We use cookies to give you the best possible experience of our website. If you continue, we'll assume you're happy for your web browser to receive all cookies from our website. See our cookie policy for more information on cookies and how to manage them.


Boat battery charging and maintenance guide

For maximising the lifespan of your marine battery, regular maintenance is essential. Inspection, testing and charging are all vital elements of this. It’s all about avoiding the need to purchase a new battery earlier than necessary - or, even worse, a complete outage when far from shore. With these aims in mind, read on to discover the essentials of keeping your battery going…

Follow the manufacturer’s guidance

Your battery requirements are governed by your specific demand requirements: i.e. the level of energy your boat needs to feed its electrical systems. Coupled with this is the capacity element: making sure you are able to store enough energy to meet your usage needs.

So your on-board battery set-up will be determined by a combination of things: notably, how power-hungry your vessel happens to be, how you use your boat - as well as the recommendations of the boat manufacturer. Almost all boat batteries fall into one of two categories: flooded cell batteries (including marine deep cycle boat batteries) and Valve-Regulated Lead Acid (VRLA) batteries. The majority of points covered in this guide apply to both types of battery - unless otherwise stated.

You should also follow the specific recommendations of the battery manufacturer, especially when it comes to optimum charge levels and how often to test your battery.


  • With reference to both the battery instructions and the charger instructions, ensure that the charger voltage settings are correct before charging.
  • Avoid overcharging, as this can cause water breakdown, overheating and premature battery aging.
  • Avoid undercharging, as this can lead to stratification (a buildup of acid) and can also wear out your battery
  • Charge in a dry, well-ventilated area, away from sparks or flames.

Visually inspecting your battery

It is usually recommended that the following checks are carried out at least once a month:

  • Inspect the container for cracks.
  • Ensure the top of the battery, all posts, terminals and connections are free from corrosion, dirt, dust and liquid. A clean terminal should have a metallic shine. The top of the battery can be cleaned using a cloth and solution of water and baking soda. Use a clamp cleaner on the terminals.
  • Replace any broken or frayed cables.
  • If the container is cracked or if the battery is leaking, it needs to be repaired or replaced.
  • Ensure all wiring connections are tightened to enable good contact with the terminals. But be careful to avoid overtightening.

Testing your battery

Specific gravity testing (this applies to flooded batteries only)

This involves measuring the specific gravity of the battery electrolyte for each of your battery’s cells. For this, you will require a hydrometer. Following the specific instructions for hydrometer usage, take a reading for the electrolyte for each cell and record it.

If the reading for a particular cell is lower than the manufacturer's specification, it may be that stratification has occurred - i.e. a buildup of acid towards the top of the battery. Equalisation is a controlled overcharge of your battery, and most chargers have an equalisation mode. Follow the battery manufacturer’s recommendations for equalisation and then repeat the specific gravity testing procedure using the hydrometer.

If the reading remains low, it’s likely that the battery is either reaching the end of its lifespan - or else a weak cell is developing. Either way, you should refer to your supplier for further testing and/or for replacement.

Open-circuit voltage testing

This can be used to test both types of battery and you will require a DC voltmeter to perform the test. To ensure accuracy, it is usually recommended that the battery remains idle for at least 6 hours prior to testing. Having disconnected all loads from the battery, take a reading using your voltmeter. If the battery registers below manufacturer’s recommended minimum levels, this is likely to be a sign either of a bad cell or of potential end-stage deterioration. Again, refer to your supplier for repair or replacement.   

Watering (applies to flooded batteries only)

To ensure optimum performance and longevity, a flooded battery should have water added at the right time. The manufacturer will issue specific guidance on this, but most manufacturers advise that the procedure should be carried out after a full charge.

  • Check water levels once a month - or until you get an idea as to how quickly water levels tend to deplete (this can vary from boat to boat depending on factors such as climate and usage levels). There should always be enough water within the battery to cover the plates.
  • Only use distilled or deionised water.
  • Topping up beyond the fill mark (i.e. right up to the cap) can cause overflow, loss of capacity and corrosion.
  • Ensure vent caps are properly tightened after replacing them.


Any views or opinions expressed in this briefing are for guidance only and are not intended as a substitute for appropriate professional guidance. We have taken all reasonable steps to ensure the information contained herein is accurate at the time of writing but it should not be regarded as a complete or authoritative statement of law.