Mod­ern in­vert­ers/charg­ers trans­form di­rect cur­rent to run AC ap­pli­ances — and they keep the bat­tery bank healthy.

Cruising World - - Contents - BY ED SHER­MAN

By Ed Sher­man

It seems sailors these days just can’t get along with­out some source of al­ter­nat­ing-cur­rent elec­tric­ity on board, ei­ther from shore power, which re­quires stay­ing at the dock all the time, or by run­ning an AC gen­er­a­tor or pos­si­bly an in­verter, which draws power from the boat’s di­rect-cur­rent bat­tery bank.

I love com­par­ing to­day’s boats to what my wife and I used on board when we were cruis­ing back in the 1980s. Our boat didn’t have any AC cir­cuitry! We read at night us­ing gim­baled oil lamps; we used a sun shower for hot wa­ter. Cof­fee was brewed us­ing a French press heated on the propane-fu­eled stove-top, mu­sic came from a bat­tery-pow­ered AM/FM cas­sette player and we dried our hair in the breeze as we sailed. Oh, and our re­frig­er­a­tion sys­tem was pow­ered by a strange thing known as block ice. My, have times changed!

Last month in Hands on Sailor, we dis­cussed se­lect­ing a gen­er­a­tor for your boat (“Power On,” Oc­to­ber 2018). Now, let’s take a look at a closely re­lated de­vice, the in­verter/charger. We’ll con­sider mod­els from Master­volt, Mag­num En­ergy, Pro­mariner, Vic­tron and Xantrex, all well­known play­ers in the ma­rine mar­ket­place, and com­pare their ad­van­tages and disad­van­tages, with an eye to­ward adding just a lit­tle bit more con­ve­nience to your sail­ing.


Once you’ve de­cided that blow-dry­ing your hair in the wind is not the ap­proach you want for your cruis­ing life­style, you need to care­fully fig­ure out how much power you need. Mod­ern in­vert­ers are avail­able with 5,000 watts or even more power po­ten­tial, and many mod­els can be con­nected in par­al­lel, ef­fec­tively dou­bling — and then some — their out­put in kilo­watts.

Sail­boats equipped with an AC gen­er­a­tor will usu­ally have one in the 4 to 10 kw size range, de­pend­ing on the gear sailors in­tend to use reg­u­larly. When in­stalling an in­verter, you may have to de­cide to elim­i­nate some of the items you would power ei­ther from shore or your AC gen­er­a­tor. It all de­pends on how much bat­tery power you have on board.

The pluses of in­verter power drawn from the bat­tery bank are silent op­er­a­tion and not hav­ing to smell diesel ex­haust. But, there are trade-offs. The bat­ter­ies needed to power an in­verter are heavy and take up a lot of on­board real es­tate, es­pe­cially if your boat has older-style lead-acid bat­ter­ies. Think of bat­tery power as the fuel for an in­verter. The greater your elec­tri­cal ap­petite, the more stor­age ca­pac­ity you’ll need. In some in­stal­la­tions I’ve seen, the lack of space for bat­ter­ies has been the lim­it­ing fac­tor on the elec­tri­cal sys­tem.

Gen­er­ally, in­verter power is not used for items that run con­tin­u­ously, such as re­frig­er­a­tion and air con­di­tion­ing, which are in­stead run ei­ther di­rectly from the bat­tery bank, in the case of re­frig­er­a­tion, or run from shore power or gen­er­a­tors. That said, it can be done, but you need to un­der­stand that the amount of bat­tery power you’ll need to make this hap­pen will be con­sid­er­able and might bring you to a point of di­min­ish­ing re­turns. The ca­pa­bil­ity will de­pend upon whether you al­ready have up­graded to new tech­nol­ogy, such as thin-plate pure-lead ab­sorbent glass mat or per­haps even lithium tech­nol­ogy.

