Deep discharge, fast recharge

For the past 30 years, lead-acid batteries have been the principal limiting factor in designing a high capacity DC system for a boat. Over these years, we have seen a number of technologies that could potentially circumvent the lead-acid roadblock &mdash NiCads, Nickel Metal Hydride, Lithium-ion, fuel cells &mdash but none has had sufficient life expectancy at the kind of price that is necessary to become a viable everyday product.

The hybrid and electric vehicle industries have been stumbling over the same obstacle, but unlike the recreational boating industry, they have had the money to do something about it. Now new high performance products that look to be affordable are being released into the marketplace. We may finally be on the cusp of a revolution in DC systems performance and design.

Enter thin plate pure lead (TPPL) technology. This has been brought to the marketplace 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 whereas AGM batteries (and all other conventional lead-acid batteries) have cast lead plate grids, which conduct current into and out of the battery, and into which the active material of a battery is pasted, the TPPL batteries have plates stamped out of a roll of pure lead.

In order to make cast plates strong enough to withstand the physical stresses in a battery over time, and to resist acid corrosion at higher states of charge (from the sulfuric acid in the electrolyte, which increases in concentration as the state of charge rises), the plates must be relatively thick (a typical AGM plate is 2 to 4-mm 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, while the alloying of the lead in the plate grids results in a certain amount of internal resistance that translates to heat under high recharge and discharge rates. If this heat exceeds a certain threshold, battery plates buckle and short circuit, and other damage occurs. In other words, the relatively thick, high-resistance plates limit discharge and recharge rates, while the heat generated is indicative of significant energy losses (over a full discharge/recharge cycle, these can be as high as 30 percent).

The TPPL plates are stamped out of a 1-mm thick roll of 99.99 percent pure lead with a very low internal resistance. The rolling process, I am told, changes the grain structure of the lead at a microscopic level such that it is highly resistant to acid corrosion, making it possible to have much thinner-than-normal plates. The combination of ultra thin, densely-packed plates with low resistance greatly reduces the time it takes for current to percolate into and out of inner plate areas while also greatly reducing the heating effect. As a result, the batteries will support much higher discharge and recharge rates than conventional batteries with lower losses. In particular, the recharge rates are truly astonishing &mdash at a 50 percent state of charge, I have verified that these batteries can be charged at a rate of up to six times their rated capacity, as opposed to 40 percent of rated capacity with conventional AGM technology: that’s a recharge rate up to 15 times higher than we have been used to!

High recharge rates can be sustained up to much higher states of charge, radically reducing the time it takes to get to a full charge. EnerSys has a graph showing that with an initial charge rate of three times a battery’s rated capacity, from a fully discharged state these batteries can be 100 percent charged in 30 minutes. Testing what the factory and I have done verifies that the charge acceptance rate (CAR) at a 90 percent state of charge is around 30 percent (once again, much higher than conventional lead-acid batteries).

Preliminary testing also suggests that 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, so this gives the DC systems designer a choice of deeper discharges with the same cycle life as previously, or similar discharges with greater cycle life.

Ceramic and foam

The Odyssey batteries represent a refinement of existing AGM technology. A more radical adaptation of AGM technology, using something known as bi-polar porous lead-infiltrated ceramic (LIC) plates, is slated to hit the marketplace in mid 2009. The driving force has come from Volvo and a Swedish battery company (Gylling Optima Batteries). The resulting Effpower batteries ( are being produced in 24-volt and 150-volt variants. They are reputed to have similar performance to nickel metal hydride (fast discharge and recharge rates and long cycle life) at one fifth the cost. The focus is on hybrid cars, but there may be a useful spin-off in the boat world.

Then there are companies such as Firefly Energy ( Firefly emerged from a search by Caterpillar for better battery technology for its earth-moving equipment. Firefly has developed a process that replaces the lead plate grid in a conventional battery’s negative plate with a lightweight conductive carbon-graphite foam (in the first generation batteries, the positive plate is a conventional plate). 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 results in a much greater utilization of the active material (Firefly claims it is up from 20-50 percent utilization in a conventional battery to 70-90 percent utilization), with higher discharge and recharge rates than with a conventional battery (largely because the diffusion path for the electrolyte from negative to positive material is reduced from the millimeters found in conventional batteries to microns). The discharge/recharge losses are lower than in a conventional battery, with less heating effects. The carbon-graphite matrix pretty much eliminates sulfation while also substantially reducing the weight of a battery.

Firefly released a prototype 107 amp-hour (Ah) (at the C20 rate) Group 31 Oasis battery in late 2008. The battery is slated to be in production by the end of 2009. According to the specifications sheet, it can be charged at up to 300 amps, and can be fully recharged in an hour. It has a reported cycle life of 800 cycles to 80 percent depth of discharge, and 700 cycles to 100 percent depth of discharge. Firefly is working on a second generation battery in which the conventional positive plate grids will also be replaced with carbon-graphite foam grids, resulting in additional performance improvements.

