Lithium-ion technology has many advantages over lead-acid batteries. Nigel Calder looks into them
Nigel Calder looks at the latest marine Lithium-ion batteries
Lithium-ion batteries have several times the energy storage capacity of an equivalent volume and weight of lead-acid batteries, and can be charged at extraordinarily high rates of charge to very high states of charge. They can be discharged almost totally without damage anywhere from hundreds of times to thou- sands of times. They are immune to sulfation and as such can be operated permanently in a partial state of charge.
Whereas the most efficient lead-acid batteries (AGM) are only 85 percent efficient at converting electrical energy into chemical energy and vice versa, lithium-ion is better than 95 percent efficient, resulting in far less heat generation during high-rate discharges and recharges—an important consideration with my current high charge rate experiments. This is an amazing set of positive characteristics. There are, however, some potential negatives.
Every lithium-ion battery currently in the
marine marketplace contains a flammable electrolyte, and all lithium-ion batteries can be driven into an exothermic state—one in which the battery generates heat internally even when disconnected from charging sources and loads. Once initiated, this exothermic reaction can be hard to stop, resulting in a rapid temperature rise (thermal runaway) that only aggravates the situation. Depending on the chemistry, the battery may get hot enough to set itself on fire (as happened on the Boeing aircraft), and even if the chemistry cannot get this hot there will be a pressure rise that frequently leads to venting of the electrolyte, after which, if there is any kind of an ignition source, the electrolyte will then catch fire. Once the electrolyte lights up, it generally cannot be extinguished—the usual result is the loss of the boat, as has already happened in a number of cases.
Conditions that can initiate thermal runaway include over-charging, over-discharging followed by a recharge, charging in freezing temperatures, operating in high ambient temperatures, manufacturing defects, external heat and physical damage. Several of these conditions are not uncommon in marine applications! What is more, whereas in automotive and other mass applications the battery builder and installer have near complete control over the installation for its entire life, once a boat gets into the hands of its owner there is no telling who will mess with what installation and its wiring over the life of the boat.
To ensure the safety of lithium-ion batteries in the boating world, a sophisticated battery management system (BMS) is required that, at a minimum, monitors voltage and temperature at the individual battery cell level, and has mechanisms to shut the battery down if it is abused or if any one cell begins to drift outside designated parameters. Unfortunately, an effective BMS is expensive to develop and implement. However, without it, the battery can threaten the health of the boat.
To put this in perspective, we had similar problems with gasoline and propane when these were first introduced to boats, with numerous fires and explosions. However, we learned how to handle these substances, with various organizations promulgating standards to ensure safe installations. Notable among these is the American Boat and Yacht Council (ABYC). The ABYC is currently working on a standard for lithium-ion batteries and installations. In time, problems will become as rare as they now are with gasoline and propane.
Cost is the other major drawback of lithiumion batteries. The individual cells may not be that expensive, but they have to be packaged into a battery and the aforementioned BMS added: a BMS that needs to be custom developed for the relatively low-volume marine marketplace. The net result is that it is rare to find a marine lithium-ion battery that retails for less than $1,000 per kilowatt-hour (kWh) of capacity, with some running as high as $2,000. By comparison, a 100 amp-hour (Ah), 12 volt, lead-acid battery has a nominal capacity of 1.2 kWh with a cost of one tenth to one fifth that of the same capacity in lithium-ion.
Happily for lithium-ion, this cost comparison is grossly misleading! At best, the lead-acid battery will only deliver half its capacity each time it is discharged and recharged (cycled), whereas lithium-ion will easily deliver 80 percent of its capacity. The lead-acid battery can only be cycled hundreds of times before it fails whereas lithium-ion, depending on its chemistry and construction, may be cycled thousands of times. A true measure of the cost of a battery is how much energy can be cycled through it during its lifetime (what I like to call the lifetime kilowatt-hour throughput), divided by the battery’s purchase price. This will yield a cost per kWh of throughput. In many applications, if the capabilities and cycle life of lithium-ion can be fully exploited (although in practice, this is often not the case) even at today’s prices lithium-ion will have a lower cost per kWh of throughput than any lead-acid battery.
