At what engine speed should a boat ideally run for optimal battery charging?

For optimal battery charging, an engine speed of at least 1500 rpm is ideal because many marine alternators are designed to produce maximum output at this range. At lower RPMs, the alternator may not generate sufficient voltage or output to effectively charge the batteries, especially under load.

Running the engine at or above 1500 rpm ensures that the alternator's magnetic field is strong enough to produce the necessary electrical current, allowing batteries to charge efficiently. Additionally, operating in this range strikes a balance between providing enough charging capability without over-revving the engine unnecessarily, which can lead to excessive fuel consumption or wear.

While some charging might occur at speeds lower than 1500 rpm, it is often not as effective. Specifically, charging efficiency can significantly dip below this threshold, making it less reliable for ensuring batteries are adequately replenished.