Sev­eral im­por­tant tech­ni­cal lim­i­ta­tions play into the whole in­verter-bat­tery re­la­tion­ship. One is the fact that mod­ern in­vert­ers are ap­prox­i­mately 90 per­cent ef­fi­cient, so there is a 10 per­cent loss of power right from the start. Sec­ond, bat­tery amp-hour ca­pac­ity will be a lim­it­ing fac­tor. The term I use to de­scribe this is “cur­rent den­sity.” Sim­ply, this means how much bat­tery ca­pac­ity you can squeeze into the space avail­able aboard your boat. This is an area where sig­nif­i­cant im­prove­ment has oc­curred over the past decade. His­tor­i­cally, we have used a 50 per­cent level of dis­charge as a max­i­mum for flooded-cell lead-acid bat­ter­ies. So, a bat­tery with a 100-amp-hour rat­ing would only have 50 amp-hours avail­able if you wanted to max­i­mize its cy­cle life.

Many of the new AGM bat­tery ven­dors are now claim­ing an 80 per­cent level of dis­charge as ac­cept­able and not dam­ag­ing to cy­cle life

ex­pectancy. So, that is a 30 per­cent im­prove­ment in avail­able amp-hour ca­pac­ity, if you are will­ing to spend the money for the ini­tial pur­chase of these new bat­ter­ies. More­over, you can the­o­ret­i­cally get more avail­able power into the space you have avail­able com­pared to us­ing less ex­pen­sive flooded-cell bat­ter­ies. This could be

the dif­fer­ence be­tween be­ing able to run an air con­di­tioner all night on in­verter power and en­dur­ing a hot, sweaty night on board.

The take-away: You are go­ing to have to per­form an hon­est eval­u­a­tion of how much power you in­tend to use in a 24-hour pe­riod. Next, you must de­ter­mine whether you will have bat­tery-recharg­ing ca­pa­bil­i­ties. Your op­tions in­clude shore power, an on­board AC gen­er­a­tor sup­ply­ing power to a per­ma­nently in­stalled bat­tery charger and run­ning your en­gine-driven al­ter­na­tor long enough to get your bat­ter­ies back up to charge.

This brings up an­other point re­gard­ing mod­ern bat­ter­ies: They can be recharged more quickly than older flooded-cell bat­ter­ies. Bat­tery ac­cep­tance rate is the term here. Flooded-cell bat­ter­ies have a recharge ac­cep­tance rate of around 25 to 30 per­cent of their am­phour rat­ing as a max­i­mum. This means that no mat­ter how many amps you try to shove back into them, they will only ac­cept power at this fixed rate. Agm-tech­nol­ogy bat­ter­ies have a slightly higher ac­cep­tance rate, in the 30 to 40 per­cent range, sav­ing en­gine run time if your recharge method is ei­ther with an AC gen­er­a­tor or propul­sion-en­gine al­ter­na­tor.

With these fac­tors in mind, you can see that se­lec­tion of an in­verter and its rat­ing is go­ing to re­quire a bit of cal­cu­la­tion to de­ter­mine your ex­pected loads, the amount of time you ex­pect to use them and what your real-world bat­tery ca­pac­ity is go­ing to be.

Also, if you are go­ing to be pow­er­ing air con­di­tion­ing or re­frig­er­a­tion equip­ment us­ing a com­pres­sor mo­tor, you need to take into ac­count mo­tor start-up cur­rent. All in­vert­ers have a peak out­put rat­ing in their spec­i­fi­ca­tions. Just make sure your peak am­per­age de­mands for mo­tor start-ups don’t ex­ceed your in­verter’s peak out­put rat­ing. Most, but not all, have a peak rat­ing that is twice the nom­i­nal rat­ing. (One of the Mag­num 3,000-watt units we looked at has a max­i­mum rat­ing of only 3,900 watts, an ex­cep­tion to the rule.)


