With autumn slowly giving way to winter, I’m surveying the electrical systems on Jean-du-Sud, in particular to see what condition they are in, but also to deepen my understanding of how they are installed and gauge any repair needs.
This post is the first in a series about electrical systems on board Jean-du-Sud. It focuses on charge controllers with maximum power point tracking, popularized by the abbreviation “MPPT”. Technology dictates that we also look at how batteries work.
Charge Controllers in Three Paragraphs
For the majority of marine installations, Ohm’s law (V = RI) provides all the relevant theory necessary to understand how to install cables, change electrical equipment and test electrical circuits. The internal (demand side) circuits are very close to idealized systems taught in basic electricity courses.
On the other hand, when voltage and amperage are constantly changing, the notion of idealized circuit quickly becomes irrelevant. This is particularly true when the energy source is constantly fluctuating or because batteries require specific charging profiles adapted to their state.
In essence, a charge controller aims to bridge the gap between the fluctuating power source, regulating both voltage and amperage. It is a technology that converts a continually changing current into an idealized one. In so doing, it helps prolong battery life. MPPT charge controllers have an additional quality, namely that they provide the charging environment with maximized power so as to accelerate the chargeing process. They not only protect batteries, but also make the most of changing conditions.
In Details
Charge controller technology is based on advanced electronics. Opening a charge controller shows that they are closer to computers than to inert circuits (Awesome Tech, 2020). However, one can understand what these instruments do with a few general notions of electricity and optimization.
The role of an MPPT charge controller is based on four important notions inherent to any on-board circuit: energy sources vary, energy requirements vary, batteries don’t like these variations, and finally, it would be best to make the most of the energy available to charge the batteries faster.
The first three ideas explain why any charge controller exists. But it’s on the last point that MPPT charge controllers differ from the main alternative technology, PWM (“Pulse Width Modulation”) charge controllers: they maximize the power availlable. If you understand these four ideas, you understand the role of an MPPT charge controller.
The Energy Supply Fluctuates
On any boat, energy sources vary in power and amperage. This is particularly true of natural energy sources such as solar, water or wind power: an hour in the sun will produce more electricity than an hour in the shade. Similarly, a windier hour will provide more electricity than one with light winds. Nature randomly affects electricity production, so voltage and amperage will also fluctuate.
The Energy Demand Fluctuates
Unsurprisingly, switching equipment on or off affects power requirements (whether measured in watthour or “amp-hours”, which is nothing more than a watthour divided by 12). Turning on a piece of equipment therefore changes the amperage on a circuit. But turning a piece of equipment on or off also creates a suddent variation in voltage by changing the balance between the circuit’s various internal resistances. At constant power, the voltage on the circuit decreases with increasing amperage. Vice versa, as amperage decreases, voltage increases. This negative relationship generates a trade-off between voltage and amperage on a circuit. This is so because at any instant, the power source yields a fixed amount of energy.
The specifics of amperage and voltage will therefore depend on the weather, but also on the technology on board: the quality of electrical cables, equipment faults and, more generally, the state of circuits will affect the state of voltage and amperage. In this way, establishing the negative relationship between voltage and amperage is best done empirically, using system measurements… and may even change according to the state of the circuits on board. Behind the apparent constancy of an installed electrical system, its internal state is quite dynamic!
Batteries Require Specific Charge Profiles
A battery is nothing more than a collection of mini chemical factories called cells. A battery cell is an arrangement of two types of metal plates – anode and cathode – soaked in a conductive material (usually acid or a gel mat). When these plates are connected by an electrical circuit, a chemical reaction transfers electrons from the anode to the cathode, producing an electric current.
Basically, it is a chemical reaction within the battery cells that generates the current. Initially, the chemical reaction occurs very quickly, because electrons on the surface of metal plates are very easy to pull off. However, as the reaction proceeds, it takes longer and longer to extract the electrons. This degradation of the reaction is at the root of a charge profile that changes with the state of the battery.
Each time the battery is discharged, the cathode becomes a little less willing to let go of the electrons it has just acquired. In so doing, each time the battery is discharged, it becomes a little more unusable. This is one of the reasons why batteries have a life cycle, after which they become inoperable. If the battery is allowed to discharge completely – a “deep discharge” – the cathode becomes even less willing to let those electrons go, further reducing the battery’s lifespan. The usual heuristic rule, more applicable to lead-acid or gel batteries, is to avoid discharging the battery by more than 50% to prolong its life.
Charging a battery involves reversing the chemical reaction, i.e. storing energy by returning electrons to the anode. This is done, of course, by forcing a charging current. Initially, charging is quick, as electrons can easily stick to the surface of metal plates. After a while, however, the process becomes slower, as these electrons have to be stuck to less accessible areas of the plates, i.e. below the metal surface.
This is why the initial charge of batteries is fast, while full charges take longer. The usual rule of thumb (for lead-acid or gel batteries) is that the first 80% of the battery’s stored energy is charged quickly. The remaining 20% takes longer. Finally, if the battery is already charged and a current is foolishly applied in an attempt to charge it again, the battery may eventually be heated to the point of degrading its condition and therefore reduce its ability to store a current.
