How To Efficiently Charge Batteries On-Board Your Boat

When away from shore, being able to charge batteries quickly and efficiently becomes more important so as to not waste fuel and to limit the time charging. Boat manufacturers will often only install sufficient batteries and charging for inshore use, where optimising battery charging is not a priority. When considering offshore sailing and passage making, upgrading your charging system is likely to halve the amount of time required to charge your batteries making charging less of a chore.

Cristec YPOWER+ Battery Charger

How do you work out what is your optimised charge demand from the battery bank? Most battery manufactures will tell you this in their specification document but as a general guide for conventional (Lead Acid, AGM, Gel) batteries, divide the total AH capacity by 5 and you will be pretty close. A 200AH bank would therefore need a 40A charger. Lithium batteries are different, they will typically take everything you give them, and so are usually specified with a charger at 1/3 of the capacity (60A for a 180AH bank).

Charging batteries is best done with a battery charger, driven by shore power or by a generator when at sea. This is the most sophisticated method and is often the most powerful. Think of your batteries as a tank of water. Beginning to fill the empty tank will be easy but as the tank gets nearer too full, the pressure in the tank starts to increase making it harder to push the water in. In electrical terms, the increase of resistance generates heat, so the process needs to be controlled more carefully.

Charging is thus a complex process. The different stages of charging a battery are split into 3 stages: Bulk, absorption and float:

Paguro 4 SY 4KVA Generator

• In the Bulk stage, the charger delivers a maximum level of current (Amps) while the battery is accepting charge easily (from 50% to 80% charge for an AGM battery).

• During the Absorption stage, the charger delivers a fixed voltage to top up the battery (80% to 100%), while the amount of current accepted by the battery reduces down to a trickle.

• For the Float stage, the voltage is reduced, so that a small current is accepted to maintain the battery at 100% charge.

Better quality chargers will control the process better adding more stages, extending the batteries life.

A small 4KVA generator will deliver more than enough power to charge most if not all battery banks. For example, a 220A charger, which would supply a large 700AH (12V) lithium battery bank, would only draw 2,640 Watts from the generator, leaving power for water heating, water making, Starlink broadband or boiling a kettle, and supplying 220V AC powered devices.

Engine alternators are the most common source of charging when at sea. They can be inefficient and not very good at regulating heat, so consider upgrading your alternator alongside a good battery splitter to ensure all battery banks get the maximum charge. High power alternators will run cooler with better output power. In the case of lithium batteries, further regulation should be added to the charging circuit to limit output, protecting the alternator from damage from overheating.

Renewable power such as solar, wind and hydro give you supplementary charge to reduce total charging times. Although they provide relatively small amounts of power compared to a generator or an engine alternator, a continuous stream of power can have a very beneficial effect and can often be paired with high quality multistage chargers. Often, the need to recharge the battery banks using engines or generators can be reduced to once a day or less, when making water and/ or heating water for showers.

A good battery and charging system will make life onboard far less stressful giving you more time to enjoy the experience of life at sea.

Also, checkout our power auditing guide to ensure you have an adequate size battery bank on-board as well as sufficient power generation sources.

Feel free to contact us if you have any questions. Don’t forget to visit us at the Southampton Boat Show on stand J336 in Ocean Hall.