Author: Phil Eagleton, VoltSport
Maverick, the new Infiniti 46 is the latest high performance boat from Infiniti Yachts. This 46 foot machine has been designed and engineered with a single purpose – to go fast. Optimising weight in the yacht is important to this goal. Batteries are a necessity. They are big, they are heavy. Placement in the yacht is important, and maintaining the long term performance of them more so. How does the battery impact the fuel usage? And how can we optimise the whole thing when the brief is - as always - smaller, lighter, cheaper, better…
Sizing the pack
Before thinking about chemistry, form factor, and placement of the battery, a full understanding of the power requirements is needed. For Maverick, we performed a full system power analysis, taking into account all charge and discharge sources, anticipated usage, and possible optimisations.
The single biggest energy user is the hydraulics system. The boat has keel, traveller, DSS, vang, and staysail rams running hydraulically. Since this is the main consumer, we have selected our system voltage to match - In this case, its 24V. This leaves two parameters to select: 1) the required current delivery rate of the battery, and 2) the overall capacity of the battery.
Again, using the energy survey for the yacht, we calculated the peak instantaneous power required for the yacht. For example, a gybe, at night with nav lights, cabin lights, hydraulics, instruments, VHF, and PC all taking power means that the battery must be able to deliver all this power in a short time. On Maverick, the night-time gybe manoeuvre would require around 150A of current, at 24V, for up to 30s!
But how does this affect the battery choice?
A good lead acid battery, capable of delivering 150A, would need to be around 300Ah in rated capacity. The energy delivery is inefficient, and less than 50% efficient during the gybe power delivery time. These losses mean the capacity drops by almost double the energy we take during the manoeuvre! Putting this much lead into a boat like Maverick is also impractical. There isn’t the space to put them, and the weight would be huge.
Lithium becomes the only choice. It is power dense, able to deliver high power, has good long term storage life, but needs managing. The biggest advantage we have with Lithium is that because it can deliver the high instantaneous power we need, we can carry less of it. The battery size, and capacity are reduced, the efficiency of power delivery is increased. For Maverick, we are down to a size of only 144Ah @ 24V. Lifespan is also increased. 2000 cycles are typical, versus 500 for a good lead acid battery.
Living with the battery
Lithium, although ideal for size and weight, does need to be managed well, and maintained. Within the battery there are individual cell monitoring electronics, which provide essential balance control between the cells, and also signal when charging should stop or start. To make sure that the crew have as little management to do as possible, this cell information from the battery is collected, and presented with a simple “ok”, “charge now”, or “stop charge” instruction. Integration with the yacht instruments has also been included, giving access to state of charge via the on deck screens. Full information is available for maintenance and monitoring as needed.
While racing, there are two sources of charge current. As with any boat, an alternator on the engine is used. This directly charges the 24V battery, and is a replacement for the standard 12V unit.
It is not quite as simple as that though, as whilst charging the battery, the boat is still sailing and using energy. To maximise charging efficiency, we run the engine a little harder by combining the battery charge current and the drain current, and delivering this combined value from the alternator.
Imagine, while charging the battery at 80A, the hydraulic system runs. The extra discharge demand is passed to the alternator in addition to the charge current. The alternator output ramps up to the total, but the battery current is maintained near constant. It is really a simple hybrid system, and aims to reduce the charge time by regulating the alternator in this way.
While sailing fast, the yacht has a hydro generator. In terms of power to weight, over a longer offshore race, this generator is a great way of powering the systems on board. Sailing at around 12kts will generate all the power the yacht needs.
Back in the marina, we use a simple 24V battery charger. As with everything else, this is controlled to maintain optimum charge conditions.
Is there a place for 12V?
With a full 24V install, we still need 12V for engine cranking. In Maverick, we use a compact Lithium starter battery, but instead of charging from the alternator (that’s 24V now…) we charge from the main 24V pack. Charging is done with a voltage converter, and this gives us another added benefit – when the boat is charged from the mains shore power, the 12V starter battery is charged too! In fact, the starter is maintained topped up all the time. Yes, many frequent restarts in a short space of time will reduce the charge level, but the averages work in our favour. For that doomsday situation when that battery fails, we can also start from a mid point on the main 24V battery. This is strictly a last resort option, but there in case.
Many systems on board are also 12V. Watermaker, lighting, instruments, radio, and so we generate 12V from the 24V battery with another voltage converter. Both converters are the same, so there is an easy emergency replacement should either fail!
How does it all stack up?
Given that the systems installed that we have to power are the same, regardless of the battery choice for the boat, what are the implications for the battery weights?
The boat can now carry more crew, or more gear, for the same weight. This brings an increase in performance in long offshore events.
The fuel? Well since we are using the same power in both cases, we should use a similar amount. Not quite true though, as the increased efficiency when charging the Lithium Battery versus the lead battery mean that we use less fuel too.
Is it all about the batteries?
Yes! - The batteries enable all the other saving to go on around them.
The reduction in fuel usage means you can go further.
10 to 15% reduction in fuel use.
The reduction in size means you have more storage space.
650 mm x 150 mm x 250 mm, for the complete 12V and 24V battery pack.
The increased lifespan, and performance means they cost you less to run.
Less to buy upfront, 4x the lifespan.
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