Starting in the 2026 season, SUNCE will be equipped with new auxiliary lithium-ion batteries in addition to its standard batteries (which were last fully replaced in 2025). This not only significantly increases the available battery capacity on the ship, but the combination of lithium and lead-acid batteries also compensates for the weaknesses of one battery type with the strengths of the other.
Lead-acid batteries deliver very high currents and offer good value for money in terms of current output and storage capacity. However, lead-acid batteries do not tolerate deep discharges, and—especially when they are nearly empty—they cannot handle high charging currents.
As a result, the lead-acid battery cannot fully absorb the charging current provided by the generator or engine; charging, especially at the end of the night, takes much longer than expected, and with only short periods of engine operation, the battery becomes increasingly depleted day by day.
The lithium battery, on the other hand, can handle very high charging currents and can therefore be recharged very quickly. Furthermore, it is not at all bothered by small charging and discharging cycles, making it ideal for being repeatedly charged briefly at maximum current in combination with the PV module and then discharged again. This is all the more true since it is a modern lithium iron phosphate (LiFePO4) battery.
What it does not particularly like, however, is the draw of very high currents, such as those generated when starting very energy-intensive loads.
If both battery systems are connected together, high charging currents flow entirely into the lithium-ion batteries and are then slowly transferred to the lead-acid batteries, so that short, high charging currents are fully absorbed. High discharge currents, on the other hand, are drawn briefly from the lead-acid batteries and are balanced out over the long term by the lithium-ion batteries.
Overall, this results in a much faster charging process and much better coordination with the PV modules and the generator. At the same time, the service life of the lead-acid batteries is significantly extended, and of course the total capacity is increased.
For the crew, the entire process is completely transparent; nothing needs to be switched or adjusted—the batteries coordinate automatically. The crew only notices the faster charging process and the longer service life.
Nevertheless: Extremely high currents, such as those required for the air conditioning, can still only be provided when the generator is running. Care must also continue to be taken not to deeply discharge the system; therefore, the automatic shutdown remains active when the voltage falls below the minimum level to protect the batteries.
However, a night with the heater running or a day of sailing with the autopilot should now be easily managed with a few hours of sunshine or a brief run of the engines or generator.
Further information about the system is also available here on the manufacturer’s website: