Battery revolutionFeb 28, 2011
Ever since electricity has been put on boats, in one way or another batteries have held back the development of effective DC and AC systems. As a result, in general our electrical systems perform poorly and often astonishingly inefficiently (I’ll quantify this in a future article).
Luckily for the boating world, in its quest for hybrid and electric vehicles the car industry has run into many of the same obstacles that have bedeviled us for decades. Unlike the boating world, the automotive world has the resources to crack these problems. We are seeing the emergence of several new battery technologies that promise to turn marine electrical systems’ design upside down.
The life expectancy of conventional lead-acid batteries (which include wet-cells, gel-cells and AGMs) is tied to how deeply they are discharged at each use cycle, and the extent to which they are fully recharged between discharges.
Even with quality “deep cycle” batteries, repeated deep discharges dramatically reduce life expectancy, so typically systems are designed to limit discharges to no more than 50 percent of rated capacity. This takes care of the deep discharge issue. It’s not so easy to ensure adequate recharges.
Unfortunately, irrespective of the power of a charging device (alternator or battery charger), a battery’s charge acceptance rate (CAR) declines rapidly as it comes to charge. This makes it difficult to fully recharge batteries without either plugging into shorepower for several hours, or else running an engine (the boat’s main engine or an AC generator powering a battery charger) for long hours. For much of this time there is minimal electricity production.
In order to avoid these unproductive engine run hours when away from the dock, it is common to cease charging at around the 80 percent state of charge (SOC) level, but then the failure to fully recharge on a regular basis leads to something known as “sulfation,” which results in premature battery death. Like it or not, and regardless of how inefficient it is, if the batteries are to stay healthy, periodically they must be driven to a state of full charge.
The two SOC numbers above — 50 percent on the discharge cycle and 80 percent on the recharge cycle, with a periodic boost to 100 percent to minimize sulfation — provide the core design parameters for a conventional DC system’s design. In effect, it is assumed the batteries will be operated over only 30 percent of their rated capacity (50-80 percent SOC). A daily load calculation is made for a boat and then multiplied by three or four (because we are only using 30 percent of the battery capacity) to derive the total battery capacity that is needed to meet this demand.
When discharged to 50 percent of rated capacity, depending on the internal chemistry (wet-cell, gel-cell or AGM) and physical construction (cranking or deep cycle), conventional lead-acid batteries will accept from 25 to 40 percent of their rated capacity as charging current. This gives us the maximum practical output for charging devices (alternators and battery chargers). In reality, there are a couple of additional ‘fudge factors’ that are taken into account, but we don’t need to address these here.
By the time a battery is 80 percent charged, its CAR is down to around 10 to 15 percent of rated capacity, and from then on the CAR declines rapidly. As a result, even charging devices sized as described rarely remain at full rated output for more than a few minutes.
Two increasingly available technologies break free of most existing battery constraints. One is thin plate pure lead (TPPL), a variant of AGM technology that is sold by EnerSys under the Odyssey label (www.odysseybatteries.com), and the other is lithium-ion, which is coming into the marine marketplace from an ever-increasing number of suppliers in an ever-increasing variety of chemistries.
The single most important characteristic of these batteries is their incredibly high charge acceptance rates. I have, for example, verified that at a 50 percent state of charge a TPPL battery will accept a charging current that is up to 600 percent of its rated capacity (e.g., a 100 amp-hour battery will accept 600 amps of charging current, as opposed to 24 to 40 amps with conventional lead-acid batteries). Lithium-ion has similar astonishing charge acceptance rates. In the case of TPPL batteries, the CAR tapers off at higher states of charge, but it is still running at better than 30 percent of rated capacity to well above a 90 percent SOC. With lithium-ion, the high CAR continues until close to 100 percent SOC.
Why is this significant? Consider a voyaging boat that is charging its batteries while at anchor. The daily load is 200 amp-hours (Ah). The battery bank has a capacity of 600 Ah. The alternator is sized for 200 amps (33 percent of the battery capacity). Charging commences at 50 percent battery SOC. The alternator kicks in at 200 amps (in reality, it will be less than this because of heating and other effects), but almost immediately begins to taper down its output because the battery CAR is declining as the SOC rises. It will take around two hours to get to 80 percent SOC, at which point the CAR will be down to 50 amps or less. After accounting for efficiency losses in charging the battery (these losses are as high as 15 percent on both the charge and discharge cycles) even after two hours we will have returned less than 200 Ah.
