Electric Meter

There are two types of electric meter, shunt-based and voltage-based. The most simple type is the voltage-based monitor and frequently comes standard on most mobile power applications. This type of monitor just measures the real-time voltage of the battery and uses it to estimate its state of charge.

Voltage-based monitors are not very accurate because the real-time voltage of a battery will fluctuate based on temperature changes and other environmental conditions. If you change the battery type the voltage meter may even be completely inaccurate altogether.

The shunt-type monitor is much more accurate because it measures the actual energy flowing into and out of the battery. These electric meter require a shunt to be installed on the negative side of the battery terminal. The shunt measures the real-time voltage of your battery system as well as the current draw.

V = Volts. Voltage is used to assess the approximate state-of-charge and to check for proper charging. For example, an at-rest, fully charged 12V battery bank will show about 12.6V to 12.8V. A 12V battery is 100% discharged when it reaches 10.5V. A 12V battery is 50% discharged at 12.2V assuming there are practically no loads on the battery. When charging, you are looking for an increase in voltage. For instance, if your batteries were at 12.1V before charging, you will want to see that number increase. It's hard to predict where the voltage will rise, as it depends on the size of your battery bank and the charge rate. A good analogy is topping off a pool with water, how quickly it takes for the water level to reach full, depends on the combination of the hose filling the pool and the size of the pool. Imagine a battery bank at 11.9V, a 100A alternator will get 2 golf cart batteries to 14.4V in little time, conversely a 55A alternator will take hours to get 8 golf carts batteries to 14.4V.

A = Amps. Amps is the flow of current in or out of the battery. Current is analogous to speed, it's the rate at which electricity is flowing. While driving a boat, you would say I'm doing 10 knots, and you would cover 10 NM if you travelled for one hour. Amps are similar to boat speed. For example, I am drawing 10 amps and, if the current was constant for one hour, you would say I used 10 amp-hours (more info on amp-hours below). Your fridge may draw 6 amps of current and this is displayed as -6.0A. Discharge is shown as a negative number and shows the number of amps that are being consumed. This is an important function to teach your crew as it serves as a good reminder to turn off unused lights, navigation equipment, etc. If you had no loads on your batteries, any charge going into the batteries (e.g. solar, alternator, charger, etc) would show up as a positive number.

Ah = Amp-Hours. This shows the amount of energy stored or removed from the battery. If you run a 10A load for one hour then 10Ah are consumed. The electric meter will show -10 in the Ah display. During charging, the electric meter will compensate for charging efficiency and count back up toward zero. A full battery is displayed as zero amp-hours or 0Ah. Any draw from the battery is reflected as negative amp-hours or – 0.4Ah and recharging will bring the number back to 0Ah.

H or t = Time. This is an estimate of how long (in hours) the battery will sustain a load, assuming the load is constant over time. It is on a selectable, time averaged, rate of discharge. The default on most monitors is the average of the last four minutes of use. This would only be useful if the loads on your boat where constant and did not fluctuate. A good example of this is predicting the distance you will travel in one hour if you are driving on the highway or in the city. On the highway, assuming little traffic, your speed is 100 km/h and you will travel 100 km in one hour. But in the city, your speed fluctuates constantly and it's hard to know the distance you will cover by knowing the speed you are going over the last 4 minutes. It's for this reason that we don't encourage boat owners to pay attention to time predications.

Set Your Battery Capacity. When you install your electric meter, it is important to remember to set your battery capacity. Many electric meter are preset, for example, the Xantrex Link 10 assumes you have 200 amp-hour, lead acid batteries. Your actual battery capacity and type will, most likely, be different so you must change these settings for the electric meter to give you an exact reading.

Location of the Shunt. The other important installation tip is the location of the shunt. Ensure that there are no loads (e.g. diesel heater or bilge pumps) or charging (e.g. solar panels or EFOY) connected directly to the batteries as these will bypass the shunt and will not be seen by the electric meter. This is the biggest mistake we see in electric meter installations.

Several key factors can help to sift through what electric meter is correct for your project. The main factors to focus on are the intended battery chemistry, application requirements, accuracy and precision, communication interface, and the integration and form factor.

Battery Chemistry
When dealing with different battery chemistries, there are a variety of electric meter that you can choose from. Other battery chemistries come with distinct characteristics, and your electric meter must be compatible with the specific chemistry you're working with. Understanding each chemistry's intricacies is essential to ensure accurate battery monitoring. Various chemistry types can impact the performance and longevity of your system.

Number of Battery Cells
You can significantly narrow your electric meter selection depending on your application requirements. Another major factor is the number of cells in the pack. Whether working with a single-cell or multi-cell configuration, your electric meter choice must align with the overall pack voltage and capacity requirements. Also, the load placed on the battery plays a prominent role. You must ensure the electric meter can accurately monitor the battery pack in its intended usage range.

Communication Protocol
Another critical factor to remember is the communication protocol used by the overall system. Standard protocols include I2C, HDQ, SPI, and One-Wire.

Methods Of Measuring
Electric meter also have three main methods of measuring the batteries' state of charge. Those three methods are coulomb counting, voltage-based, and hybrid electric meter.

Coulomb Counting Electric Meter
Coulomb counting electric meter are some of the most fundamental electric meter available. They allow for a high precision state of charge estimate and are less affected by the self-discharging of cells within a battery. However, they require precise calibration to be reliable and can accumulate minor errors over time that lead to inaccuracy during the end of the battery pack's life cycle.

Voltage-Based Electric Meter
Voltage-based electric meter are some of the simplest on the market. These rely on monitoring the battery voltage to estimate the remaining state of charge. Typically, these gauges consume minimal power and are suitable for simple fuel gauging scenarios.
These generally are less precise than coulomb counting gauges and may take inaccurate readings while the battery is under load. Voltage-based electric meter are most accurate when the batteries have had time to rest after being under load or charging.

Hybrid Electric Meter
The final type of electric meter is a hybrid gauge. These typically combine multiple measurement methods and are the most accurate of the three gauges. They are the most common type of electric meter available on the market today. They perform their monitoring using a combination of coulomb counting, voltage monitoring, and temperature sensing to provide a more comprehensive view of battery health. These typically are more robust and fluctuate less with variations in battery health over time.