The deeper we delve the better it gets for lithium-ion. Let’s say I am running my main engine or generator to battery charge at anchor. We have to include the engine run time, fuel and maintenance in the cost of the energy being produced, along with the battery cost per kWh of throughput. If I have an 8kW charging device and I have lithium-ion batteries that can absorb the full 8kW to a high state of charge (whereas lead-acid batteries will, on average, only absorb half this, and likely far less than half this), then I can cut the engine run time for battery charging in half. If I run the numbers, I will find this dramatically reduces my cost of energy, more-or-less regardless of the cost of the lithium-ion batteries. In this situation, ensuring the safety of the lithium-ion installation becomes more important than its cost.
Two chemistries predominate in the marine lithium-ion world: lithium-ion iron phosphate (LFP), and nickel manganese cobalt (NMC). If LFP is driven into thermal runaway it will not generate high enough temperatures to set itself on fire, whereas NMC can. For this reason, LFP has often been described as intrinsically safe and has been promoted as the only suitable chemistry for marine applications. However, as noted above the electrolyte is still flammable and there have been some notable fires and boat losses.
Within the LFP and NMC families, there are literally dozens of variations in terms of things like construction, chemical doping and protective measures. Given the correct BMS and packaging, it is arguable that NMC can be made as safe as LFP, and there are certainly some NMC batteries that are safer than some LFP batteries. Ultimately, for the consumer, regardless of chemistry, the only real protection is to buy from a recognized marine vendor with an excellent track record.
Surprisingly, perhaps the biggest impact on what will come to predominate in the marine world—LFP, NMC or some other chemistry—may be the fall-out from the VolksWagen “dieselgate” scandal, which has caused a major re-think in Europe regarding diesel cars in general. Whereas tax structures have for decades favored diesels, with the result that 60 percent of new car sales have been diesels, there is now talk of banning diesels altogether from city centers. Automotive manufacturers are therefore scrambling to adjust to this new reality, with a massive re-orientation towards electric cars, which in turn, will require lithium-ion batteries in large numbers at low costs. In order to achieve this, the industry will have to settle on a specific chemistry and format, and tool up the factories for volume production. Once this happens it will, to some extent, lock in for some time to come what will dominate the marketplace.
As of now, it looks like NMC will be the chemistry. We are already seeing at least two marine lithium-ion battery players—Torqeedo and Volta—partner with automotive suppliers to repackage NMC cells for boats. Lithionics, formerly a strong proponent of LFP, is also developing an NMC offering. However, we have also seen Yanmar issue a (June 2017) Technical Bulletin that only allows LFP to be used with their factory-installed alternators.
AMAZING ELECTRICAL SYSTEMS
Regardless, both LFP and NMC have the properties we need for the massively-powerful, 8-plus kW alternator-type device I am currently testing. A relatively small battery pack, rated at around 10 kWh capacity, will be able to absorb the full output of the machine up to almost a 100 percent state of charge. The more than 95 percent battery efficiency will ensure that little charging energy is wasted as heat. The ability to withstand near 100 percent discharges for thousands of cycles will enable us to utilize at least 8 kWh of the 10 kWh battery capacity at each cycle. The immunity to damage from sulfation will permit operation in a partial state of charge whenever, and for however long, we want.
On our boat, with our current energy needs of under 3 kWh a day, one battery charge will keep us going for three days, or alternatively, 20 minutes spent setting or retrieving the anchor and getting in and out of a slip will give us all the energy we need for 24 hours. I even am thinking of converting to electric cooking and getting ride of the boat’s propane system! Bottom line: lithium-ion, coupled to the generating device we have developed and tested over the past several years, will give me the energy system I have always dreamed of. s
A lithium-ion battery bank may be the best solution for many long-distance cruisers
Lithium-ion batteries are available in many sizes