Author Topic: DC Net Current Metering  (Read 2202 times)

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Offline MadScientist267

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DC Net Current Metering
« on: June 04, 2014, 10:57:34 AM »
Note: This thread is about DC net current measurement with a battery bank, and while the concept is analogous, the similarities with AC net metering for grid tie inverter systems ends there.

It comes up from time to time in IRC, what is net current metering with a battery and how is it accomplished?

It's a relatively simple concept, but isn't the easiest thing to describe with text only, so here's the basic rundown with a couple of schematics for clarification...

Net metering as covered here is a simple way to extract the most useful information out of a single ammeter connected to a battery. On larger systems, charge and load currents are generally monitored separately, as this provides greater resolution and behavior is easier to spot. But what if your system is relatively small, and for one reason or another, you only need, have, or want a single ammeter to measure current? Net is the way to go.

Connecting a single meter in line with only your charge source or a load is fine if you want to see what's coming in or going out, and as mentioned, both simultaneously is fairly standard practice on larger systems. But alone, one or the other can be deceptive. Monitoring charge current may tell you how your panel(s) and charge controller are functioning, but if your load is greater than your charge source, you're not charging your battery, and this will never show up on that meter. The same applies for load current. You can gather what you're drawing, but are you producing enough? Simply not there.

With net current measurement, you get the effective current going into or coming out of the battery with a single meter. For charge current exceeding a load, the values are positive; when the load current is greater than the charge source, they go negative.

As the general rule, digital meters of today can read both positive and negative current flow, and nothing special about them is required to use them in this manner. If you're looking at using an analog meter movement instead of digital, you'll need a meter specifically designed to measure current in both directions transparently (0 is in the center of the scale, as the veteran crowd may remember from alternator gauges of lore on vehicles). The shunts for both are connected to the system in the exact same manner, however.

The next consideration is high or low side measurement. Simply put, which side of the battery is best to put the meter on? There are pros and cons to each, and other aspects of the system may dictate that one is preferable over the other. The measurement is the same either way. One of these considerations is the design of the meter itself.

Analog meters don't care where they are in the chain. There's no reference to anything else that would cause issues on a small system, and as such, the decision can be made with almost no other considerations. But while digital meters measure the same attribute, the way they are designed internally may require that they be connected on one side or the other. Some don't care, some need one or the other to read correctly, and others yet may be permanently damaged if improperly connected. Consult the documentation that comes with your meter to find out.

Once you know which side you need or want to be on, you can connect your meter. Below are the two general schematics for doing this.

High Side:





Low Side:





Note that the volt and ammeter connections are directly across the battery and shunt, respectively. In fact, shunts have specially designated terminals for connecting the meter. This is important, as mis-connection of the ammeter across the shunt causes incorrect readings. Likewise, each volt meter connection is made on the battery-most side of each terminal so that current flow has the least direct effect on readings as possible. The only thing worse than no information is bad information. ;)

One final note: Not drawn in the schematics are fuses for safety. The exact location of fuses depends quite a bit on the design of the system, but regardless, should never be omitted! Bare minimum, a fuse between the shunt and the battery will provide protection that is better than nothing at all. But you knew that already :)

That does it for this episode. Any questions/additions/corrections/oversights etc, post as desired...

Steve
Wanted: Schrödinger's cat, dead and alive.