5,000 NAUTICAL MILES AGAINST THE TRADES WITH AN ELECTRIC MOTOR
by Keith Dickey and Rebecca Frontz
This is part three in a five-part series. See the links below for other parts:
In last month’s article, we shared how we physically installed an electric motor in our 1979 Pearson 424, S/V Vagari. This time, we’ll walk you through something a little more intricate – our charging and discharging system. We didn’t get it perfect at the start and we are still in the middle of updating the system to better fit our needs. We hope that by sharing what we’ve learned, our headaches will be ones you can avoid when updating or converting your own system.
Our electric motor requires 48VDC to run and our entire system has been built around this piece of information. We designed our system with the philosophy that it is easier to step down voltage as needed rather than to step up voltage. Therefore, when possible, our charging components come into the boat at 48V for our 300ah 48V LiFePO4 battery bank, which directly powers the electric motors. We convert it to 12V as needed for the house system, and invert it to 110VAC for appliances. The relationship between our DC 48V and 12V systems has been re-worked a few times and we’ll go into some detail later in this article to explain the nuances we’ve
learned. First, let’s discuss in detail the four ways in which we can charge our 48V LiFePO4 battery bank: solar panels, wind generator, hydrogeneration, and a battery charger via a diesel generator.
Solar: With a total of approximately 1900 watts, solar proves to be our most effective and efficient way to charge our batteries. We have four groups of solar panels on
Vagari. Two groups of four 110W 12V flexible panels are mounted above our bimini wired in series for 48V, then wired together in parallel, sharing a single Victron® MPPT controller. Mounted on our archway, we have two 100W 12V rigid panels run in series for 24V. Ideally, as mentioned above, we would have four of these panels to run in series for 48V. However, we were gifted these as a favor for helping a friend and wanted to utilize them. We found an MPPT controller made by Genasun® that could boost the voltage to 48V with up to 95 percent efficiency, so mounting two in series at 24V was a reasonable compromise. After spending some time out cruising, we strongly felt that you could not have too much solar power with an electric motor. While spending a hurricane season in the Rio Dulce of Guatemala, we built brackets over our davits and installed the fourth group of solar panels, which are two 400W 24V rigid panels wired in series for 48V controlled with a Victron® MPPT. These additional solar panels have half-cell technology, which improves performance against shading.
Regarding shading, there is a library worth of information on the internet about how best to wire solar panels; parallel versus series. There seems to be consensus that panels wired in parallel are not quite as susceptible to shading. We do not have the ability to run all of our panels in parallel as we need to bring the voltage into the batteries at 48V or higher. As such, our mix of ‘in series’ versus ‘in parallel’ is driven by necessity of voltage more than performance in shading. We achieve approximately 1600 watts at ‘peak sun’, losing about 15 percent due to shading, mostly from the archway and wind generator. A benefit, however, to wiring the panels in series to create higher voltage is that it allows for the use of smaller gauge wire, which reduces cost and weight. This reduction can be substantial if the wire lengths are long.
Wind: Our wind generator has proven to be a pleasant surprise for us. In keeping with our philosophy of charging at 48V or higher, we limited our hardwarechoices. In the end, we chose a 400W 48V Silent Air made by Primus® with a built-in regulator. Having previous experience with wind generators while downwind through the Pacific, Keith was skeptical of its value, especially as it would partially shade Vagari’s solar panels. However, it has proved beneficial while sailing in the Caribbean with the consistent tradewinds and as an alternative to solar on cloudy days and night sails. The energy production during sailing is improved due to its mounting location.
Being mounted aft and center-line, there is increased air flow when sailing on anything forward of a beam as it catches the wind that is funneling off the mainsail.
Hydrogeneration: An exceptionally fun way to recharge our batteries is using the electric motor itself. While sailing with the motor turned on, but not engaged in forward or reverse, water flows over the prop causing it to spin. The electric motor creates electricity from the spinning prop to charge the batteries.
Via the electric motor controller, we can adjust the resistance at the prop. With increased resistance, we achieve increased charging output. However, high output charging requires a faster speed through the water to overcome the resistance (drag) to spin the prop. Alternatively, the resistance can be decreased, which allows for lower charging output at slower boat speeds. The trade-off with hydrogeneration is its affect on boat speed. At lower boat speeds of around four knots, we notice the hydrogeneration can decrease our boat speed up to half a knot. If we are cruising along at six knots or more, the effect on our speed seems negligible as the boat easily overcomes the drag of the prop through the water. Of course, this all is variable as we can change the resistance settings, which affects our energy generation and boat speed. It is rather nuanced and we are still learning how to harness this energy most effectively given its relationship with boat speed, but it proves to be a very powerful charging tool, providing upwards of 1000W with a mid-resistance setting when sailing at or near hull speed.
