Bet­ter bat­ter­ies

The 14m yacht Mai Tai has just left New Zealand on a cir­cum­nav­i­ga­tion. Prior to de­par­ture the own­ers swapped her lead-acid bat­ter­ies for lithium-iron­phos­phate (Lifepo4) al­ter­na­tives. Here’s why – and how – they did it.


The Case for Lithium. A bat­tery trans­plant with zest.


We’ve owned Mai Tai for 28 years and on W av­er­age have in­stalled new bat­ter­ies ev­ery six years. That’s five sets of costly, large lead blocks, and we’d grown tired of liv­ing with their in­ef­fi­ciency, weight and ex­pense.

For most boats with lead-acid bat­ter­ies, the charg­ing process is frus­trat­ing be­cause the closer they get to be­ing full, the greater the re­sis­tance to ab­sorb charge cur­rent. This ‘di­min­ish­ing-charge-ac­cep­tance-rate’ is a prob­lem ir­re­spec­tive of the charg­ing source.

At best, so­lar pan­els are only 17 per­cent ef­fi­cient at con­vert­ing so­lar en­ergy to DC cur­rent. And if the bat­ter­ies are 75 per­cent full, most of that cur­rent is turned away by the bat­ter­ies.

The same is true with al­ter­na­tors. We use a Bal­mar 165-amp al­ter­na­tor, but most of the time it’s charg­ing be­tween five and 25 amps be­cause the bat­ter­ies turn away the avail­able charge cur­rent. Con­se­quently, the en­gine must run much longer to achieve a full charge. This be­comes even worse as the bat­ter­ies age. And there’s more. In ad­di­tion to be­ing dif­fi­cult to charge fully, lead-acid bat­ter­ies only give back about 40-50 per­cent of their rated ca­pac­ity (amp-hours) be­tween charges. So if you re­quire 300 amp-hours (ah) of use­able en­ergy for your elec­tri­cal needs, you will need a bat­tery bank rated at 600ah or more. That bat­tery bank will weigh over 175kg and re­quire a lot of space.

We didn’t want to put an­other set of lead-acid bat­ter­ies on Mai Tai. So we be­gan re­search­ing lithium-iron-phos­phate (Lifepo4) bat­ter­ies. What we found was much sim­pler than we’d ex­pected, con­sid­er­ing all the scary stuff that’s been writ­ten about them.


How are Lifepo4 bat­ter­ies su­pe­rior to their lead-acid cousins? Here’s a quick over­view.

Lifepo4 bat­ter­ies will take a charge cur­rent equal to their amp-hour rat­ing, right up to their fully-charged state. The­o­ret­i­cally, you could top-up a fully-dis­charged 300ah bank of lithium bat­ter­ies in an hour if you had a 300A charger.

They don’t care if they are fully- charged or only halfcharged. In fact, when stored for long pe­ri­ods they pre­fer be­ing half-full and they will lose less than two per­cent of their charge per month. Over six-months the bat­tery level

may drop by 12 per­cent.

Lifepo4 bat­ter­ies can give back about 80 per­cent of their rated amp-hours and not be se­ri­ously dis­charged. If you need 250ah of use­able power, a 300ah Lifepo4 bank will be plenty.

Even as Lifepo4 bat­ter­ies are dis­charged they main­tain a very steady volt­age level. This is good for your boat’s elec­tronic load though it does have one dis­ad­van­tage; it makes it very dif­fi­cult to mea­sure the state of charge (SOC) of your bank us­ing volt­age read­ings alone.

Com­pared with a same-ca­pac­ity lead-acid bank, Lifepo4 bat­ter­ies are gen­er­ally a third of the weight and less than a third of the vol­ume.


It turns out the prob­lem’s not with the bat­ter­ies but with your boat’s ex­ist­ing in­fra­struc­ture.

AC charg­ers, al­ter­na­tors, so­lar reg­u­la­tors and wind gen­er­a­tors are all charg­ing sources for bat­ter­ies. They’ve all been de­vel­oped for lead-acid bat­ter­ies – and they’re not al­ways suit­able for a Lifepo4 bank.

