Bat­tery Tech

SAIL - - Contents - In the first of a three-part se­ries on the search for ef­fi­cient on­board en­ergy stor­age, Nigel Calder in­ves­ti­gates lead-acid bat­ter­ies

Nigel Calder ex­plores the pros and cons of cur­rent bat­tery tech­nol­ogy

For the past few years I have been part of a project ex­per­i­ment­ing with mas­sively pow­er­ful al­ter­na­tor-type de­vices; the one I have now will gen­er­ate up to 9 kilo­watts (kW). To put this in per­spec­tive, that’s 750 amps at 12 volts! There’s not a lot of point in hav­ing this kind of gen­er­at­ing ca­pa­bil­ity if there is nowhere to dump and store the en­ergy, so the corol­lary to the gen­er­at­ing test­ing has been bat­tery test­ing.

The Holy Grail for which we have been search­ing is a sys­tem ca- pable of gen­er­at­ing and stor­ing, dur­ing nor­mal propul­sion en­gine run times, suf­fi­cient en­ergy to run the house sys­tems for 24 or more hours. In our case, nor­mal en­gine run times would be the time it takes me to set or pull up the an­chor, or to get on and off a dock. For those with air con­di­tion­ing, the ob­jec­tive is to be able to run the air con­di­tion­ing overnight with­out hav­ing to run a gen­er­a­tor, and then next day to be able to re­plen­ish the en­ergy used dur­ing nor­mal boat op­er­a­tions (i.e.

with­out re­quir­ing a stand-alone gen­er­a­tor).

We have the gen­er­at­ing and con­trol piece of this solved (more on this in a fu­ture ar­ti­cle), but the bat­ter­ies are a work in progress. What’s needed is a bat­tery that can store sub­stan­tial amounts of en­ergy in a rel­a­tively com­pact and light­weight for­mat (a high en­ergy den­sity), and which is ca­pa­ble of ab­sorb­ing ex­tremely high charge rates to high states of charge. It must also be able to tol­er­ate deep dis­charges so that the full ca­pac­ity can be used at each cy­cle. This is the bat­tery a typ­i­cal cruis­ing sailor needs, al­beit on steroids in our ap­pli­ca­tion. There are var­i­ous lithium-ion of­fer­ings that meet these re­quire­ments, but in gen­eral these are shock­ingly ex­pen­sive, so ini­tially we have been con­cen­trat­ing on lead-acid.

LEAD-ACID LIM­I­TA­TIONS All forms of lead-acid bat­ter­ies have sev­eral ma­jor draw­backs in a cruis­ing en­vi­ron­ment. First, they don’t like deep dis­charges. In or­der to min­i­mize the depth of dis­charge at each cy­cle, you need at least dou­ble the ca­pac­ity you will use, which trans­lates into a lot of vol­ume, weight and cost. Then when it comes time to recharge, once you get up to around a 50-60 per­cent state of charge, the bat­ter­ies’ abil­ity to ab­sorb charg­ing cur­rent steadily ta­pers off and can’t be forced with­out do­ing dam­age. Not only that, if you get fed up with the ex­tended en­gine or gen­er­a­tor run time re­sult­ing from the low charge ac­cep­tance rates and re­peat­edly ter­mi­nate the charge be­fore the bat­ter­ies are fully charged (this is known as oper­at­ing in a par­tial state of charge), the bat­ter­ies suf­fer dam­age from sul­fa­tion.

Lead-acid bat­ter­ies are also rel­a­tively in­ef­fi­cient at con­vert­ing elec­tri­cal en­ergy into chem­i­cal en­ergy and vice versa. This in­ef­fi­ciency is man­i­fested as heat. If you force the pace on the charg­ing side, a lot of heat is gen­er­ated. If the bat­tery gets too hot, it goes into a con­di­tion known as ther­mal run­away in which the in­ter­nal elec­trolyte boils, pro­duc­ing hy­dro­gen and oxy­gen. In a worst-case sce­nario, the bat­tery blows up, or the vented hy­dro­gen ac­cu­mu­lates in­side the boat, is ig­nited by a spark and the boat blows up.

AGM RULES Of all the lead-acid bat­ter­ies on the mar­ket, the ab­sorbed glass mat (AGM) fam­ily has the best over­all prop­er­ties for the oper­at­ing en­vi­ron­ment on most cruis­ing boats. In par­tic­u­lar, these bat­ter­ies have the high­est charge ac­cep­tance rate of any lead-acid bat­tery to the high­est states of charge, and the high­est ef­fi­ciency (around 85 per­cent, as op­posed to around 60 per­cent for wet-cells), and as such gen­er­ate the least heat dur­ing fast charges and dis­charges.

Within the AGM fam­ily, we have a sub-genre de­scribed as thin plate pure lead (TPPL), the best known of which are the Odyssey and SBS bat­ter­ies from En­erSys and the NSB En­ergy1 bat­ter­ies from North­star. These bat­ter­ies have the high­est charge ac­cep­tance rate of any AGM bat­tery with the best ef­fi­cien­cies. They will tol­er­ate re­peated deep dis­charges (down to around 20 per­cent re­main­ing ca­pac­ity). Over the past eight years, I have par­tic­i­pated in ag­gres­sive test­ing of dozens of these bat­ter­ies. In gen­eral, they have per­formed well, ex­cept that un­for­tu­nately they do not like be­ing op­er­ated in a par­tial state of charge, suf­fer­ing from sul­fa­tion. This means you reg­u­larly (ideally, once a week) have to have an ex­tended charge cy­cle at low rates of charge.

If you have to run an en­gine (the main en­gine or a gen­er­a­tor) solely for the full charge cy­cle, it is ex­traor­di­nar­ily in­ef­fi­cient. An an­ti­dote to this is to have suf­fi­cient so­lar on board, con­fig­ured so that it is pro­vid­ing the nec­es­sary ex­tended charge cy­cle, as op­posed to be­ing con­sumed by

house loads. An even bet­ter an­ti­dote would be to elim­i­nate the sul­fa­tion is­sue and thus the need for the ex­tended full charge cy­cle.

THE MAGIC OF CAR­BON Some years ago it was dis­cov­ered that if you sprin­kle car­bon dust into the ac­tive ma­te­rial in the neg­a­tive plates of an AGM bat­tery the car­bon in­hibits sul­fa­tion. There is now a fam­ily of these bat­ter­ies avail­able from North­star, known as NSB Blue bat­ter­ies. I have yet to test any, but this looks to be a sig­nif­i­cant step for­ward for par­tial state of charge op­er­a­tion.

The ul­ti­mate bat­tery in the car­bon dop­ing world comes from Fire­fly. In a “nor­mal” lead acid bat­tery, there are plates with a grid com­posed of lead (this con­ducts the en­ergy in and out of the bat­tery), into which is pasted the ac­tive ma­te­rial (which ab­sorbs en­ergy on charg­ing and gives it up on dis­charg­ing). A Fire­fly bat­tery dis­penses with the lead grid in the neg­a­tive bat­tery plate and re­places it with a foam formed from car­bon. The ac­tive ma­te­rial is pasted into the pores in the foam. The car­bon acts as the con­duc­tor of en­ergy in and out of the bat­tery plate.

This foam ush­ers in some ex­cel­lent prop­er­ties. The nor­mal con­se­quence of dis­charg­ing a bat­tery is to turn the ac­tive ma­te­rial into lead sul­fate. If the bat­tery is left in a dis­charged state, the lead sul­fate slowly morphs into large crys­tals that can­not be re­cov­ered by nor­mal charg­ing pro­cesses (the sul­fa­tion men­tioned above). How­ever, the pore struc­ture in the Fire­fly foam is too small to al­low the lead sul­fate to crys­tal­ize to this ex­tent, and as a re­sult these bat­ter­ies are more-or­less im­mune to sul­fa­tion. Does this sound too good to be true? I have op­er­ated these bat­ter­ies in a par­tial state of charge for months at a time, and I have dis­charged them to 35 per­cent state of charge (SOC) and left them for eight months with­out recharg­ing them. I have then re­stored them to 100 per­cent SOC.

The car­bon-foam plate grid is a re­mark­able step for­ward in the lead-acid bat­tery world. The tech­nol­ogy is patented and as such these bat­ter­ies are only avail­able from one com­pany. Un­for­tu­nately, I have found the qual­ity con­trol to be some­what un­even.

ULTRACAPACITORS Fi­nally, for a decade var­i­ous ex­per­i­ments have been made with ca­pac­i­tors em­bed­ded in lead-acid bat­ter­ies. A ca­pac­i­tor is a de­vice that can ab­sorb very high en­ergy spikes, but with a minis­cule stor­age ca­pa­bil­ity. In the­ory, a ca­pac­i­tor/lead-acid com­bi­na­tion can ab­sorb high rate charges in the ca­pac­i­tor and then over time dump the charge into the bat­tery for stor­age, main­tain­ing a high av­er­age state of charge and min­i­miz­ing sul­fa­tion. The first of these bat­ter­ies is just now be­com­ing avail­able in the United States from East Penn Man­u­fac­tur­ing (the Ul­tra bat­tery). The jury is still out on whether or not this rep­re­sents a sig­nif­i­cant step for­ward in ma­rine cy­cling ap­pli­ca­tions.

FROM LEAD-ACID TO LITHIUM-ION Af­ter more than a cen­tury of devel­op­ment, you’d think we’d know ev­ery­thing there is to be known about lead-acid bat­ter­ies, but ex­cit­ing dis­cov­er­ies are still be­ing reg­u­larly made; lead-acid tech­nol­ogy should not be writ­ten off. How­ever, there is noth­ing on the hori­zon that sug­gests lead-acid bat­ter­ies will ab­sorb the kind of charg­ing cur­rents I would like to throw at them up to high states of charge and at ef­fi­ciency lev­els that will limit the in­ter­nal heat gen­er­a­tion in the bat­tery. For this, we have to search else­where, with the only game in town cur­rently be­ing lithi­u­mion. We’ll take a look at these bat­ter­ies in the Oc­to­ber is­sue. s

“Sealed” AGM bat­ter­ies will still vent if pushed too hard

The af­ter­math of a hy­dro­gen ex­plo­sion from vent­ing lead-acid bat­ter­ies

The car­bon foam tech­nol­ogy used in Fire­fly bat­ter­ies is promis­ing

The au­thor puts a TPPL bat­tery bank through its paces

An Odyssey TPPL bat­tery used for en­gine start­ing, paired with an AGM house bank

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