Power Management

Batteries are chemical reactions inside sealed boxes, the only way to determine what is going on is to use monitoring tools.

Batteries are essentially “boxed chemistry sets.” Managing a battery system and maximizing its lifespan is a task of managing that chemical process without being able to feel or see what is going on. Once the senses of sight and touch are unavailable, the only fall-back is through monitoring equipment.

The principle piece of power management for DC battery system on yachts is the dedicated Power Manager. Essentially a power manager is a combination of a voltmeter and amp-meter with a numeric or graphical display. Now one could simply use a voltmeter and amp-meter (commonly combined into a multi-purpose device called a multimeter) to read the voltage produced and the current either entering or leaving the battery. This is an initial level of monitoring and is certainly better than nothing. People working on this level use an approximation table of voltages to determine the level of charge.

Voltage Charge Level
12.70 100%
12.50 90%
12.42 80%
12.32 70%
12.20 60%
12.06 50%
11.90 40%
11.75 30%
11.58 20%
11.31 10%
10.50 0%

The trouble with this approach is that there is not a direct correlation between a specific voltage and a level of charge in the battery. For example the voltage 12.32 might occur for various different stages of discharge. An example is 12.32 when the battery is running some lights is not the same as 12.32 when the battery is being charged – the first might mean 75% charged and decreasing while the second might mean 45% charged and increased.

What is missing is a measurement of “charge” in the battery. This unit does exist and is known as the amp-hour, 1 amp-hour meaning the ability to supply 1 amp for 1 hour, or 1/2 amp for 2 hours, or 5 amps for 12 minutes, etc. Although the most commonly used unit for measuring the level of battery charge the unit is not perfect. The problem is that being a chemical reaction, the rate of discharge does not load the battery in a purely linear way. Put another way, 10 amp for 10 hours is not the same load on the battery as 200 amps for 30 minutes. If a typical battery were expect to supply 200 amps for 30 minutes it would quite possibly fail. In addition the battery supplies more amp-hours at warmer temperatures than at cold or hot temperatures so charging a battery for 200 amp-hours at room temperature will not necessarily produce 200 amp-hours at freezing temperatures (as anyone trying to start their car in the winter can attest to). Better quality batteries list their amp-hour ratings at different temperatures (often freezing and room temperature). Also better batteries list their ratings at different loads – typical being 10 amps and 20 amps. Most batteries will deliver more amp-hours when supplying 10 amps then when supplying 20 amps.

So the amp-hour is not perfect, but it is the most workable commonly used measurement unit. Returning to measurement devices, in addition to a voltmeter and amp-meter a proper power manager will incorporate a amp-hour (Ah) counter. This measurement is made by accurately measuring the current passing in or out of a battery – as current flows in and out of the battery the sum rises and falls. Some units display the amount of discharge (e.g. -43 Ah) while others display the amount of charge remaining (e.g. 85%). Either tells you the same thing, neither number makes a lot of sense without knowing the capacity of the battery bank. The most common method for accurately measuring the current is by placing “shunts” in-line so all current in or out passes through the shunt. The measurement of electrical movement across the shunt is then used to determine the movement of the  circuit itself.

Once a power manger is installed you can start to see into the internal workings of the batteries. Mysteries remain as they will with any nondeterministic process, but your ability to manage the situation is still vastly increased. With the display of a power manager you quickly see how insufficient the voltage alone is for describing the state of the batteries. A modern battery system with multiple charge sources and multiple appliances (e.g. instruments, lights etc.) taking power from it is a complex and synergistic system. It’s behavior is complex, non-linear and sometimes self modifying. The more complex and extensive the system the more measurement points are needed to be able to determine what is happening.

  • managing an electrical system depends on accurate instrumentation
  • batteries are black boxes which you need a tool to see into
  • “you can’t manage what you can’t see”

We have had a Sterling Power manager for over 10 years and it has been flawless. We even managed to find one bug in the functions which hadn’t been described in the manual. We now run 4 shunts for measurement points:

  • house battery
  • engine battery
  • wind and solar charging
  • bow thruster battery (20 volts)

We can see what the battery is giving or receiving, what the wind and/or solar are contributing, even what the charging and voltage are of the 24 volt battery system for the bow thruster (which is totally isolated from the boat’s 12 volt domestic system). One limitation is that we can not tell what the solar panels are contributing compared to the wind generator as we have only one shunt measuring the two. We would add a 5th shunt except that the Sterling power manager only takes up to 4 shunts.

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