But by mak­ing some mi­nor ad­just­ments to your charg­ing in­fra­struc­ture, you can mas­sively im­prove power and ef­fi­ciency. Yes, this comes at a cost, but it’s a one-off.

Here’s an out­line of the changes re­quired:

The lead-acid charg­ing regime usu­ally has three steps – bulk, ab­sorb and float. A Lifepo4 charg­ing regime has two steps; bulk and off. There is no need for the ab­sorp­tion phase. Note: Lifepo4 bat­ter­ies don’t like con­tin­u­ous float charg­ing – it short­ens their life.

Al­ter­na­tors use reg­u­la­tors de­signed to charge lead-acid bat­ter­ies, but th­ese reg­u­la­tors don’t suit Lifepo4 bat­ter­ies. Some com­pa­nies, how­ever, have de­vel­oped reg­u­la­tors for Lifepo4 bat­ter­ies. Bal­mar’s new MC614 reg­u­la­tor is one of them.

So­lar pan­els use a con­troller for charg­ing bat­ter­ies and it needs to be ad­justed to suit Lifepo4. Thanks to the growth of off-grid, so­lar-pow­ered homes and RVS, there are now sev­eral suit­able so­lar con­trollers on the mar­ket.

A wind gen­er­a­tor must also be reg­u­lated for a Lifepo4 bank. It must au­to­mat­i­cally shut down be­fore the bat­ter­ies are over- charged.

The AC charg­ers we use with shore power are de­signed for lead-acid bat­ter­ies. Many tout a five-step charge regime, but this is ir­rel­e­vant for Lifepo4 bat­ter­ies.


Our new sys­tem has var­i­ous de­sign fea­tures.

First, be­cause Lifepo4 bat­ter­ies don’t like heat (or ex­ces­sive cold) we moved the bat­tery bank to an area that’s in­su­lated from the en­gine room.

There are three bus bars – one for all charge cur­rent go­ing into the bat­ter­ies (Charge Bus), one for all cur­rent leav­ing the bat­ter­ies (Load Bus) and a com­mon ground bus (Ground Bus). This ar­range­ment stops cur­rent from en­ter­ing the bat­ter­ies when fully-charged by switch­ing off the Charge Bus. It stops cur­rent leav­ing by switch­ing off the Load Bus when the bat­ter­ies are nearly empty.

The Charge and Load Bus bars are con­nected to sep­a­rate switches con­trolled by the ‘bat­tery mon­i­tor­ing sys­tem’ (BMS). The BMS con­stantly mon­i­tors the lithium cells and tells the switches to close when set lev­els are reached.

Some in­stal­la­tions sim­plify this ar­range­ment by hav­ing both the Charge Bus and the Load Bus on one switch. This can be a prob­lem: if the bat­tery is over-charged and the switch is turned off, the Load Bus is

also turned off and you can’t use the bat­ter­ies.

It might keep the cost down but it pre­vents the bat­ter­ies from be­ing used and brought back within an ac­cept­able charge range. Hav­ing two bus bars and two switches al­lows the bat­ter­ies to be kept within charge lim­its with­out re­mov­ing them from the sys­tem com­pletely.

One of the is­sues with us­ing re­lays to stop the flow of cur­rent is this: what hap­pens if the charge re­lay opens when you are charg­ing the bank? For ex­am­ple, if the al­ter­na­tor is grind­ing out 100 amps of cur­rent and sud­denly the switch is thrown? The al­ter­na­tor can’t sense the bat­tery charge level be­cause it’s been dis­con­nected.

This may cause wild spikes in volt­age lev­els through­out the sys­tem and could cause some da­m­age, in­clud­ing to the al­ter­na­tor. To solve this we put a sealed lead-acid (SLA) start bat­tery along­side the Lifepo4 sys­tem. All it does is start the en­gine.

But – we de­signed the Charge Bus re­lay so that it con­nects with the SLA bat­tery be­fore it switches off from the lithium bank. So if the Charge Bus switch is opened, the al­ter­na­tor im­me­di­ately sees the lead-acid bat­tery in the sys­tem and ad­justs its charg­ing ac­cord­ing to the new bat­tery.

The Lifepo4 bank is re­moved from the Charge Bus but is still run­ning the loads through the Load Bus. As the charge level falls back to within the pro­grammed range, the re­lay brings the Charge Bus back on­line and the SLA bat­tery is dis­con­nected.

