Question: I’m trying to figure out the best way to reduce engine running time while recharging my batteries on my cruising sailboat. I’ve been told that a high-output alternator like the Balmar unit in the photo here is the best choice for this. But I’ve also been told that just because I have a high-output alternator doesn't mean that my run time will actually be reduced. What’s the story here?
Answer: The short answer is that it depends. Battery recharge time is limited by a factor called "absorption rate." That is, a battery can only accept a charge that is so high. The internal electrical resistance of the battery controls the actual rate at which the battery can accept a charge. Historically, with conventional flooded-cell lead-acid batteries, we typically considered absorption rates somewhere in the area of 25-30% of a battery’s amp-hour rating as the maximum amperage acceptance rate for recharge. Today however, with absorbed glass mat (AGM) batteries and thin-plate pure lead batteries readily available to the marine market, this factor has changed considerably. These types of batteries have a comparatively high acceptance rate approaching 40% of their amp-hour capacity. Simply put, this means that batteries in this category can be charged at a higher rate and will in effect recharge more quickly.
So, depending upon what type of batteries your boat has, a higher output alternator might just help reduce charge time. The truth is, however, that before you spend $1000 or so for a new high-output alternator and regulator combination, you may discover that all you really need to do is upgrade your boat’s batteries to AGM and use your existing alternator, perhaps with an upgrade to an external, programmable voltage regulator.