Now we replace our batteries with high CAR batteries. At 50 percent battery SOC, the alternator goes to full output and it stays there. In addition, the charge and discharge losses are lower (7-10 percent in each direction with TPPL; negligible with lithium-ion). In an hour we have put back more energy than we did in two hours with the conventional batteries. If we upgrade the alternator, or add another one, we can put back the day’s energy use in 30 minutes, or 15 minutes, or … The limiting factor is the charging device and not the batteries. At a 50 percent SOC our TPPL batteries will accept up to 3,600 amps of charging current!
On my boat I have a large bank of TPPL batteries (1,200 Ah at a nominal 12 volts) with a powerful DC generator (rated at 22 kilowatts). We are using these for electric propulsion experiments. In a moderately discharged state, the batteries drive the generator to full continuous output (the equivalent of 1,800 amps at 12 volts). In 15 minutes we can put enough energy into the battery bank to run the boat at anchor for two days. This kind of charging performance translates into a much more efficient use of the engine when it is running, and greatly reduces overall engine run hours (with, of course, the freedom from the associated noise, exhaust fumes and maintenance).
Batteries as a ‘buffer’
It’s the efficiency aspect that I find most intriguing. I have a 75-hp engine in my boat. Even with two big alternators, when battery charging at anchor in the conventional manner, the load is at most 5 percent of the engine’s rated power, and rapidly tapers down from there. At a 5 percent load the engine is already operating chronically inefficiently and it only gets worse. By loading up the engine with more powerful charging devices coupled to high CAR batteries, I can improve my power generation efficiency at anchor by a remarkable 300 percent (I will look at the numbers behind this in a future article).
What about when I am at sea under power? All engines have one particular speed and load at which they run at peak efficiency. The nature of the way engines develop power and propellers absorb power is such that unless a boat has a controllable pitch propeller (almost none do), the engine never runs at this ‘sweet’ spot and, what is more, it cannot be made to run at this sweet spot. However, with high-CAR batteries there is the ability to soak up large amounts of energy. With the right software (which does not yet exist in the marine world) the charge rate into the batteries can be manipulated such that any time propulsion power is less than the optimum load for a given engine speed (which it is almost all the time) the batteries can be used to manipulate the load to keep it at the optimum for that engine and boat speed (this is what is done with hybrid cars). The batteries, in effect, are used as a ‘buffer’ that soaks up surplus energy to maintain peak system efficiency (for both propulsion and electrical power generation).
Now let’s consider a ‘typical’ AC generator on a boat running an intermittent load, such as an air conditioner. The generator has to be sized for the start-up load of the air conditioner, which is commonly several times its running load. As a result, it is not uncommon to find a generator’s running load is less than 25 percent of its rated output. When the load cycles to ‘off’, there is no load on the generator at all. The generator load varies between 0 and 25 percent of its rated output. The generator is operating inefficiently 100 percent of the time.
If we add a powerful enough battery charger to the mix coupled to high-CAR batteries, just as with our propulsion engine we can use the batteries to maintain a high average load on the generator, keeping the generator operating at, or close to, peak efficiency all of the time. The technology already exists to do this, using synchronizing (paralleling) inverters (I’ll look at these, too, in a future article). Once again, it is not hard to demonstrate improvements in energy production efficiency of 300 percent or more, with concomitant reductions in engine run time, noise, exhaust fumes and maintenance.
The natural reaction to these kinds of performance numbers is to say: “This is just too good to be true; there has to be a snag somewhere!” And of course there are some unintended consequences.
An immediate effect of high-CAR batteries is to drive conventional charging devices to full continuous output. In the case of alternators, almost none are built to take this. In the case of battery chargers, if the charger has a high enough output to take advantage of the battery’s CAR, the charger will trip the shoreside breaker!
At recent boat shows I have taken to asking alternator vendors if they are starting to see a rash of burned out alternators. I get a surprised look and the question: “How did you know that?” Even so-called ‘hot’ and ‘continuously’ rated alternators do not take kindly to being run at 100 percent of rated output for potentially hours at a time. The problem is alternators have a peak efficiency of not much above 50 percent. The other 50 percent of the energy supplied by the drive belt is dissipated as heat. If we have a 200-amp, 12-volt alternator running at full power, that is around 2.5 kW of heat that has to be dissipated on a continuous basis. Sooner or later the alternator gets hot enough to fry diodes and windings.
Until we get a new generation of more efficient, more heat-tolerant alternators, what is needed with high-CAR batteries is an alternator with an external voltage regulator that also allows a current (amps) limit to be set. This should be set to no more than 80 percent of rated output. Even ‘governed down’ like this, the system will still produce a given amount of energy much faster than the same alternator without a current limit charging into conventional batteries. If you want a faster charge rate with the high-CAR batteries, you can always get a more powerful alternator, or add another one, but only after considering other factors such as belt and crankshaft loading, the effect on propulsion of siphoning off this much power, etc.
So far as battery chargers are concerned, you need to look at the available shoreside supply (typically around 3 kW) and either buy a charger with an output that stays below this limit, or else one on which it is possible to set a maximum power draw. I spent the past summer in Sweden tripping the shoreside breaker until I set a 3-kW current limit on my charger.
Over the past several years I have collaborated in extensive destructive testing of TPPL batteries to try and discover a management program that will allow these batteries to be operated in a more-or-less permanent partial state of charge (i.e., to avoid the periodic “conditioning” cycle that conventional batteries need in order to avoid sulfation). We have failed to find a mechanism to do this. All our testing suggests that the TPPL batteries need regular conditioning, which requires charging them until the CAR is down to milliamps. Just as with conventional lead-acid batteries, this can lead to extended hours of inefficient, low-load engine operation. However, given the high CAR to high states of charge, the conditioning cycle takes considerably less time than with conventional batteries.
Lithium-ion batteries are in a world of their own. They can be discharged down to 20 percent SOC thousands of times, operated in a partial SOC more-or-less indefinitely, and in any case will support charge rates equal to their rated capacity up to 98 percent SOC. As noted above, there are virtually no energy losses when discharging and recharging. This is phenomenal technology that just keeps getting better. The one huge downside is the equally phenomenal price, although this is starting to come down as volumes go up and more competitors enter the marketplace.
Lithium does have one special requirement. Battery conditioning (known as ‘cell balancing’ in the lithium world) is necessary at the individual cell level. Without it, the cells get out of sync. The first time a cell is over charged or over discharged the battery is history. Cell balancing requires a microprocessor at each cell and sophisticated electronics to shunt current from higher charged cells to lower charged cells, plus the software — known as a battery management system or BMS — necessary to maintain the battery in an even state of charge and protect it against conditions that would damage it. It’s one reason these batteries are so expensive. It will take time and experience to fine tune a BMS for marine applications: you can’t simply transplant an automotive BMS into a boat. We are already seeing some expensive failures as part of the learning process.
A DC-based boat
High-CAR batteries require us to rethink both DC and AC electrical systems design. To fully exploit their high CAR, we need powerful and efficient DC charging devices. Alternators, because of their high losses, do not fit the bill. Instead, we are seeing an increasing number of DC generators emerging, with electrical efficiencies as high as 90+ percent (as opposed to ~50 percent for alternators). This is considerably more efficient than any commonly available AC generator.
We have had automatic generator stop and start capabilities for some time. Typically, with DC generators, this is based on battery SOC (e.g., crank the generator when the SOC drops to 50 percent; shut it down when the SOC rises to 80 percent). What we are starting to see are management strategies based on load: when the combined DC operating load and battery CAR are high enough to enable the generator to be run at its ‘sweet’ spot, crank the generator and hold it at the sweet spot; when the load drops below this level, shut the generator down. Properly implemented, with such a strategy the generator only runs at, or close to, its peak efficiency. If it can’t be run at peak efficiency, it is not run. There is, quite simply, no more efficient way to meet a boat’s energy needs if power has to be derived from a fossil-fueled engine. Additionally, with a strategy based on load rather than SOC, the batteries tend to get microcycled (numerous shallow cycles) rather than deep cycled, which extends their life.
A logical extension of such an approach is to run all the AC loads via inverters. There are some losses, of course, through the inverter(s), but overall such a system will be far more efficient than operating a conventional AC generator. Depending on the capacity of the battery bank, it will be possible to operate the boat, including, in many cases, high loads such as air conditioning, for long hours without cranking an engine.
The final piece in such a picture is to add electric propulsion as just another load, albeit a large one, at which point we have a hybrid boat. This is the focus of my current investigations and experiments as part of the European Union funded Hybrid Marine (HYMAR) project (www.hymar.org).
Batteries are the key
It’s the new generation of high-CAR batteries that are opening all these doors. Instead of being a roadblock to progress, batteries are now a dynamic enabling technology. It will take some time for the boating world to understand and fully exploit the potential, but one way or another we are on the cusp of substantially more powerful and radically more efficient electrical systems.
I have been tracking and writing about marine systems for almost three decades. This is the single most exciting technological development I have seen in all that time.
—Contributing editor Nigel Calder is the author of such marine books as Nigel Calder’s Cruising Handbook and How to Read a Nautical Chart.