Conversion to lithium is the subject of numerous reader enquiries. At the heart of this is always the interaction between existing and new technology as well as possible reservations. In order to be able to provide you with first-hand answers, we have been working with power supplies based on lithium batteries for some time now. Following our positive experiences with the use of the LPS system from Leab/Clayton (YACHT 6/2018), it was now the turn of a system made up of individual components.
The full name of the LPS is Lithium Power System and it is a complete power supply for small boats or motorhomes. The manufacturer Clayton has housed the battery as well as the charger, inverter and battery management in a ready-to-use housing. The connection on board is correspondingly simple: 230 volts come directly from the socket on the LPS. There are four terminals for 12 volts: two for the starter battery and two for the on-board power supply. That's it, you don't really need anything else.
However, there are two reasons in favour of a different solution: the LPS is only available in two sizes, with 70 or 100 ampere hours. And it is significantly more expensive than standard on-board batteries due to the additional components. On our test ship, these two points proved to be exclusion criteria.
The crew demanded more capacity, and the battery compartment is tailored to the height of car batteries ex shipyard, a modification was not possible because of the berths above. The obvious decision: We build a lithium power supply from individual components in such a way that it is really safe to rebuild.
The safety of lithium is an important aspect. Not because the batteries themselves are dangerous: We have opted for an intrinsically safe technology that has nothing to do with the problems with mobile phones and notebooks. The crux of the matter is rather the existing on-board electrical system. Lithium batteries can supply much more current than lead batteries. Cable cross-sections and electrical fuse protection must be designed for this, otherwise there is a risk of a cable fire in the event of a fault.
CHEMISTRY
The question of which lithium technology to use for a boat's on-board power supply is almost self-evident: The cell voltage of lithium iron phosphate batteries (LiFePo, LiFe for short) is 3.2 volts, so four cells form a battery with a nominal voltage of 12.8 volts. This is perfect for operating a conventional 12-volt on-board power supply, all consumers can continue to be used unchanged - an enormous advantage for small and medium-sized boats.
This means that the power supply to the consumers is quickly sorted out, but not the charging of the batteries: there are a few things to consider with lithium, which we will go into in detail later.
LiFe is an intrinsically safe technology. You will not experience smoke and flames, as you know them from YouTube videos and regulars' table stories, unless someone tries to do so by deliberately mishandling it. In any case, thermal runaway is prevented by design. Manufacturers of such batteries like to demonstrate the safety of their cells by perforating them with a nail, which only leads to the end of the power supply.
LiFe is also quite restrained electrically: if charged with the wrong equipment, the cells will stop working but leave your boat intact. All in all, LiFe batteries are no more dangerous than sealed lead batteries.
Lithium-cobalt cells (LiCo), which are used in many mobile devices for reasons of weight and capacity, present a contrasting programme. These react to mechanical or electrical abuse with fireworks. Background: If heated too much, such cells can be damaged internally to such an extent that their components react with each other and they continue to heat themselves up. This process is called thermal runaway. It is almost impossible to extinguish this with on-board equipment; it only ends when there is no reactive material left. This is why no reputable manufacturer offers LiCo technology as an on-board battery for the water sports market. What you can buy online is a completely different question.
BATTERY MONITORING
In connection with lithium batteries, there is always talk of a battery management system, or BMS for short. This must be present in any case, either as a single component or integrated.
Why? Lithium cells of all known designs cannot get rid of excess charge without being damaged immediately. But the cells of a battery always exhibit minute differences, regardless of the technology. As a result, their states of charge drift apart over time.
In a lead-acid battery, this is automatically equalised simply by slightly overcharging the entire battery. In a lithium battery, this is not so easy; instead, electronics must ensure equalisation between the cells. This is one of the tasks of the BMS. It measures the voltage of each individual cell and a circuit - again on each cell - diverts the entire charging current past it if necessary.
The process is called balancing and is technically the most complex part of the monitoring electronics. Without it, individual cells would become unusable within a few charging cycles due to overcharging and others due to deep discharge.
However, a complete BMS has other indispensable tasks: It disconnects the battery if there is a risk of overcharging, deep discharge, excessive current flow or exceeding the permissible operating temperature. A distinction is made between charging current and power consumption - otherwise, for example, a battery that has switched off due to a low state of charge could not be recharged.
Some modern BMS can also transfer their data to other systems, such as plotters, via a network. It is known from lead batteries that the voltage slowly decreases during discharge. With lithium batteries, on the other hand, the voltage remains almost constant over a wide range of the state of charge and then drops very quickly shortly before complete discharge.
This is good for the smooth operation of the on-board power supply - but estimating the remaining operating time is almost impossible without an electronic aid. If you already have a battery monitor on board, you may also be able to use it for a lithium system.
The principle of totalling charging and discharging currents and deriving the state of charge from this also works here - provided the monitor can be programmed to the properties of LiFe batteries. Ideally, the LiFe battery type can be selected directly in the menu. If not, you can still try to adjust the lead-acid programme using clever parameters: Set 100 per cent for the charge factor and 1.0 for the Peukert coefficient.
Whether the automatic calibration to the state of charge works is unfortunately a matter of luck, the lead criterion does not really fit. You have probably already noticed that playing with the parameters only affects the display of the remaining capacity on the battery monitor. It does not affect the behaviour of the charger or safety devices. You cannot use it to cause dangerous situations.
ACCU CHOICE
When it comes to lithium batteries for the vehicle electrical system, there is currently a choice between two concepts: building a system from individual components, i.e. cells, connectors, battery management and safety devices. This is very scalable. However, the planning alone is complex and the assembly can only be carried out by specialised personnel. You can see what something like this can look like in the picture below right: It's not for DIY.
Fortunately, as a customer you no longer need to assemble your lithium battery from individual parts; you can also obtain the batteries as ready-made battery blocks that already contain all the components required for safe operation.
Some of the dimensions are even modelled on conventional lead-acid batteries. The BMS and the most important safety devices are integrated alongside the actual lithium power storage unit. A power supply set up in this way remains reasonably manageable, which is why we opted for this concept. The batteries we use are from AMPS (Advanced Mobile Power Systems), distributed by Sterling Power.
There are similar models from different manufacturers. The batteries only have two connections: Positive and negative terminals. Access to the internal battery management data is not provided. In this case, this was not a major issue as the charge status can be monitored via an external battery monitor.
The AMPS 100Ah batteries can easily be connected in parallel. This enabled us to build a battery bank with the desired 200 ampere hours of nominal capacity. This simple procedure is not universally valid. There are also lithium batteries where the BM systems have to communicate with each other when connected in parallel.
We stuck with the lead-acid battery for the starter battery. Lithium would only have meant a small weight advantage here, but extensive modifications to the motor electrics would have been necessary: Starter motors expect the voltage of the lead-acid battery to drop to well below 10 volts during the starting process and are dimensioned accordingly.
This would not be the case with a lithium battery, so the starter would be operated at too high a voltage; if the machine did not start immediately, the starter motor would be in danger of burning out. In addition, we would have had to replace the regulator of the alternator with a version for LiFe batteries, but this was not mechanically possible.
REPLENISH STOCKS
Charging can be complicated. The technology from the lead installation does not fit without further ado: Lithium batteries require - actually - a different charging process than lead. For example, there must be no trickle charging; the power supply must be terminated definitively when the battery is full.
The word "actually" is at the top because there are now lithium batteries whose built-in electronics adapt the charging process themselves. This property is then explicitly labelled, for example with the note "Interchangeable with AGM/GEL without changing the charging structure".
If this information is missing, a lead loader must not be used under any circumstances. In our case, this was easy to solve for the shore connection: The Sterling charger on board already has a mode for LiFe batteries, so only a short change in the menu was necessary. Of course, it is no longer possible to charge the starter battery from the shore power supply at the same time, but this is not necessary in practice.