Ten years ago, the mat­ter of whether you needed a true sine wave or mod­i­fied sine wave in­verter was a de­ci­sion with a sig­nif­i­cant price dif­fer­en­tial. You were go­ing to pay big-time for the true or “pure” sine wave in­verter com­pared to a mod­i­fied square-wave model. To­day, that price dif­fer­en­tial has shrunk con­sid­er­ably, and de­pend­ing on whose in­verter you choose and its rated out­put, you will only be look­ing at a $200 to $300 price in­crease.

The es­sen­tial dif­fer­ence be­tween the two types is the AC wave­form that gets cre­ated as part of the con­ver­sion from bat­tery-sup­plied di­rect cur­rent to al­ter­nat­ing cur­rent. The pure sine wave AC is nearly in­dis­tin­guish­able from util­ity-de­liv­ered power, and it is able to run the most sen­si­tive of equip­ment. The mod­i­fied square-wave op­tion in­tro­duces what is de­scribed in the world of elec­tron­ics as “noise” into your elec­tri­cal sys­tem. This con­ducted elec­tronic noise can af­fect the per­for­mance of some on­board ap­pli­ances that are be­com­ing quite pop­u­lar on mod­ern cruis­ing boats. Au­dio am­pli­fiers might turn the elec­tri­cal noise into an au­di­ble hum, for in­stance, and noise can cause tele­vi­sion screens to flicker. As more and more elec­tri­cal de­vices ac­quire dig­i­tal con­trols as a part of their de­signs, the po­ten­tial for is­sues with mod­i­fied square-wave in­vert­ers be­comes more real.

On the other hand, de­vices such as hair dry­ers, toaster ovens and most cof­fee mak­ers are not as finicky about their power source, as long as volt­age and fre­quency re­quire­ments are met. In to­day’s global mar­ket­place, many ap­pli­ances are rated for 120 and 240 volts, and dual fre­quency at 60 Hertz or 50 Hertz.

If in doubt about a par­tic­u­lar ap­pli­ance, check the la­bel on the de­vice to pro­vide you with vi­tal in­for­ma­tion on wattage, amps, fre­quency and volts needed. All you need to do to fig­ure out how many amp-hours of ca­pac­ity you are go­ing to need is con­sider how long you in­tend to use the equip­ment. Mul­ti­ply that value by 1.1 to ac­count for the ef­fi­ciency loss in the in­verter, and you should be close to your in­verter-rat­ing needs in watts.


The new­est in­vert­ers have in­cor­po­rated some im­pres­sive fea­tures that you may use, de­pend­ing on how far you want to go with in­te­grat­ing the in­verter into your on­board elec­tri­cal sys­tem. For ex­am­ple, in­verter/charg­ers are now com­mon­place, and I would cer­tainly rec­om­mend one, es­pe­cially if you are up­grad­ing to mod­ern bat­ter­ies and your ex­ist­ing bat­tery charger is more than five or six years old. Of the pop­u­lar brands in our roundup, we fo­cused on the Pro­mariner in­verter-only de­vices, but when you look at our com­par­i­son chart, “In­vert­ers by the Num­bers,” page 84, you can see that the price dif­fer­en­tial be­tween its in­verter and all the other brands with in­te­grated charg­ers is such that pur­chas­ing a Pro­mariner charger that will match nicely with these lat­est-ver­sion in­vert­ers is a very doable op­tion. Pro­mariner does of­fer two slightly old­ertech­nol­ogy com­bi­na­tion units at its on­line out­let store; these of­fer both in­verter

and bat­tery charger in one case. The pure sine wave Pro­mariner in­verter has a 2,000-watt con­tin­u­ous and 5,600-watt peak out­put rat­ing. The mod­i­fied sine wave model has a 2,500-watt con­tin­u­ous rat­ing and a 7,000-watt peak rat­ing. They are avail­able with re­tail prices of $1,495 and $1,149, re­spec­tively. With those two units, the charger tech­nol­ogy in­te­grated into the units is quite state-of-the-art. The in­verter side of the de­vice does not of­fer the same ca­pa­bil­ity as the in­verter-only units in our com­par­i­son. So, it re­ally de­pends upon your spe­cific needs and prod­uct avail­abil­ity.