Keep in mind that the re­lays for the Charge and Load Bus are for ‘sys­tem fail­ure’ in­ci­dents (per­haps the wind gen­er­a­tor fails to shut down when it should). Each of the charg­ing sources should be pro­grammed to keep the Lifepo4 pack within its lim­its with­out throw­ing the re­lay.

Note: In the best pos­si­ble de­sign we want the whole sys­tem to op­er­ate with­out ever throw­ing one of th­ese re­lays.

I’ve also seen con­fig­u­ra­tions that use an SLA bat­tery in the sys­tem and use a bat­tery iso­lat­ing re­lay to keep the bat­ter­ies sep­a­rate while al­low­ing the Charge Bus to keep both bat­ter­ies charged. With this sys­tem it’s dif­fi­cult to sense true bat­tery volt­age be­cause the iso­la­tor blocks the volt­age from trav­el­ling back through the iso­la­tor to the post where it can be de­tected.

Our sys­tem has the ad­van­tage of al­low­ing both bat­ter­ies to be charged by the Charge Bus, but they cause a lot of dif­fi­culty when try­ing to ac­quire an ac­cu­rate bat­tery volt­age for the al­ter­na­tor. For this rea­son we haven’t used a bat­tery iso­lat­ing re­lay. In­stead, we have a ‘make-be­fore-break’ sin­gle­pole-dou­ble-throw (SPDT) Mos­fet solid state re­lay that will com­fort­ably han­dle 250 amps at 12 volts con­tin­u­ous.

This won’t al­low us to si­mul­ta­ne­ously charge the lead-acid bat­tery through the Charge Bus. But we have a so­lar charge reg­u­la­tor, which will put a small charge to the lead-acid bat­tery in­de­pen­dent from the Charge Bus. This should keep it well charged as the cur­rent draw on this bat­tery will be quite low. In ad­di­tion, we have placed a man­ual over­ride switch in the sys­tem, which will al­low us to bring the SLA bat­tery on to the Charge Bus if it ever re­quires a quick top-up.


Our GBS bat­ter­ies (made in China) are dis­trib­uted by Elite Power So­lu­tions in the USA. They come with their own BMS so­lu­tion, where each lithium cell is in­di­vid­u­ally mon­i­tored by a net­work of small com­puter chips.

Th­ese con­stantly send volt­age and tem­per­a­ture in­for­ma­tion to the main BMS unit, while also per­form­ing cell bal­anc­ing on the in­di­vid­ual cells. The BMS unit also counts the num­ber of amps-in and amps-out so it can give an ac­cu­rate state of charge (SOC) for the bank with­out re­ly­ing on the volt­age read­ing.

The in­for­ma­tion is sent by video link to an LCD display. Ev­ery time the bank is fully charged the BMS re­sets its amp count and thereby re­mains ac­cu­rate. They also come strapped to­gether in 100ah packs and the guys at Elite Power So­lu­tions will pro­gram the BMS for you, tak­ing into con­sid­er­a­tion size of your bank. You may want 400ah or 600ah. We feel com­fort­able with 300ah. There are out­put pins (spades) on the BMS unit for pow­er­ing un­der volt­age (UV) and over volt­age (OV) switches and alarms as well as the video out­put.


We wanted a make-be­fore-break re­lay for the Charge Bus, which would al­low the charge bus to sense the SLA bat­tery be­fore los­ing con­tact with the lithium bank. This will pro­tect the sys­tem from the charge bus throw­ing out stray cur­rent with nowhere to go. We also wanted the re­lay to have its own set of cri­te­ria for switch­ing off in case the BMS unit lost power or in some way failed.

Per­fect Switch in South­ern Cal­i­for­nia helped me out. It put to­gether a Slave and Mas­ter unit for the make-be­fore­break re­lay. You can see in the wiring di­a­gram that the Slave unit is con­nected to the lithium bank and is nor­mally “on”, which means it is closed.

