SAFE CHARG­ING

SAIL - - Features - In the third in­stall­ment of his se­ries on bat­tery tech­nol­ogy, Nigel Calder looks at fire preven­tion

Nigel Calder looks at man­ag­ing the heat cre­ated by al­ter­na­tors

One way or an­other we now have bat­ter­ies in the mar­ket­place that can ab­sorb very high lev­els of charg­ing cur­rent, en­abling us to op­ti­mize elec­tri­cal sys­tems in ways not pre­vi­ously pos­si­ble. How­ever, this high charge ac­cep­tance rate has the ef­fect of forc­ing charg­ing de­vices (al­ter­na­tors and bat­tery charg­ers) to max­i­mum rated out­put for ex­tended pe­ri­ods of time. It also sub­jects the ca­bles in the sys­tem to high con­tin­u­ous cur­rents, cre­at­ing some in­stal­la­tion chal­lenges.

No charg­ing de­vice is 100 per­cent ef­fi­cient, nor is any cable 100 per­cent ef­fi­cient at con­duct­ing charg­ing cur­rents. In­ef­fi­cien­cies trans­late into heat, which in turn trans­lates into dam­age to sen­si­tive elec­tron­ics and a po­ten­tial fire risk for wind­ings, ca­bles and con­nec­tions. Con­ven­tional al­ter­na­tors are the worst, be­cause the av­er­age al­ter­na­tor is, at best, only 60 per­cent ef­fi­cient at con­vert­ing me­chan­i­cal en­ergy into elec­tric­ity (al­though there are some spe­cial­ized al­ter­na­tors that are con­sid­er­ably more ef­fi­cient than this), with the re­main­ing 40-plus per­cent of the in­put en­ergy con­vert­ing to heat. If the heat is not re­moved, the wind­ings will fry and the diodes will fail.

In a “tra­di­tional” charg­ing sit­u­a­tion, the bat­ter­ies ac­cept the al­ter­na­tor’s full rated out­put for only a short pe­riod of time, af­ter which the bat­tery charge ac­cep­tance rate de­clines, the al­ter­na­tor’s out­put re­duces and the fan within the al­ter­na­tor is adequate to han­dle the re­main­ing heat. How­ever, as soon as we con­nect that same al­ter­na­tor to a bat­tery bank ca­pa­ble of ab­sorb­ing high charge rates for ex­tended pe­ri­ods of time, we are po­ten­tially in trou­ble.

In fact, to han­dle just this kind of sit­u­a­tion, a (June 2017) Yan­mar Tech­ni­cal Bul­letin re­quires that its 125-amp, 12-volt al­ter­na­tor to be lim­ited to 100 amps if con­nected to a lithium-ion bat­tery, and that its 65-amp, 24-volt al­ter­na­tor be lim­ited to 50 amps. Sim­i­larly, Bal­mar, a ma­jor sup­plier of high-out­put al­ter­na­tors, has for years pro­vided a tem­per­a­ture sen­sor that at­taches to the back of the al­ter­na­tor, with the volt­age reg­u­la­tor cut­ting the al­ter­na­tor’s out­put in half if the tem­per­a­ture thresh­old is reached. One way or an­other, all ex­ist­ing al­ter­na­tors con­nected to high charge rate bat­ter­ies need tem­per­a­ture pro­tec­tion.

CABLE BURNOUT Even with tem­per­a­ture pro­tec­tion, al­ter­na­tors can run ex­tremely hot, with some rated to op­er­ate at tem­per­a­tures of al­most 400 de­grees F. It is also not un­com­mon for on an al­ter­na­tor to be at 230 de­grees F or higher in the course of nor­mal op­er­a­tion. Most boat­builders in the United States use wiring that has a tem­per­a­ture rat­ing of 221 de­grees F, while in Europe much cable found on boats is rated just un­der 200 de­grees F. As a re­sult, when this cable is at­tached to an al­ter­na­tor with a case that is run­ning at 230 de­grees or higher, the heat from the al­ter­na­tor will pre-heat the cable to close to its rated tem­per­a­ture, at which point the cable’s cur­rent car­ry­ing ca­pa­bil­ity the­o­ret­i­cally re­duces to close to zero, even as we will fre­quently run well over 100 amps through it. Un­der th­ese cir­cum­stance, the in­su­la­tion close to al­ter­na­tors that are reg­u­larly run hard can be se­ri­ously de­graded, cre­at­ing a risk of in­su­la­tion fail­ure and sub­se­quent short cir­cuits. Any time you get a short cir­cuit with high lev­els of cur­rent flow, you have a sig­nif­i­cant fire risk.

Given the scarcity of ca­bles with a tem­per­a­ture rat­ing above 221 de­grees F, there is no good an­ti­dote to this sit­u­a­tion other than to make the al­ter­na­tor ca­bles as large as is prac­ti­ca­ble; mount them in free air to max­i­mize cool­ing; and en­sure that the pos­i­tive cable is not run in con­tact with or ad­ja­cent to any grounded sur­face un­til the cable is well clear of the al­ter­na­tor, giv­ing the cable the op­por­tu­nity to dis­si­pate some of its heat.

SHORE­POWER MELT DOWN

The shore­power cord and its end fit­tings are an­other weak link in our high-charge-rate sys­tems. For one of my ex­per­i­ments, we had a mas­sive lead-acid bat­tery bank that was ca­pa­ble of ab­sorb­ing for hours on end any charg­ing en­ergy I could throw at it. We were op­er­at­ing in Europe where the stan­dard shore­power out­let is rated for 230 volts and 16 amps, which equates to 3.6 kW (pretty much the same as a U.S. 120-volt, 30-amp shore­side sup­ply). I had a 3.5 kW bat­tery charger that would have run at full out­put for hours if I had let it, but in­stead

I set it to 2.8 kW (around 80 per­cent of the shore­power out­let rat­ing) in or­der to pro­tect the shore­power cir­cuit. I still melted down the dock­side out­let.