The Advanced Lead Acid Battery Consortium (ALABC &mdash a worldwide research and development alliance of AGM battery manufacturers that includes Effpower) is another entity focusing on modified plate grid designs that will deliver high-rate discharges and recharges with minimal sulfation even if a battery is operated in a partial state of charge.

Cell balancing and safety

At the present time, for truly astonishing performance we still have to look outside the realm of lead-acid batteries, and in particular at lithium-ion. Lithium-ion results in energy densities, and energy delivery rates (power densities), that are several times higher than those of lead-acid. It does this at a fraction of the weight. Whereas a conventional lead-acid battery has discharge/recharge losses of around 30 percent (the Odyssey, Effpower and Firefly technologies are significantly below this), lithium-ion is close to zero percent, and whereas lead-acid has a CAR that tapers down to minimal levels as a battery comes to charge, lithium-ion accepts a very high charge rate to almost a 100 percent state of charge. Lithium-ion is an immensely attractive technology which has long since caught the eye of hybrid automotive developers. Unfortunately, it’s also hard to handle in the real world, and comes with an exotic price tag, which is why we have not yet seen any significant implementation in high-powered applications (it is, of course, commonplace in lower-powered applications such as laptops, cell phones, and portable electrical tools).

In the laboratory, almost all lithium batteries have terrific cycling capabilities &mdash most can be discharged by 80 percent of rated capacity and recharged 2,000 times with little loss of capacity &mdash but it’s not so easy to achieve these performance levels in real life. It requires cell balancing, which is a form of computer-controlled charging and discharging at the individual cell level (as opposed to at the battery level, or battery bank level, as with other technologies).

To create the kind of powerful batteries needed in hybrid applications, you need large capacity cells. Some are now advertised at up to 200-Ah. Typically lithium batteries can be charged and discharged at a rate equal to, or greater than, the cell capacity &mdash i.e. 200 amps or higher in the case of a 200-Ah cell. With individual cell balancing, we now need computer-controlled charging and discharging for each cell at 200-plus amps.

In effect, you need an individual 200-amp battery charger on each cell, but have you ever seen a 200-amp charger, and if so, how big was it? Now consider putting one on a car or boat for each cell in a battery. It’s a daunting prospect. The problem with creating large-scale lithium batteries is not so much finding suitable cells as it is figuring out how to charge them.

Then there’s the safety issue. If you hammer on, or pierce, the case of many lithium-ion batteries, they explosively catch fire. You can bang nails into others without much effect. Many lithium-ion batteries will also catch fire if overcharged. Others will not.

The rush to market
Just in the past few months alone several major battery companies (e.g., Saft and Ener1) have announced lithium developments in the hybrid field, including setting up factories for the production of large-scale, cell-balanced, lithium battery packs with deliveries slated to begin in 2009. One of the more interesting developments is the Arc Lite battery from EnergyTech Marine (

In November 2008, Mastervolt ( beat everyone into the marine marketplace by releasing a cell-balanced, 24-volt, 160 -Ah battery at the Marine Equipment Trade Show in Amsterdam.

We really do seem to be on the cusp of having viable high-capacity lithium batteries, although the price is still shocking.

Even a lithium battery cannot be indefinitely deeply cycled. Supercapitors (ultracapacitors) have the potential to greatly reduce cycling.

A supercapacitor is a device that can absorb limited amounts of energy at very high power levels, and then rapidly discharge this energy back into the system, with very little loss along the way. It can do this hundreds of thousands, and sometimes millions, of times without damage. However, if left in a charged state, it has, relative to battery technologies, a high self-discharge rate, so it is not suitable as a storage device for anything other than short periods of time.

It has been recognized for some time that the characteristics of supercapacitors perfectly complement those of batteries in applications where rapid cycling of batteries would otherwise take place. For example, in hybrid cars supercapacitors can absorb the high energy spikes created by braking events, and then immediately deliver this energy back to the drive train, thus protecting the batteries from short-term cycling. Another application is to continue to absorb high charging currents when the CAR of a battery begins to taper down (this will keep any charging device well loaded, minimizing engine run times), and to then slowly discharge this current into a battery once the charging device has been turned off.

There’s a good deal of experimentation going on in the supercapacitor field, with some promising results. Batteries are being constructed with supercapcitors built into the battery terminal posts. Maybe not in 2009, but quite soon we may see these devices filtering down to practical applications in the boat world.

The practical implications

What are the implications of these new technologies? First off, if batteries will support a deeper level of discharge, and/or can be more fully recharged on a regular basis, as compared to conventional batteries, then for a given level of performance the battery bank can be down-sized, or else for a given size of battery bank, performance can be enhanced.

However, as useful as this is, this may not be the principal benefit of these batteries. From a design and performance perspective, the single biggest impact may well come from the enhanced CAR. The limiting factor in a DC system will now be the charging current that can be supplied to the batteries and not the batteries themselves.

Take my last boat, with a 450-Ah, 24-volt AGM battery bank. With discharges limited to 50 percent of capacity, and the CAR being a maximum of 40 percent of rated battery capacity at a 50 percent level of discharge, the maximum CAR was 450 x 0.4 = 180 amps at 24 volts (which is 4.5-kW). I had a 180-amp, 24-volt alternator on the main engine, and a 220-amp, 24-volt 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.