The Mas­ter unit is con­nected to the SLA bat­tery and is nor­mally “off” or open. They are both con­nected di­rectly to the Charge Bus. When there is an “event” where the BMS over-volt­age pin (OV) sends out a shut-off sig­nal, the Mas­ter unit closes just be­fore the Slave unit opens, thereby pro­vid­ing a path to the SLA bat­tery for any charge cur­rent com­ing in.

Since the Mas­ter and Slave units are both bi-di­rec­tional

the al­ter­na­tor is al­ways able to get an ac­cu­rate volt­age read­ing of the bat­tery be­ing charged. Th­ese re­lays are able to be pro­grammed to switch off at a cer­tain volt­age so I have set them up to shut off at just a bit higher volt­age than what the BMS is set for. If the BMS fails, the re­lays should shut off the charge bus be­fore any da­m­age is done to the bat­ter­ies.

The Load Bus is pretty straight for­ward. It uses one re­lay, which switches off the loads when the BMS sends a mes­sage that the volt­age level in the lithium bank is too low (UV). We ended up us­ing 90 per­cent charged for ‘Full’ and 20 per­cent charged for ‘Empty’. This gives us use­able am­per­age of roughly 70 per­cent of 300 amps – or 210 amps of use­able power while keep­ing the Lifepo4 bat­ter­ies well within their best op­er­at­ing mode.

Ob­vi­ously, it is very im­por­tant to have all loads from the boat con­nected to the Load Bus and all charg­ing sys­tems con­nected to the Charge Bus in or­der for this sys­tem to work prop­erly.


To make sure none of the charg­ing units would kill our new bat­ter­ies, we did the fol­low­ing: So­lar – we have two 160-watt pan­els and set­tled on a Votronic MPP 350 Duo Digital so­lar reg­u­la­tor, which has switch­able set­tings for charg­ing lithium bat­ter­ies. I also set up a switch on the panel, which al­lows me to shut off the so­lar pan­els com­pletely so they won’t try to trickle charge the bat­ter­ies to death when we are not on­board. Wind gen­er­a­tor – ours is an Air-x-ma­rine unit with new Blue Blades (much qui­eter!). This unit has an in­ter­nal reg­u­la­tor, but also an ex­te­rior ‘pot’ screw on the body that can change the cut-out volt­age on the unit. I ad­justed the cut-out volt­age to 14V, which should shut the unit down long be­fore the BMS senses a crit­i­cal volt­age level in the lithium cells. Al­ter­na­tor – this Bal­mar 165 has an ex­ter­nal MC-612 reg­u­la­tor. Bal­mar says it has de­vel­oped a new reg­u­la­tor with a Lifepo4 charg­ing regime all set up. It also says my older model MC-612 can be pro­grammed to suit Lifepo4 bat­ter­ies. Bal­mar is­sued a ser­vice bul­letin in De­cem­ber last year which ex­plains how to re-pro­gram the MC-612 for lithium. Job done! AC Charger – Elite Power So­lu­tions dis­trib­utes a small AC charger, where a con­trol ca­ble plugs di­rectly into the BMS unit and charges the bat­ter­ies ac­cord­ing to the BMS read­ings. It only puts out 15 amps. So this unit is aboard and con­nected, ready to use if we ever use shore power. BNZ

LEFTThe Lifepo4 bat­tery bank –much smaller and lighter, and far eas­ier to charge, than con­ven­tional lead-acid bat­ter­ies.

RIGHTThe Mas­ter, Slave and Load re­lays. The Charge re­lay al­lows the bus to sense the SLA bat­tery be­fore los­ing con­tact with the lithium bank. Note the Ground Bus across the top right of the im­age.

LEFTLane and Kay Fin­ley have over 30 years blue­wa­ter cruis­ing ex­pe­ri­ence.

OP­PO­SITE BE­LOW The Elite Power BMS. There are out­put pins (spades) for pow­er­ing un­der volt­age (UV) and over volt­age (OV) switches and alarms.RIGHT & FAR RIGHT The Ground bus and one of the re­lays. Mai Tai’s cir­cum­nav­i­ga­tion will be a lit­tle eas­ier with more re­li­able power.

RIGHTThe Charge Bus – the com­mon fac­tor for the var­i­ous charge sources. BE­LOWThe two 160-watt so­lar pan­els have ben­e­fit­ted from a Votronic reg­u­la­tor.

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