The prob­lem here is the fric­tion-type con­nec­tors we have at both ends of a shore­power cord, as op­posed to the bolted con­nec­tions we nor­mally have with high-cur­rent cir­cuits. If you go around any large ma­rina and in­spect the shore­power pedestals you will see signs of burn­ing; sim­i­larly, if you look at the two ends of shore­power cords. We are ever more of­ten push­ing th­ese cir­cuits to con­tin­u­ous high cur­rent lev­els for which they are not well suited. At the boat end, there is a con­nec­tion avail­able from Smart­Plug in Seat­tle that takes care of the con­nec­tion is­sues. Other than this, the only an­ti­dote is to be aware of the prob­lem, not to push the shorecord to its rated cur­rent for ex­tended pe­ri­ods of time and to reg­u­larly in­spect the con­nec­tions at both ends of the cord, on the dock and the boat.

IM­PER­FECT CON­NEC­TIONS

In truth, we should have bolted con­nec­tions wher­ever we have high cur­rents. But even here we run into an­other po­ten­tial prob­lem. On boats, we com­monly use stain­less steel to bolt th­ese con­nec­tions to­gether. How­ever, stain­less steel has very poor elec­tri­cal con­duc­tiv­ity. If the stain­less be­comes part of a con­duct­ing cir­cuit with high con­tin­u­ous cur­rents, it can be­come hot enough to cre­ate a fire risk. Not that there is any­thing wrong with us­ing the stain­less: you just have to make sure that it is only be­ing used to clamp one con­duct­ing sur­face di­rectly to an­other. Even a stain­less washer be­tween the two can cause prob­lems.

At high am­per­age lev­els the ter­mi­nals them­selves can also be prob­lem­atic, as th­ese are in­vari­ably crimped. For a given stranded cable size (gauge) there are vari­a­tions in the di­am­e­ter of the cop­per de­pend­ing on the stan­dard used for siz­ing (SAE, AWG or ISO) and the num­ber of strands in the cable, and there are also a have half-dozen dif­fer­ent types of cop­per crimp-on ter­mi­nals, vary­ing in cop­per thick­ness and di­am­e­ter. If the ca­bles and ter­mi­nals are not prop­erly matched both to each other and the dies in the crimper, poor crimps will re­sult. For ex­am­ple, I have pulled 1/0 ter­mi­nals off their ca­bles by hand af­ter they had been crimped with a mis­matched die in a hy­draulic crimper.

Fi­nally, heat shrink is of­ten added to th­ese ter­mi­nals. If even a tiny cor­ner of heat shrink ex­tends into the con­duct­ing sur­face, and even if the con­nec­tor is torqued down hard in a bolted con­nec­tion, the con­nec­tion can be re­sis­tive enough to gen­er­ate a great deal of heat. You get the idea. Bot­tom line, on cir­cuits car­ry­ing con­tin­u­ous high cur­rents, all con­nec­tions need to be elec­tri­cally per­fect and ex­tremely tight.

USE­FUL TOOLS

An ef­fec­tive means to check for po­ten­tial heat prob­lems in high-cur­rent DC cir­cuits is to switch a mul­ti­me­ter into its DC volts mode and place one probe of the me­ter at one end of the cir­cuit (e.g. the pos­i­tive out­put ter­mi­nal in the back of an al­ter­na­tor) and the other at the other end of the cir­cuit (e.g. the pos­i­tive ter­mi­nal on the bat­tery be­ing charged), us­ing ex­ten­sion leads if nec­es­sary. That done, fully load the cir­cuit by run­ning your charg­ing de­vices flat out). Any volt­age that is reg­is­tered rep­re­sents volt­age drop caused by re­sis­tance in the cir­cuit. In prin­ci­pal, the volt­age drop on fully-loaded charg­ing cir­cuits should not ex­ceed 3 per­cent of the rated volt­age, which would be 0.36 volts on a 12-volt cir­cuit. In any case, you should never ex­ceed a 10 per­cent drop, or 1.2 volts on a 12-volt cir­cuit.

If you per­form this test and find that the volt­age drop is too high, it may be the re­sult of the cu­mu­la­tive re­sis­tance in un­der­sized ca­bles, but is fre­quently the re­sult of re­sis­tive con­nec­tions, which in high cur­rent cir­cuits will form hot spots. For­tu­nately, we now have some tools that are par­tic­u­larly use­ful for de­tect­ing th­ese kinds of things. One is an in­frared laser heat gun, which will give pre­cise tem­per­a­ture read­ings from very spe­cific lo­ca­tions. An­other is a ther­mal imag­ing cam­era, which will show tem­per­a­ture gra­di­ents within a larger area. The lat­ter is a par­tic­u­larly use­ful de­vice. Th­ese cam­eras can now be bought for un­der $200 and will plug into any smart phone.

STEP CHANGE IN DC SYS­TEMS

We are en­ter­ing a new era in terms of en­ergy sys­tems on boats, with ex­traor­di­nar­ily high rate charg­ing de­vices and bat­ter­ies that can ab­sorb pretty much any­thing we can throw at them. To­gether, th­ese will give us greatly im­proved ef­fi­cien­cies and life­styles, but along with this comes new in­stal­la­tion chal­lenges. For­tu­nately, we also have the tools to help us through th­ese chal­lenges and en­sure safe and re­li­able in­stal­la­tions. The pieces are com­ing to­gether rather nicely! s

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