choose a location for the meter display, choose a location for the shunt, and string light gauge wires.  It actually does qualify as a *small* boat project.

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