Electric Yacht Charging

Electric power entices us with its flexibility and adaptability. And that extends to our charging sources. We gain the ability to recharge our batteries from nearly any source, assuming everything works together. Discover the critical issues to combine multiple charging sources for electric propulsion.

Merging Charging Sources

1.0 Introduction

Without a method to recharge them, electric batteries just make expensive paperweights.  A major allure behind electric propulsion is the ability to recharge the batteries from multiple sources, leading to vastly extended range.  But this isn’t easy.  Most electric yachts require multiple charging sources.  How to make them all work together nicely?  Today I review methods of charging on electric yachts, and how to combine them.

2.0 DC Charging

When recharging the massive battery bank, go straight to the source.  Direct current (DC) chargers.  Many electric yachts include multiple DC sources for charging: 

1. Solar panels
2. Engine alternator
3. Methanol fuel cell
4. Regeneration from propulsion motor
5. Wind turbine

How to combine them?  Charging on the DC circuit offers the simplest option.  Generally, each source includes its own battery charger.  The battery charger takes the raw output from your charging source and controls it to provide a tempered current flow to the battery.  The current controls the rate of recharge for the battery, and we want to slow down that recharge as the battery gets full.  Recharging heats up the battery; too much heat and we damage the battery.  For some batteries, too much heat also means fire or explosion.  This is why the good battery chargers are programmed with different charging profiles, depending on your battery chemistry.  Smarter chargers make for healthy batteries.

Charging too fast makes batteries go boom; so, combining multiple chargers makes for a terrible idea, right?  Nope.  There is nothing wrong with connecting all these battery chargers to the same point, going straight into the battery.  Chargers work by sensing the voltage at the battery.  As the battery recharges up to full, the voltage from the battery increases slightly.  A nominal 12V battery may generate 13 – 13.6V when fully charged.  This is how a charger tracks the state of the battery.  But it also means that to put power into the battery, the charger needs to supply that full voltage, or even slightly higher.

Now play this out with two battery chargers going to one battery. (Figure 2‑1)  Charger 1 reads the battery voltage as 12.5V.  It starts recharging the battery, supplying a voltage of 14.2V.  The voltage now reads as 14.2V everywhere on that circuit.  Charger 2 came into the game a little too late.  Now it reads 14.2V, thinking that is the battery voltage.  Charger 2 thinks the battery is full and turns off.  This method of coordination means multiple chargers will not overload the battery.  You can happily connect dozens of chargers, and only one turns on at any given time.  But sometimes we want more than one charger. 

Electric propulsion frequently requires smart battery chargers.  To get a reasonable recharge time on those massive batteries, we generally need more current than one charger can muster.  Smart battery chargers work in parallel. (Figure 2‑2)  Rather than just sensing the voltage at the battery, these chargers communicate with each other directly.  Normally, one the chargers becomes the master, with a charge profile that specifies current limits for each stage in the charge cycle.  The master charger then coordinates with the others to ensure the combined total stays within that charge profile.

How do they coordinate?  Not sure on the details.  That lies beyond my knowledge, and the specific details usually depend on the manufacturer.  This often ties you to one manufacturer.  When coordinating with multiple smart charges, expect they all need to come from the same manufacturer.  Probably even models from the same series of products.  Carefully select your manufacturer, since you may buy a LOT of equipment from them.

2.1 Charging Lithium Batteries

When charging lithium ion batteries, be especially cautious.  Ensure your battery charger has a profile for lithium ion batteries.  If a lithium battery gets too hot, that can lead to fire in the wrong circumstances.  Temperature monitoring becomes critical.  With lithium, the wrong settings mean more than just a damaged battery. 

And be sure any engine alternator is designed for lithium ion batteries.  Lithium batteries often come with internal circuity to protect the battery during recharging.  If the battery gets too hot, that internal circuit disconnects the battery to protect it, just like flipping a switch.  Engine alternators don’t like it when they suddenly lose the battery.  It tends to damage or destroy the alternator.  Be sure your alternator is protected against this if recharging a lithium battery. 

3.0 Diesel Generators

Diesel generators are great for long term storage of energy.  Yes, they consume diesel fuel, which has negative environmental effects.  But batteries can’t compete with the energy density of diesel fuel.  Even adjusting for the weight and size of the generator, diesel fuel holds much more energy in a smaller space than any battery.

So go out and buy a DC generator, right?  I just extolled the simplicity of DC charging.  A massive DC generator can supply huge quantities of power and quickly recharge the batteries, right?  Not so easy.  Here we suffer from supply limits.  Most large generators are AC supply.  They were built for a traditional yacht market, where the largest loads come from the AC side:  refrigeration, air conditioning, etc.  Most likely, you need to buy an AC generator and then convert that into DC power for battery charging.

This is normally where people talk about inverters.  A standard yacht comes with an inverter to convert from DC power to AC, and the inverter can go the other way, converter your AC generator into DC battery charging.  Seems normal to connect that generator with the inverter.  Maybe not.  This depends on cost. 

Remember that you need to convert a huge amount of power to supply electric propulsion.  Large power means large, expensive equipment.  A simple battery charger costs less than the fancy inverter.  You can potentially save some money by running the generator through a dedicated battery charger and then buy a smaller inverter for the regular hotel loads.

4.0 Merging Power Supplies

There is a myth that ABYC standards do not allow boats to combine AC charging sources.  You only get to select the internal generator or the shore power.  Seems like a major problem on an electric yacht where charging sources abound.  Let me reassure you, AC charging absolutely allows you to combine sources, but it gets a little more complicated than the DC side.  We need to tackle the problem of synchronization.  (Figure 4‑1)

AC power works with electrons moving back and forth through the wires.  They travel in both directions, working in a rhythm.  That rhythm is the frequency of the power, normally 60 Hz in North America.  When adding a new AC source in, we need to match that rhythm in three ways: 

1. Match voltage
2. Match frequency
3. Match phase

All these items need to match (within tolerance) before connecting two or more AC sources together.  This synchronization takes specialized equipment, which is why ABYC will not let you just throw a switch and connect the wires from two AC sources.  But add synchronizing equipment as a bridge between the two AC sources, and its perfectly fine to combine them.  This is covered in ABYC standard A-32, part 32.12.  Thankfully, you don’t need to learn the process of manually synchronizing your generators.  We have equipment now to automatically synchronize AC sources.

In fact, you may already have an AC synchronizer.  Some modern inverters allow parallel operation of shore power and an AC generator.  How does the inverter combine these?  It has an internal synchronizer.  So multiple AC sources are very possible, with synchronization.

5.0 Conclusion

Electric power entices us with its flexibility and adaptability.  And that extends to our charging sources.  We gain the ability to recharge our batteries from nearly any source.  When properly wired with the correct equipment, this is a nearly flawless process.  It happens automatically, a silent symphony of synchronized equipment.  Just take the time to ensure you buy the right equipment.

6.0 Acknowledgement

Many thanks to Jeff Cote from Pacific Yacht Systems for helping with much of the background and practical knowledge of these articles.  For more information on yacht electrical installations, check out their YouTube channel at:  https://www.youtube.com/c/PacificYachtSystems

7.0 References