I replaced these batteries with Odyssey batteries on the new boat. Let’s say the batteries will truly support a 600 percent charge rate at a 50 percent state of charge. This translates to 450 x 6 = 2,700 amps at 24 volts, which is a staggering 65-kW! If the CAR is a more modest 300 percent, I’ll still want a 1,350-amp charging device at 24-volts (33.5-kW). In practice, I have a 22-kW generator on the boat (for my hybrid propulsion system), which gets driven to continuous full output. In 20 minutes, I can put enough energy into the batteries to keep the boat going at anchor for 24 hours, including running my laptop all day.

On most boats, it won’t be possible to establish the kind of charging capability that I have on my boat. What is going to happen is that whatever charging capability there is will be driven to continuous full output by these batteries for extended periods of time, stressing the charging devices and their associated voltage regulators to the maximum. As high charge rate batteries find their way onto boats, I suspect we are going to see a rash of burned out charging devices until we figure out how to properly integrate the batteries into the system (the Odyssey people report they are beginning to see the first burned out alternators).

Hybrid boats
The new battery technologies have special relevance in the realm of hybrid boats. At present, I have a conventional inboard diesel engine on my boat with an auxiliary diesel-electric system in parallel so that I can collect hard numbers on relative fuel efficiencies. In order to get reasonable diesel-electric performance in adverse conditions, I have a 16-kW (21-hp) electric propulsion motor. The 22-kW (continuously rated) diesel generator for this system is the default generator for battery charging and house loads at anchor. Its most efficient operating point (its ‘sweet’ spot) is at 16-kW.

Because of the low CAR of conventional batteries, most generators are very lightly loaded when charging batteries, making it extremely inefficient. High CAR batteries transform this picture. The batteries soak up whatever charging current is thrown at them up to a high state of charge. Given a sufficiently sophisticated control system (this is still under development), the generator can always be loaded at its sweet spot, which is not only fuel efficient but also greatly reduces generator running hours, with a concomitant reduction in maintenance. The net effect will be a lowered fuel and maintenance bill, with the cost of the generator amortized over a longer time span.

The pieces all fit together rather nicely so long as the batteries will tolerate this kind of use with a life expectancy that is at least as great as conventional batteries. At the present time, this is one of the big unknowns that I intend to explore.

Relative costs
Currently, there is no pricing information on the Effpower and Firefly batteries. The Odyssey TPPL batteries have been available for a year or two.

I have done some Internet searches for conventional AGM batteries and Odyssey batteries. I have found that the Odyssey batteries are typically 25 to 30 percent more expensive then what is already a relatively expensive technology (AGM batteries tend to cost more than other conventional batteries). Is it worth paying this kind of a premium?

If the Odyssey batteries have a longer life expectancy for similar performance, then the numbers immediately pencil out. Similarly, it may be possible to trade off enhanced performance, resulting, for example, in a down-sized battery bank, against the extra per-battery cost. The other major cost issue that should be looked at, but which is rarely considered, is the real cost of charging batteries.

Let’s say I have a 50-hp inboard diesel that I run for 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. The all-up replacement cost will be somewhere between $15,000 and $25,000. The capital cost per hour of running time (i.e., excluding fuel and maintenance) is therefore between $1.50 and $5.00 an hour – a good ballpark figure is $3.00 an hour. Fuel and maintenance costs will add another dollar or so, depending on fuel prices (in the summer of 2008 &mdash and at any time in Europe &mdash it would have been $2).

If the Odyssey batteries, or some other new technology, cut engine running hours for battery charging in half (which is easy to project – in many applications, the savings could be much greater), there will be a considerable saving that can be set against the added cost of the batteries. This will vary from application to application according to boat use. Many times, it will make the batteries look positively cheap.

So far as lithium is concerned, the only suitable battery currently available is the Mastervolt battery. It is priced around $4,000! Given that it is a 24-volt battery with a 160-Ah capacity, its capacity is 24 x 160 = 3,840 watt-hours (Wh), so the cost is approximately $1 per Wh, as compared to, for example, high end AGM batteries at $0.20 per Wh (based on an 8D battery with a 225-Ah capacity at 12 volts and a $550 price tag).

This would seem to be an insurmountable price differential, but in fact if you do an analysis of the amount of energy the Mastervolt battery can deliver over its lifetime, and the cost of putting that energy into it (i.e. recharging costs) in many applications, even at this price, its lifetime cost will be less than that of the AGM, while its performance will always be superior. Of course, this presupposes that it will live up to its projected life cycles in the real world, which is something we don’t yet know.

Costs for lithium are predicted to come down to $0.50 per Wh, and perhaps even $0.30 per Wh, over the next few years, at which point the technology should be extremely competitive for high-end boats with demanding DC applications.

Entering a new era
I’m always reluctant to predict radical breakthroughs in technology. In fact, until recently I have bemoaned how little things have fundamentally changed in the past 30 years with respect to boat 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 it’s fair to say we are on the cusp of the most radical change in DC systems design that we have seen in a generation.

By Ocean Navigator