In short, batteries are capable of accepting a maximum charge voltage (beyond which they heat up) and have voltage and amperage charge profiles that vary according to their state of charge. Charging them efficiently therefore requires controlled profiles according to their state of charge. These profiles are specific to the battery technology used (lead-acid batteries, lithium batteries and lead-gel batteries).
An example of a charging profile is shown below, with fast charge shown in the middle (“CC Fast Charge”) and slower charge (“Constant Voltage Charge”) on the right. Immediately after this is a trickle charge (often called a “floating charge” on chargers) which keeps the battery charged. This trickle charge is just enogh to keep the battery charged but low enough to avoid damages.
The 50% to 80% Range is the Sweet Spot
Although unrelated to charge controllers, this is a good time to point out that the 50% to 80% usage range is a good heuristic rule of thumb for a battery system. Staying above 50% discharge preserves battery life. Staying below the 80% threshold facilitates rapid recharging.
The idea is therefore to have a sufficient number of batteries such that their total capacity (in watt-hours or ampere-hours) will ensure that the energy available in the 50% to 80% range is greater than the energy requirements between charge cycles. For example, a daily requirement of 200 ah of energy, dictated by on-board needs and the fact that you don’t want to start the engine more than once a day, will retain enough batteries for a total storage of 667 ah (≈200/(0.8-0.5)).
Charge controllers
Connecting a fluctuating power source directly to batteries can damage them. The idea of introducing an intermediary, a charge controller, takes the fluctuating amperage and voltage and regulates it to bring it within the ranges tolerated by a battery’s different charge profiles. This extends a battery life.
What distinguishes MPPT charge controllers from “ordinary” charge controllers is their ability to regulate current optimally, i.e. to deliver maximum power to the circuit regulated by the controller.
MPPT: Optimizing the Power Delivered
An MPPT charge controller will adjust the amperage so that power is maximized. By varying its amperage, the MPPT charge controller can also affect the voltage on the circuit to the power source.
Since there is a negative relationship between voltage and sudden amperage variation, this point of maximum power is (theoretically) well defined and empirically detected by the charge controller. So, over very short time intervals (on the order of nanoseconds), the charge controller tries out different amperage values and retains the one that maximizes power. In so doing, they improve the power delivered to the batteries, enabling them to do more charging work than ordinary controllers.
For math geeks
Recall that power is the product of amperage and voltage (P = I \cdot V). Because it is fixed at any instant, there is a decreasing relationship between voltage and amperage as amperage is varied. So P =I \cdot V(I) with V'(I)<0. Maximizing power by choosing the amperage therefore requires the first-order condition:
V'(I)I + V(I) = 0 \Leftrightarrow -\frac{V'(I)}{V(I)} = \frac{1}{I}.
The MPPT is therefore implicitly determined by the above expression, where the voltage variation, in percentage, is equal to the inverse of the amperage. MPPT charge controllers identify this point numerically as conditions change.
MPPT Charge Controllers: Are They Worth It?
It is a long term investment. In essence, an MPPT charge controller extends battery life and improves charging. The measured improvement of controllers varies between 30% and 14%, depending on the source (Laguando & al, 2019; Renogy, n.d.). Models sold generally allow for different battery profiles, adapting to the technology on your boat, through a configuration menu.
With prices ranging from $150 to $500, depending on maximum amperage and brands, these intermediate products are establishing themselves for the savings they can generate in the long run. For instance, if two batteries generating a reserve of 400 ah cost a total of around 2000 CAD, the investment pays off if the controllers extend battery life by 25%.
Two Videos to Further Understand the Concept
The two videos below are of great help to understand how an MPPT controller works. The video by GreatScott! (2017) is exemplary in its pedagogy, especially when it comes to understanding solar panels and charge controllers. Unlike videos that focus on installation, the author concentrates on how they work from a technical point of view. I personally understood the fundamental optimization feature with this video. The AKIO TV video (2022) is perhaps a little more conceptual and is a good complement.
References
AKIO TV (2022). MPPT explained, YouTube video retrieved online in november 2023 at this address.
Awesome Tech (2020). What’s Inside MPPT Solar Charge Controller, YouTube video retrieved online in november at this address.
Great Scott! (2017). Electronic Basics #29: Solar Panel & Charge Controller, YouTube video retrieved online in november at this address.
Laguanda M.A., Luna Paipa I. A., Bustos – Márquez, L. F. et S. B. Sepulveda – Mora (2019). Performance comparison between PWM and MPPT charge controllers, Scientia Et Technica, vol. 24, no. 1, pp. 6-11, 2019.
Monolithicpower.com (s.d.). Battery Charger IC Fundamentals, webpage retrieved in November 2023 at this address.
Renogy Canada (s.d.). Rover 20/30/40Amp MPPT Solar Charge Controller 12V/24V, webpage retrieved in November 2023 at this address.
Wikipedia (s.d.). Ohm’s Law, webpage retrieved in November 2023 at this address.