The new­est charg­ers from the man­u­fac­tures listed in “In­vert­ers by the Num­bers” come with three- or four­phase charge ca­pa­bil­ity — step one in en­sur­ing long bat­tery life — and are pro­gram­mable to more ef­fec­tively match a spe­cific bat­tery tech­nol­ogy. If you are go­ing to in­vest in the lat­est bat­ter­ies, you will want to max­i­mize your bat­tery cy­cle life, and these new­est in­vert­ers/charg­ers have that ca­pa­bil­ity. In my view, this is the com­pelling rea­son to pay a bit more and get this more so­phis­ti­cated charger ca­pa­bil­ity. It’ll max­i­mize your re­turn on in­vest­ment when it comes to the bat­tery bank.

An­other rel­a­tively new

in­verter fea­ture is the abil­ity to au­to­mat­i­cally switch on and sup­ple­ment gen­er­a­tor or shore-power out­put when the loads on the boat reach their peak. This “co­gen­er­a­tion” ca­pa­bil­ity, pop­u­lar in Eu­rope for some time, has only been al­lowed on U.S. boats un­der Amer­i­can Boat and Yacht Coun­cil Stan­dard A-32 since 2012. Among the in­vert­ers men­tioned in this story, the Mag­num units, both Master­volt de­vices and the larger Xantrex in­verter of­fer this ca­pa­bil­ity.

Some in­vert­ers will au­to­mat­i­cally switch to shore power when it is sensed, pre­vent­ing un­nec­es­sary bat­tery drainage. In some cases, this will also work in re­verse: When shore power is lost for what­ever rea­son, the in­verter will sense this and turn on au­to­mat­i­cally. All the in­vert­ers and in­verter/charg­ers we’ll men­tion here have pro­gram­mable low-volt­age shut­down when bat­tery state of charge reaches crit­i­cal lev­els. This will also help pre­vent dam­age to your ex­pen­sive bat­ter­ies.

To sum up, to­day’s in­verter/ charg­ers of­fer a list of fea­tures that can make them and the bat­ter­ies that fuel them work more ef­fi­ciently than ever. There is con­sid­er­able ini­tial cost, how­ever, in­volved in de­sign­ing a sys­tem that can con­ceiv­ably power up the av­er­age 35- to 45-foot cruis­ing sail­boat. This is es­pe­cially true if you want to have silent AC power avail­able away from the dock and more of the con­ve­niences from home avail­able to you while on the hook.

That said, I am more and more con­vinced that it is pos­si­ble, with am­ple stor­age space for bat­ter­ies, to de­sign a sys­tem us­ing al­ter­na­tive bat­tery-charg­ing means, such as so­lar, wind and hy­dro power. This means you can have the lux­ury of home with­out the need to run AC gen­er­a­tors or main-propul­sion en­gines just to re­plen­ish your bat­ter­ies — and I love the con­cept of min­i­miz­ing the need for fos­sil fu­els to ev­ery ex­tent pos­si­ble. Long term, I be­lieve that ca­pa­bil­ity far out­weighs the ini­tial in­vest­ment re­quired.

From left: The Mag­num En­ergy, Xantrex and Master­volt in­vert­ers of­fer pure sine wave tech­nol­ogy and state-of-the-art bat­tery-charg­ing tech­nol­ogy that will help en­sure the well-be­ing of an ex­pen­sive-to-re­place bat­tery bank.

The lat­est Pro­mariner in­verter tech­nol­ogy is of­fered in in­verter-only units (left). Vic­tron in­vert­ers come with a five-year war­ranty, the longest of the units dis­cussed.

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