Lithium Batteries for Small Boats

We are now all familiar with the primary advantages (life expectancy, weight, usable capacity etc.) of lithium iron phosphate (LiFePo4) batteries. What is absent, however, is a broad appreciation of the many other features of LiFePo4 batteries, that fall outside the normal promotional material. These features are important and affect how the 12 V electrical system of a sailboat needs to be reconfigured to accommodate LiFePo4 technology.

I am a full-time cruiser on my 1975 CAL 34, Moonrise, now in Panama and about to cross the Pacific. I once owned my own renewable energy company, however, trained as an electrical technician engineer and managed complex electrical systems throughout my career. As I began to investigate converting my own boat’s system, however, I was disappointed to find how ill-informed most user on-line commentary was. Moreover, as I began to consult various battery and equipment manufacturers, I soon realized that their attention is spread across the needs of multiple users (off-grid installations, RVs, commercial vehicles etc.) and not necessarily on small boats.

Nonetheless, some manufacturers gave me thoughtful, detailed, technical advice, whereas others couldn’t get beyond the standard marketing talking points. When I pressed one manufacturer, for instance, on how to best integrate their product, I was told that it was not their expertise and that I needed to hire a licensed marine electrical contractor.

LiFePO4 battery technology is now of a maturity and at a cost where it should be the logical choice for many sailboat owners, but hiring a licensed electrical contractor would likely be prohibitively expensive for many, even if you are fortunate enough to live near one. The purpose of this series of articles, therefore, is to give sufficient information, such that the average, competent, sailboat owner is aware of all the key issues, sufficient to safely manage their own LiFePO4 conversion.

Difference Between Lithium Iron Phosphate (LiFePo4) and Lithium-ion

Before we go there, however, we need to explain the difference between the term Lithium iron phosphate (LiFePo4) and Lithium-ion. As the table in figure 1 illustrates, Lithium iron phosphate (LiFePo4) is within the Lithium-ion family but has its own unique characteristics. Although subject to the same HAZMT shipping restrictions as the other battery types, LiFePO4 distinguishes itself by being the most chemically stable. If subject to abuse (overcharging, excessive discharge, temperature etc), however, it can become unstable and thereby pose a fire risk, which is why it also needs a Battery Management System (BMS), a device which manages key battery functions.

Why LiFePo4 Are Not Drop-In Replacements for Lead Acid Batteries

A common mantra, often heard, is that LiFePO4 batteries are not a “drop-in” replacement for lead acid batteries, but what exactly does that mean? Well, the first key difference is that whereas conventional lead acid systems might be viewed as analog, this BMS system effectively makes a LiFePO4 battery a digitally controlled energy system. The function of the BMS is to manage the battery, both internally and externally, protecting it from risk events. Its function is not to manage the boat’s electrical system (although it might send a message to it).

What does that mean for a small boat owner? It means that the BMS can decide to shut down all loads, including critical loads, at a time it determines the battery to be at risk, regardless of the boat’s circumstances at that time. Having a means to continue to serve such critical loads is therefore a key consideration, as we think about reconfiguring our boat’s electrical system.

Volt Drop

A second and rarely discussed issue is the sensitivity of digital devices to volt drop (in cables and/or connections). Put very simply, volt drop can be one of three things: acceptable, frustrating or dangerous. Acceptable is where we acknowledge the loss, but it doesn’t noticeably affect things; Frustrating is when you begin seeing noticeable effects in the system; Dangerous is when the voltage loss is so significant that it is indicative of something seriously wrong. The digital devices that we need to operate LiFePO4 batteries correctly are sensitive to incorrect voltage information that can arise from a voltage drop.

Hot Alternators and Wiring

Next, and also rarely discussed, a LiFePO4 battery’s internal impedance is possibly around one-tenth that of a lead-acid battery (the difference between a few to tens of milliohm (mΩ) to a few hundred mΩ). Because of this, a standard alternator regulator would deliver significantly higher current into a LiFePO4 battery than the alternator was designed for. This is why you hear stories of alternators and wiring running hot, when directly charging a LiFePO4 battery.

At worst, this is a fire risk and, at least, it will reduce the life expectancy of battery and equipment plus invalidate any warranties. There is a partial solution, by fitting an external regulator, adjusted for LiFePO4, but that does not resolve several other alternator issues that we discuss later in this article.

Short Circuit Fault Currents

The lower impedance of LiFePO4 has another, even more significant implication, however, in that short circuit fault currents on a LiFePO4 battery are multiples higher—and therefore multiples more dangerous—than with a similar sized lead acid battery. To put it in graphic terms, if you dropped a wrench across the terminals of a lead acid battery, you might see 8,000 amps and the wrench would probably vaporize. If you did the same with a LiFePo4 battery, you might see 20,000 amps or more (if you could even measure such a phenomena).

If you have multiple LiFePo4 batteries, in parallel, you will get multiples of this. Regular fuses do not work with such currents, as the current would simply create its own ionized path, much as lightning does. Battery manufacturers may honestly claim that their BMS is capable of responding to such a fault condition, but this may be an example of what is adequate for a land-based installation being inadequate for a marine installation. The new AYBC standard takes the view that the only solution is to install a “high AIC ” (Amperage Interrupt Capability) fuse, often known as a Class T fuse, fitted immediately adjacent to the positive terminal of the LiFePo4 battery. The cost of this is not insignificant.

Discharge Profile

Next, we need to consider the discharge profile of a LiFePo4 battery, as compared to a lead acid battery. You are probably familiar with the standard lead acid terminology of capacity being quoted with reference to a standard 20-hour test. That test and its terminology does not apply to a LiFePo4 battery primarily because of something called Peukert’s Law. In simple terms, Peukert’s Law states that a 100 ah lead acid battery (even when new and perfect condition), that is capable of delivering 5 amps for 20 hours will only deliver 20 amps for 4 hours (i.e. 80 ah), before reaching 50 percent discharge (in both cases).

With LiFePO4 batteries, Peukert’s constant is 0.97, or higher, which means that you get (close to) the same ampere-hour’s out of the battery, regardless of the rate of discharge. Instead of the familiar 20-hour test, therefore, LiFePo4 manufacturers simply test and determine the capacity of their product, and that capacity should be routinely accessible.

How Much Capacity To Install When Upgrading to LiFePo4?

Speaking only of the house bank, the old lead acid rule was to buy a multiple of your daily load, as the more capacity you had, the less frequently you would subject it to deep discharge and therefore the more cycles you would get out of the battery. That logic does not apply to LiFePO4, which might therefore lead you to conclude that you can save money and install less capacity. Conversely, manufacturers will advise you to install as much capacity as you have available space for and no less than the name plate capacity of the lead acid system it is replacing. That may sound self-serving but it’s a view I have come round to.

Once you have such a reliable source of capacity, you will find that you start considering all sorts of additional loads, such as a microwave, a bigger inverter etc., i.e. you will start using your electrical system in different ways. The key issue, however, is that LiFePO4 does not lend itself to gradually adding capacity over time, as you want a single BMS to be in control of your battery bank, which means that one big battery is preferable to two smaller batteries.

Note: There is an exception to that rule if, due to space limitations, you install two smaller batteries of the same size and from the same manufacturer in parallel, if it is established that they are designed to communicate with each other and therefore act as if they were a single BMS.

Maximum Current Discharge Capability

An alternative approach is to focus on your new battery’s maximum current discharge capability. You need to study the specification of the LiFePo4 battery that you are considering buying, but its discharge capacity is likely limited to “1c” e.g. max 100 amps from a 100 ah battery, and it may be less. Ignoring the engine starter motor for a second, your greatest load is potentially a windlass or bow thruster, maybe consuming a 100 amps or more, which means that you need a battery that has at least that discharge capacity. Putting it simply, you might think in terms of there being small batteries and large batteries and if you have a large load, then you need a large battery.

At this point I should mention that Dakota offer a 130 ah LiFePo4 battery with a claimed 1,000 CCA—which, by definition means 1,000 amps for 30 seconds at 0 C—which might offer an alternative approach for small boat installations. Their more detailed specs, however, state that it can only achieve 1,000 amps as a 2 second pulse (900 A for a 5 second pulse), which may not be sufficient for diesel engine starting. Moreover, it would require a BMS that could safely control that level of current, plus it would require cells that are rated for those very high current levels.

As we have discussed, the risks of LiFePo4 are not zero and the new AYBC standard leans against such applications (and towards retaining lead acid) for engine starting duties, for very good reasons.

Charging Characteristics

The charging characteristics of LiFePO4 batteries are different from lead acid and potentially different from each other too. A simple table illustrates the issue:

If you have purchased your shore-based smart charger or a solar converter in recent years, it likely has the capability of switching between these various battery types. If you have an older charger or controller, you may need to change it for a device that is capable of being programed to LiFePO4 operation. One key thing to understand, however, is that when smart chargers offer multi-bank charging capability, what they mean is batteries of the same type. They are not designed for charging lead acid on one bank and LiFePO4 on another.

Some of the less informed online comments suggest that such a relatively minor voltage difference doesn’t really matter, but it does. If you are under or over charging your battery you are ultimately shortening life expectancy and probably invalidating warranties. This also means, if you are considering one bank of lead acid (say for start purposes) and another LiFePO4 (for house purposes), that you can no longer have an old fashioned ACR in your electrical system, as that would act to tie the two together in parallel and force one to be overcharged and the other undercharged.

One additional difference between LiFePO4 and lead acid batteries is that the charge and discharge profiles are relatively flat. Consequently, while there’s nothing wrong with continuing to have a standard (digital or analog) voltmeter on your panel display, it won’t prove as useful as it did before. The better solution is a “Battery Monitor,” which measures both voltage and current (by means of a shunt installed on the main return circuit) and therefore SOC.

Combining Alternators and LiFePo4 Batteries

Now let’s focus on the elephant in the room, which is the various issues arising with combining alternators and LiFePo4 batteries. We have already discussed that a standard alternator, and its regulator, would have been designed for the impedance of lead acid operation. A second issue is the impact of any unplanned battery shutdown, if your engine (and therefore your alternator) is running at the time of shutdown. Then, your alternator would be delivering into a “no load” condition and potentially create a voltage spike that might damage the alternator regulator diodes.

There are yet more alternator issues to consider. As a LiFePO4 battery gets close to being fully charged, it approaches in effect, a similar “no load” situation. Alternators become unstable, when operating into a near no-load system, as it has zero stabilizing current draw, which means that the alternator output can oscillate (a ripple effect), which is also potentially damaging to equipment. Moreover, being electromagnetic, the alternator needs a field voltage to excite the alternator, otherwise it rotates without generating any electricity. Should the BMS go to sleep, that voltage is not available.

Bottom Line

There are solutions for some of these alternator issues (such as surge protection and aftermarket external regulators) but currently there is no “perfect,” fully integrated solution to address all these issues. Also, small boat owners must think about how much digital complexity they are willing to introduce into their boats electrical system. For these and other reasons, retaining lead acid for start battery operation (fed by the alternator) is often the preferred approach.