Lithium has recently experienced a boom in popularity. Until a few years ago, the metal with the lowest density of all solid materials was more likely to be used in the sinfully expensive rims of Formula 1 cars or as an antidepressant; even the band Nirvana sang about the metal. But then came the new battery technology, in which the material can fully utilise its properties - to the benefit of on-board technology too.
When it comes to batteries, even on yachts, lithium has become the measure of all things when it comes to storing electricity. Lead can also do this, albeit in a slightly different way. The heavy metal has been used as the basis for rechargeable batteries for more than a hundred years. However, it is heavy, toxic and is now out of fashion. But it is cheap, costing only around a tenth of its light counterpart. What's more, it can be recycled again and again.
The best of two worlds
The battery technologies complement each other very well from a technical point of view: lithium batteries like to absorb a lot of charge quickly, maintain an almost constant voltage until they are empty, and they really like to be charged and discharged frequently; they are very cycle-resistant. However, delivering large amounts of current quickly is not their favourite discipline, nor is remaining fully charged and unused for long periods of time. They also need to be protected against too much and too little voltage by a battery management system (BMS).
This BMS disconnects the batteries from the mains when they are discharged and reach a defined, low voltage. The BMS also interrupts if the charging voltage is too high in order to protect the battery. Lithium batteries cannot be used without a functioning BMS. In fact, they are not entirely safe, as they can catch fire in the worst case scenario.
Lead batteries, on the other hand, take up their charge slowly and can release it again quite quickly. The voltage always drops with the charge, they like to be stored as fully charged as possible and many cycles are not their strong point. On the other hand, these heavy energy storage devices are quite inexpensive and do not require electronic protection.
Sensible combination
As the technologies complement each other very well, the idea of combining them on board is obvious. The cheap and safe reserve is the lead-acid battery, which is always fully charged and therefore has a long service life and only comes into play when the required current is particularly high or the lithium batteries are empty. These in turn are rather small in size and cover the usual demand day in, day out in many cycles, if possible without touching the lead storage. In this way, both batteries operate in their optimum operating states: The heavy metal battery usually remains fully charged without being damaged, while the lithium accumulator is fully utilised and thus operates in the mode that makes the most sense for it. Nevertheless, the full capacity is always available, even if it is not possible to charge the battery sufficiently during a longer period at anchor.
The overall system is cheaper than a pure lithium set-up and more durable than a battery bank consisting only of lead-acid batteries. This is because the lead batteries are always kept full by their light metal partners, which prevents sulphation and increases the service life. In everyday life on board, it looks something like this: All batteries are full. If a charge is now removed, the lithium battery is the first to kick in. When charging current is available again and the lead-acid battery is still 100 per cent full, the lithium battery is recharged first. Without charging current, it keeps the vehicle electrical system permanently at 12.8 volts and thus the lead-acid battery at full charge until it is empty and its BMS disconnects from the vehicle electrical system. If further current is then drawn, the lead-acid battery kicks in.
Charging cycles under laboratory conditions?
The user does not notice any of this or, if at all, only a slight drop in the on-board voltage. If the battery is then recharged, the lead-acid battery is charged first. Only when this is sufficiently charged, i.e. a certain voltage is reached, does the lithium counterpart begin to charge. Finally, both batteries are full and ready for the next cycle.
In practice, such cycles are very rarely achieved. You are connected to shore power and the batteries are full. After casting off, the engine provides a constant charging current. When you stop for a swim or at anchor, all consumers suck on the battery for a few hours, only to be recharged by the engine when you continue your journey or the next morning, and so on. The lithium battery copes very well with these short cycles. Due to the high charging current it can absorb, even short engine running times are often enough to fully charge it. The same applies to a generator. The lead-acid battery is only charged during longer journeys or several days at anchor.
Proven technology
The motorhome manufacturer Hymer has also recognised the advantages of such a hybrid system and has developed its own system together with Bosch and the company BOS. The Neu-Ulm-based company has now sold over 25,000 of its LE 300 lithium hybrid systems, the majority of which are used in motorhomes.
The Bavarians now want to enter the water sports market and are working together with the company Ferropilot, which is organising sales. The fully encapsulated battery pack, developed solely for use in a hybrid system, comes with its own BMS and can be easily connected to an existing battery bank.
An additional display provides information about the current, voltage and charge status. Initially, the capacity of the lead-acid battery must be programmed in, as up to 24 such battery packs can be connected in total. One of them has a nominal capacity of 25 ampere hours - or 300 watt hours at 12 volts, hence the name LE 300. The BMS regulates the charging current from the lead-acid battery to the LE 300 in line with its voltage, which protects the alternator. This is because the high possible charging currents of lithium batteries can otherwise push them to their performance limits if they mercilessly demand power from the dynamo. At low speeds and therefore low cooling, these can be damaged.
Good old lead batteries as a safe bank
Not so with the LE 300: "If the requested power is too high, the voltage of the alternator drops and therefore also in the lead-acid battery. The LE 300 recognises this and reduces the power consumption. This means that nothing can happen," explains Benjamin Seckinger, CEO of BOS-AG. This ensures maximum operational safety. Thanks to the modular design, each module can be removed individually from the package in the event of a fault without impairing its function.
As already described, lithium batteries require a BMS to control them. This is an electronic component. However, many owners want to minimise the amount of electronics in important components such as the batteries. In a hybrid system, the good old lead batteries are therefore used, which function simply and without electronics, as a safe bank, so to speak.
Then there are the lithium batteries with all their advantages such as fast charging and enormous cycle stability. This means that you have the simplicity and therefore the operational reliability of conventional batteries on the one hand and the convenience of modern batteries on the other, which also ensure a longer service life for lead batteries. A sensible and effective combination of both battery worlds.
For whom is it worthwhile?
The calculation example shows that a new on-board system made of lithium is hardly more expensive. The only remaining argument is the additional operational reliability of the lead batteries. However, if you only want to expand your existing lead bank on board a little and also want to extend its service life, the lithium system as a hybrid is a good choice. However, the LE 300 currently costs around twice as much per kilowatt hour as a lithium battery from Liontron, for example. On the other hand, you save on the purchase of lead batteries, if available.
What does it cost?
AGM batteries can deliver around half of their nominal capacity without being damaged. Lithium batteries can even manage 80 per cent. To have a usable capacity of around 150 Ah, you therefore need a 200 Ah LiFePo battery. This currently costs 1600 euros. As a pure AGM pack, you therefore need 320 Ah nominal capacity for the same usable capacity. Two 160 Ah batteries from Victron together currently cost 850 euros. However, the lithium battery can manage around four times as many cycles - so the light metal battery costs less per cycle. An example of a combination of lead and lithium with high cycle stability: 2 LE 300 batteries for approx. 400 euros each with 20 Ah usable capacity plus an AGM battery with 110 Ah usable capacity for approx. 525 euros. You therefore get 150 Ah of usable capacity for approx. 1323 euros.