Charging by machine is more complex, but still manageable. As mentioned above, we have left the lead starter battery in place and it is now charged directly by the alternator. The simplest DIY solution for charging the lithium on-board batteries is a battery-to-battery charger (B2B charger) with a characteristic curve for LiFe. We used the Sterling Power model, but the principle is the same for all manufacturers. The motor electrics remain exactly as supplied by the manufacturer: The starter battery is connected directly to the starter motor and alternator, any isolating relays or diode distributors are omitted.
The B2B charger is connected in parallel to the starter battery. Its electronics recognise whether the battery is sufficiently charged and whether charging current is available; only then does the integrated voltage converter go into operation and provide the mains battery with the optimum charging voltage for it.
In order to draw a lot of current from the alternator despite the standard regulator, the B2B charger causes its voltage to drop significantly, but at around 13.0 volts it still remains above the open-circuit voltage of the starter battery - so the latter is not stressed. The system remains inactive when the engine is stationary or the starter battery is flat.
As the B2B charger does not care who generates the electricity, it can also make a lead-acid charger or the wind generator lithium-compatible. These power sources simply need to be connected to the starter battery.
Winter operation
The behaviour at low temperatures is not a decisive factor for most boaters, but is definitely worth knowing at the beginning or end of the season.
Lithium iron phosphate batteries can be used from around -5 to +50 degrees Celsius. In practice, the ability to deliver very high currents drops just above the freezing point. This effect is only temporary and does not cause any permanent damage. The danger of freezing only occurs at -20 degrees Celsius, which would destroy the battery. Lithium batteries should therefore not be left in the ship during outdoor winter storage.
The storage conditions during the season are ideal: the battery is happy with a charge level of between 40 and 70 per cent, which is ideal for someone who wants to get off the boat quickly after embarking and wants to save the shore power cable.
Behavioural research
Lithium batteries can supply and absorb any amount of current without the voltage fluctuating noticeably, and there is no familiar flickering of the lights when the cooling cap is switched on. However, this also means that in the event of a fault, the current would only be limited by the cable cross-sections. The first fuse must always be connected directly to the battery. If you skimp here, you risk losing your ship in the event of a short circuit. There are only two states when charging: either the maximum available charging current flows or none at all when the battery is full.
The fact that the battery can simply switch off the power if there is a risk of deep discharge may seem suspicious at first. However, this danger only exists if you do not pay any attention to the charge level - and then you are no better off with lead. What's more, there really is virtually nothing left when the forced switch-off occurs; the system does not hide half-full batteries.
COSTS
LiFe batteries cost between 900 euros and 1300 euros per 100 ampere hours of nominal capacity. When buying from the relevant online retailers, you should bear in mind the shipping costs: Lithium batteries are classed as hazardous goods and are correspondingly expensive to transport.
The only additional technology required on our sample boat was the battery-to-battery charger for around 250 euros. A further 100 euros were invested in installation materials such as cables, cable lugs and better fuse protection, and around 170 euros were added for a LiFe-compatible battery monitor.
Practice
The installed 200 ampere hours of nominal capacity result in 170 ampere hours of usable power supply, which is 80 more than the lead batteries they replaced. This increases the endurance from four to almost seven days without charging. It took six hours to fully recharge the completely empty batteries, compared to 24 hours for lead-acid batteries. Due to the charging characteristics of lithium, even short motor running times deliver a lot of power to the batteries.
As a result, it's hard to believe that on the 17-week cruise around the Baltic Sea, despite the constantly running compressor refrigerator and other consumers, shore power was only rarely necessary, if at all. Two points need to be improved: In accordance with the installation instructions, we had only connected the B2B charger to the starter battery with a fuse, without a main switch.
This has not proved to be a good idea if the boat is then left for a longer period of time. In this case, you should proceed according to the version in the diagram on the right on page 58 and switch off if the boat is left for a longer period of time. And the parameters on our older battery monitor cannot be set to lithium-compatible values; the device must be replaced with a newer one.
In the next issue, we will describe what additional safety measures are available and how to extinguish battery fires reliably.
