Batteries in the On-Board Electrical System: Types, Design, Charging

The battery is the most expensive single component in the onboard power system and the one where wrong decisions hurt the longest. This specialized area covers everything from the question of which technology suits your usage profile to the complete design of a battery bank, including charging infrastructure. You can jump in at any point.

Which technology suits which application

The decision between LiFePO4 and lead-acid rarely comes down to price per ampere-hour. It comes down to three characteristics that differ widely in the datasheet: usable depth of discharge, charge acceptance, and weight.

A lead-acid battery can be discharged by about half in continuous operation before its lifespan noticeably suffers. So, from 200 Ah nominal capacity, only about 100 Ah remain usable. A LiFePO4 cell tolerates 80 to 90 percent depth of discharge without significant losses. From 200 Ah, this becomes about 170 Ah. For the same usable capacity, you need about double the nominal capacity with lead-acid, about four times the weight, and significantly more storage space.

The second difference is charge acceptance. Lead-acid only accepts charging current to a limited extent and spends the last 20 percent in a long absorption phase. If you have two engine hours daily, a lead-acid battery bank is practically never fully charged. LiFePO4 charges with full current until shortly before full and is ready to accept charge again in a fraction of the time. For cruising with short engine phases, this is often the decisive argument.

What still speaks in favor of lead-acid: purchase price, uncritical behavior in cold temperatures, and the fact that an existing charging infrastructure continues to operate without modification. For a long-term berth holder with shore power, low consumption, and a tight budget, AGM is still a reasonable choice.

Correctly dimensioning capacity

The dimensioning always follows the same sequence. Anyone who skips a step regularly ends up with a battery bank that is too small.

1. Create a consumer list. Every consumer with current consumption in amperes and daily runtime in hours. The product gives the daily consumption in ampere-hours.
2. Don't forget quiescent current. Gas detectors, alarm systems, onboard computers in standby, and displays run 24 hours. In practice, this is often 10 to 15 Ah per day, which is more than all the lighting.
3. Define autonomy days. How long should the vessel operate without shore power and without engine running? Two to three days is a common approach.
4. Convert to usable capacity. The requirement divided by the permissible depth of discharge of the selected technology gives the nominal capacity.
5. Add reserve. 10 to 20 percent for aging, cold, and consumers that may be added later.

Example calculation: Cruising sailor, 12 V

Typical values for a 38-foot vessel in summer operation. The refrigerator accounts for about half—this is the norm, not the exception.

With three days of autonomy, this results in a demand of 270 Ah. With a 10 percent reserve, approximately 300 Ah of usable capacity should be assumed. This leads to:

• LiFePO4 at 80 percent depth of discharge: 300 / 0.8 = 375 Ah nominal capacity, approx. 45 kg
• AGM at 50 percent depth of discharge: 300 / 0.5 = 600 Ah nominal capacity, approx. 180 kg

NoteThe 180 kg of AGM is not just a weight problem. It also requires space, standard compliant fastening, and an alternator that can recharge such a large battery bank within a reasonable time.

Direct comparison of technologies

Typical ranges for marine applications at 25 °C. Cycle counts are valid up to 80 percent residual capacity and always only in conjunction with the depth of discharge to which they refer — a cycle specification without depth of discharge provides no information. Manufacturer specifications take precedence in individual cases.

The last line is often underestimated: The flat discharge curve of LiFePO4 makes a voltage display practically useless. Without an ampere-hour counter with a shunt, you won't know your lithium battery's state of charge.

Charging and charging infrastructure

The most common mistake when changing technology is just swapping the battery. A battery bank made of LiFePO4 cells places different demands on every charging source than lead-acid, and it behaves fundamentally differently in case of a fault: If the BMS shuts down, the battery suddenly disappears from the onboard power system.

This particularly affects the alternator. A LiFePO4 battery bank accepts current until the regulator limits it—a standard alternator will then run at its limit permanently and overheat. And if the BMS disconnects under load, a load dump occurs, whose voltage spike can destroy the rectifier. Both can be managed with an external charge regulator, an alternator temperature sensor, and a buffer battery or a charge converter, but it must be planned.

WarningDo not charge LiFePO4 in freezing temperatures without an enabling cell heater. Below approximately 0 °C, metallic lithium is deposited on the anode instead of being intercalated. The loss of capacity is permanent and not outwardly recognizable. Manufacturer specifications for the lower charging limit range from −5 to +5 °C — adhere to the value specified by your cell manufacturer. Discharging, however, is permissible in sub-zero temperatures.

What typically needs to be replaced or supplemented during the changeover: charge controller or charger with lithium characteristic, external regulator for the alternator or a charge converter, solar charge controller with appropriate charging end voltage, a battery monitor with shunt, and the fusing, which must be designed for the significantly higher short-circuit currents.

Typical errors

• Capacity based on gut feeling. Without a consumer list, the refrigerator is almost always underestimated, and quiescent current is completely forgotten.
• Old and new batteries in parallel. The weaker one pulls the stronger one down to its level. A battery bank is always replaced completely.
• Different technologies in one battery bank. Different charging voltages and internal resistances mean that one of them is permanently handled incorrectly.
• Shunt in the wrong branch. If it is not positioned so that every current really flows through it, the monitor will consistently measure inaccurately.
• Fusing not adapted. LiFePO4 delivers significantly higher short-circuit currents than lead-acid. Fuses and disconnect switches must match this.
• Unequal cable lengths in parallel connection. The battery with the shorter path is subjected to greater stress and ages faster.

Standards and regulations

The decisive standard for electrical installations on recreational craft is ISO 13297:2020. Since this edition, it covers both: DC systems up to 50 V nominal voltage and single-phase AC systems up to 250 V. The former ISO 10133, which was responsible for the DC side, was withdrawn at the end of 2020 and incorporated into ISO 13297 — anyone still referring to ISO 10133 is working with a withdrawn document.

Relevant for battery installation are primarily: secure fastening against slipping and capsizing, protection of terminals against accidental contact and short circuits by tools, ventilation of the battery compartment for gassing lead-acid batteries, and overcurrent protection as close as possible to the positive terminal.

The main fuse belongs at the battery, not at the switch panel. It protects the line between the battery and the distribution—and precisely this line is the problem in the event of a short circuit. For lithium systems, the cell manufacturer's and BMS's specifications also apply, which are regularly stricter than the standard. In North America, ABYC E-13 additionally regulates the requirements for lithium systems and explicitly demands a BMS that keeps the battery within its safe operating range and disconnects it otherwise.