Generator: With wind, solar, and hydrogeneration being our primary charging sources, our generator is not often used, but does serve as a back-up and provides peace of mind. We chose a two-cylinder 5.5kW NextGen® diesel with a full sound enclosure. It is primarily used when we need to motor in otherwise poor charging conditions, i.e. minimal sun. While we would have loved a larger generator, at the time we converted to an electric motor, sourcing an appropriate battery charger was a constraint. With the input voltage on Vagari being 110V and output voltage at 48V, we did not have many options. The only off-the-shelf commercially available battery chargers were those for electric bicycles. We were able to find a 20A charger that drew around 1100W that would work with our system, but knew it would be quite undersized. We used it for a short period before seeking the assistance of a manufacturer overseas for a custom-built charger. They were able to build a 60A charger that pulls 4200W at peak draw. As our generator is 5.5kW, that leaves approximately 1000W unused. To optimize the use of our generator, we run the 12V house system directly off the generator while simultaneously charging the 48V batteries. Now a few years later, a quick internet search yields a 9000W 110V battery charger with 150A 48V output. Though quite expensive, this battery charger paired with a 9.5kW generator would be ideal for our system if we were to design it today now that we have insight into the electric motor energy consumption-to-speed curve.
Discharging: The obvious use of our battery bank is powering our electric motor. The energy consumption in relation to boat speed is complex and how we optimize
the energy use is quite nuanced. With that said, we will discuss this in detail in the next article. Aside from using our batteries to power our electric motor, we also discharge the batteries via an inverter for 110V appliances. More complicated is how we use our 48V batteries in conjunction with our 12V house system, which has gone
through a bit of an evolution and is worth a discussion.
Initially, we chose to use a standard converter to step down the 48V to 12V. However, our electric winch uses upwards of 90A, so we needed a rather large
converter. Unfortunately, the only one we could find fitting these specifications produced exactly 12V, which caused intermittent low voltage issues with our house
system. Following a friend’s suggestion, we now use an MPPT controller to keep a 12V 40ah sealed lead acid battery topped up by drawing from our 48V lithium
battery bank. The lead acid battery provides electricity at a sufficient voltage to run our house system without the low voltage issues, as well as allowing a 90A draw for
our electric winch. The drawback to this system is that the single 12V battery needs to be constantly charging from the 48V system as the 12V amp hour capacity is not sufficient to run our house system for any length of time. While this generally is of no concern, there are times that we would prefer to have a separate house battery bank, such as during the night prior to a passage when we ideally would leave the anchorage with a full motor battery bank, or while on the move and needing to conserve our battery bank for motoring.
In a previous article (“5000 Nautical Miles Against the Trades with an Electric Motor, Part 2: Installation Overview,” by Keith Dickey and Rebeca Frontz, Compass January 2023) we mentioned we had a 48V 100ah lithium battery with a bad cell. We’ve reconfigured the healthy cells of the battery to create a 12V 300ah battery. We are awaiting the delivery of an appropriate battery management system (BMS) and plan to use this as our new house bank. We will continue to charge the battery with
a MPPT controller via the 48V bank. We are hopeful this will solve most of our needs and wants. The voltage will be high enough to avoid voltage drop issues, the BMS will allow for a large enough draw to run the electric winch, and given the battery will be 300ah, it should allow for multiple days between needing to be charged giving us the ability to conserve the 48v battery bank for the motor when needed. We will keep you posted about the success of this approach.
Lastly, a large consideration when designing the charging and discharging system is the need for all of the associated computers/controllers to integrate with one another. We found out how important this was when our wind generator had to work in conjunction with the solar panels, batteries, and the electric motor computers. If our batteries were nearly fully charged and the wind generator was producing electricity, as well as the solar panels, it would trip the BMS of the lithium batteries. This had a downstream effect of shutting down our electric motor computers. The motors then needed to be manually reset. This was especially fun to sort out when entering a narrow channel to an anchorage and planning to motor! After some experimenting, we later learned this phenomenon would not occur if the solar panels were not also actively producing electricity. Rather, the wind generator would shut itself off as expected as the batteries approached fully charged. We believe the additional charge from the solar panels entering the batteries confused the wind generator. We found that sightly turning down the voltage setting in the internal regulator of the wind generator fixed our issue. Now when the batteries reach that lower voltage, which is just prior to being fully charged, the wind generator shuts itself off, while the solar continues to top off the batteries.
While this was likely specific to our system, it is only one example of several challenges we have had to muddle through in order to get all the sometimes steep, but from our experience, nearly always surmountable with patience and an intelligent troubleshooting approach. In our next article we will talk in detail about the electric motor discharging, along with the energy consumption-to-speed curve and its significance. In the meantime, if you have any questions please email us at [email protected].
This is part three in a five-part series. See the links below for other parts: