Charging Into the Future
For the past 30 years, lead-acid batteries have always been the principal limiting factor in the design of high-capacity DC systems for sailboats. Over the years a number of technologies have been developed that attempt to circumvent this roadblock—NiCad, nickel metal hydride (NiMH), lithium ion (LiI), fuel cells—but none has had sufficient life expectancy at a price affordable enough to be viable in the marine market.
The auto industry has been stumbling over the same obstacle in the design of hybrid and electric vehicles, but unlike the recreational-boating industry, it has sufficient resources to spend hundreds of millions of dollars on research and development. Now, finally, new high-performance products that may be affordable are reaching the marketplace—perhaps the cusp of a revolution in DC systems performance and design.
Deeply discharging a conventional lead-acid battery always produces stresses that result in some internal damage. To protect against such damage, the first rule of thumb when operating a conventional DC system is to limit battery discharges, generally to no more than 50 percent of capacity.
When recharging a battery, the amount of charging current it will safely accept (its charge acceptance rate, CAR) rapidly declines from around 30 to 40 percent of rated capacity at a 50 percent state of charge to 10 to 15 percent of rated capacity at an 80 percent state of charge. Above 80 percent, the CAR steadily tapers off to about 2 percent of rated capacity as the battery nears a full charge. (Note that the actual CAR varies with battery type.)
Because of the declining CAR, it can take up to 7 hours to fully recharge a well-discharged battery. This leads to a second rule of thumb. If charging times are limited—for example, on a cruising boat that runs its main engine at anchor—it normally makes little sense to recharge past an 80 percent state of charge. After this, the amount of engine running time is completely disproportionate to the amount of power being accepted by the battery.
Because of these limitations, only 30 percent of a battery’s capacity is used on a regular basis—from an 80 percent to a 50 percent state of charge and back again. Hence there’s a third rule of thumb: Battery capacity needs to be three to four times the load experienced between charges.
The internal chemistry of a battery that is left discharged or partially discharged changes over time, and the battery becomes increasingly harder to recharge. This is known as “sulfation.” To avoid this, the fourth rule of thumb when operating a conventional DC system is to ensure a full recharge at least once a month.
Given that a battery is rarely taken below a 50 percent state of charge, at which point its CAR is no more than 30 to 40 percent of its rated capacity, there is little point in having a charging capacity that exceeds 30 to 40 percent of the battery’s capacity. This is the fifth rule of thumb. There are some qualifiers relating to temperature and alternator speed, but these can be ignored for the purposes of this discussion.
Putting heavy sustained loads on a battery produces yet another limitation. The more ruggedly a battery is built to handle deep discharge and cycling stresses, the more slowly it releases its stored energy. As a result, its voltage falls faster under the load, and there is less available capacity before voltage falls to unacceptably low levels. This leads to a sixth rule of thumb: Sustained discharge rates should not exceed 25 percent of a battery’s rated capacity.
Searching for the Holy Grail
Batteries in hybrid cars need to be capable of supporting very high discharge rates, especially when accelerating under battery power alone. Also, pressing the brake pedal activates a generator that charges the car’s batteries. The harder you brake, the higher the generator’s output. For the car’s batteries to accept the charge, they must be partially discharged and able to absorb high charge rates at this state of charge. Once partially charged up, the batteries need to be discharged (i.e., used for propulsion) in order to prepare for the next braking event.
Conventional batteries don’t work worth a damn in this application. They’ve got to be well discharged before they can accept even moderate amounts of power from the brakes, in which case they can support only very limited propulsion loads and will be damaged by the depth of discharge. If kept well discharged, they will die from sulfation. Hybrid cars need batteries that can be discharged rapidly, can be cycled for extended periods of time in a partial state of charge, have high charge-acceptance rates to relatively high states of charge, and, of course, have a long service life at an affordable price. This set of attributes is also ideal for boats, but has generally been thought to be unattainable using inexpensive lead-acid technology.
Thin Plate Pure Lead (TPPL) Technology
Enter thin plate pure lead (TPPL) technology. This is being introduced under the Odyssey brand name by EnerSys, successor of Gates Energy, the original developer of absorbed glass mat (AGM) batteries. TPPL batteries are a variant of AGM technology. But where AGM batteries (and all other conventional lead-acid batteries) have cast-lead plate grids into which the active acid material of the battery is pasted, TPPL batteries have plates stamped out of rolls of pure lead.
To withstand the physical stresses in a battery, conventional cast plates have to be relatively thick (a typical AGM plate is 2mm to 4mm thick) and must contain additives, such as calcium or antimony, to strengthen the lead. The thicker a plate, the longer it takes for current to percolate into and out of inner plate areas during charges and discharges. Also, the alloys in the lead create internal resistance, which creates heat that can damage the plates during heavy charges and discharges. Both factors limit discharge and recharging rates.
TPPL plates are just 1mm thick and are 99.99 percent pure lead with very low internal resistance. This greatly reduces the time it takes for current to percolate through inner plate areas and greatly reduces the heat generated. As a result, TPPL batteries can support much higher discharge and recharging rates. Indeed, recharge rates are truly astonishing. I have verified that TPPL batteries at a 50 percent state of charge can be charged at a rate of up to six times their rated capacity, which is 15 times faster than what we are used to!
High recharge rates can be sustained up to much higher states of charge, radically reducing the time it takes to achieve a full charge. According to EnerSys, these batteries can be 100 percent recharged from a fully discharged state in just 30 minutes. Preliminary testing also suggests these batteries will have a higher cycle life at deep-discharge levels than conventional AGM batteries. However, as with any lead-acid battery, cycle life is still a function of depth of discharge. DC systems powered by TPPL batteries can therefore support deeper discharges with the same cycle life as conventional systems, or can support similar discharges and enjoy greater cycle life.
EFFpower, Firefly, Altairnano, and Ultracapacitors
Odyssey batteries represent a refinement of existing AGM technology. A more-radical adaptation of AGM technology using bi-polar porous lead-infiltrated-ceramic (LIC) plates is also about to hit the marketplace. The driving force here is Volvo and a Swedish battery company, Gylling Optima Batteries. Their EFFpower batteries are available in 24-volt and 150-volt variants and reportedly perform as well as nickel metal hydride batteries, with fast discharge and recharge rates and long cycle life, at one fifth the cost. The focus is on hybrid cars, but they may prove useful on boats.
Another pioneer is Firefly Energy, which has partnered with Caterpillar to create better batteries for earth-moving equipment. Firefly is working to replace the lead-plate grid in a conventional battery with a lightweight conductive carbon or graphite foam. The active material in the battery, in the form of a paste or slurry, is contained in the foam. The cellular structure of the foam allows much greater utilization of the active material, with higher discharge and recharge rates. The carbon/graphite matrix pretty much eliminates sulfation and reduces the weight of the battery by one third. These may appear on the market in 2008.
Meanwhile, the Advanced Lead Acid Battery Consortium (ALABC), a worldwide research-and-development alliance of AGM-battery manufacturers that includes EFFpower, is also focusing on modified plate-grid designs that permit heavy discharges and recharging with minimal sulfation, even if a battery is operated in a partial state of charge.
Truly astonishing performance, however, will likely come from beyond lead-acid technology. A lithium ion battery introduced by Altairnano in September 2006 can reportedly be discharged at up to 100 times its rated capacity, can be recharged from a full discharge in less than 10 minutes, will tolerate up to 15,000 full-discharge cycles, and has a predicted life expectancy of 20 years or more. Unfortunately, it’s shockingly expensive.
Finally, a word about ultracapacitors. These can be can be charged and discharged at very high rates, but have limited capacity and self-discharge over time faster than batteries. In the automotive world they are useful for absorbing high-rate, short-term charges from brakes and for delivering lots of power for short periods of time. It’s not yet clear what role they might play on boats, although one that springs to mind is as a power source for a windlass or bow thruster.
