Boat­works

Lithium-ion tech­nol­ogy has many ad­van­tages over lead-acid bat­ter­ies. Nigel Calder looks into them

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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.

PRE­VENT­ING FIRES

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 COM­PAR­ISONS

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.

CHEM­ISTRY CHOICES

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

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