The practical implications
Assuming these technologies are viable for marine DC systems, the implications are profound. For example, if batteries can be discharged more deeply and/or be fully recharged more often, then battery banks can be downsized to maintain a system’s current level of performance, or performance can be enhanced by maintaining a bank at its current size. The biggest impact, however, is that a system’s limiting factor will be the amount of charging current that can be supplied to the batteries and not the batteries themselves.
Take my last boat, which had a 450-amp-hour, 24-volt AGM battery bank. Its maximum CAR was just 180 amps at 24 volts. I had a 180-amp alternator on the main engine and a 220-amp auxiliary generator, which was never fully loaded. After a few minutes’ charging, the CAR would taper down to 100 amps or less and continue falling. If I replace this battery bank with Odyssey batteries, as I intend to do on my next boat, and the batteries will truly support a 600 percent charge rate at a 50 percent state of charge, my new 450-amp-hour battery bank will have a CAR of 2,700 amps at 24 volts. If the CAR is a more modest 300 percent, I’ll still want a charging device putting out 1,350 amps at 24 volts.
Of course, I won’t be able to create this kind of charging capability on my boat. Instead, whatever charging capability I do have will be driven to continuous full output for extended periods of time, stressing both the charging devices and their voltage regulators. As these new batteries find their way onto boats, I suspect we will see a rash of burned-out charging devices until we figure out how to make proper use of them.
The new batteries will be most useful on hybrid boats. I am installing both a conventional diesel engine on my new boat and a diesel-electric system so I can compare fuel efficiency. In order to get reasonable diesel-electric performance in adverse conditions, I need a system putting out at least 15kW (20 horsepower). The diesel generator for this system will also be the default generator for battery charging and house loads at anchor.
Converting the 22kW continuous-rated output of the generator I will be using into amps at 24 volts yields 917 amps. At my old maximum CAR of 180 amps, the generator would be very lightly loaded, making it inefficient, so whatever benefits I may see on the propulsion front will likely be lost on the battery-charging front.
With Odyssey batteries or other high-CAR batteries, this picture is transformed. The batteries will soak up whatever charging current is thrown at them up to a high state of charge. This will not only enable me to load the generator at its most efficient point, but will also greatly reduce running time for battery charging. The cost of the generator will be amortized over a longer time span. The pieces all fit together rather nicely, as long as the new batteries have a real lifespan at least as long as conventional batteries. Whether they do or not is what I intend to find out on my boat.
I have found that Odyssey batteries are typically 25 to 30 percent more expensive than AGM batteries (which are themselves relatively expensive). Is it worth paying this kind of premium, and will the market support it?
If Odyssey batteries live longer and perform as well as AGMs, then the numbers immediately favor them. Similarly, it may be possible to increase performance with a downsized Odyssey battery bank that lives as long as an AGM battery bank.
The other major cost issue, which is rarely considered, is the real cost of charging batteries. Let’s say I have a 50-horsepower diesel engine I run an hour a day at anchor to charge my batteries. The life expectancy of this engine is somewhere between 5,000 and 10,000 hours. Its replacement cost is somewhere between $15,000 and $25,000. The capital cost per hour of running time (excluding fuel and maintenance) is therefore between $1.50 and $5 an hour—say, $3 an hour on average.
If Odyssey batteries, or some other new technology, cut my engine-running hours in half, there will be a considerable saving that can be set against the extra cost of the batteries. This will vary from application to application according to how the boat is used. Many times, it will make the batteries look positively cheap.
I’m always reluctant to predict radical breakthroughs in technology. Indeed, I have often bemoaned how little things have changed over the past 30 years with respect to marine electrical systems. But if these new battery technologies pan out, as I think they will, and if they are coupled to the new digital-switching and power-
distribution systems, I believe we may be on the cusp of the most radical change in DC systems design we have seen in a generation.