I Love Classes That Make You Think! Part II

We left the previous installment of I Love Classes That Make You Think! wondering how long one needs to run the motor in order to recharge batteries that have been in use while at anchor.  That actually brings up a whole other topic.  Calculating electrical consumption, battery capacities, and battery charging.  Sadly, I’m not enough of an expert to turn this into a true technical discussion, so this tip includes a few assumptions and is more of a layperson’s guide to battery usage and charging while at anchor and on charter.

Personally, I believe the first things you need to know are how many batteries, what are their sizes, and how is the system set up.  The only way to get this information on a charter is to ask the charter company.  Then, double check what you have been told.  In Part I, we ended the tip while on a 42′ Catalina during a two week charter in the Sea of Cortez.  Let’s continue that discussion and see if we can approximate how long per day to run the motor in order to charge batteries back to a usable state.

A pretty common set up on this boat is three batteries.  Two set up as two separate house banks, and one as the starter and windlass bank.  Because the starter/windlass battery isn’t used much and should charge back up pretty quickly we won’t worry about it.  The house bank(s) on the other hand are well used while at anchor.  Every boat potentially has different batteries.  In this case lets assume there are two Group 8D AGM batteries rated at 225 amp hours, giving a total of 450 amps of capacity.  Unfortunately, you should never go below 50% of maximum charge, so you have about 225 amp available to you.  We have a starting point.  Now for consumption.

Here are some common devices and their consumption requirements while at anchor for 24 hours:

  • Anchor light … 0.8 Amps … 12 hours per day … 9.6 Daily amp-hours
  • Cabin Fan (two of them) … 0.2 Amps each … 6 hours each per day … 2.4 amp-hours
  • Cabin Light (three) … 2.1 Amps each … 4 hours each per day … 25.2 amp hours
  • Fresh water pump … 6 Amps … 5 min per day … .5 amp-hours
  • Refrigerator … 5 Amps … 12 hours per day … 60 amp-hours
  • Music player … 1 Amp … 6 hours per day … 6 amp-hours
  • Monitoring a SSB … 2.5 Amps … 1 hour per day … 2.5 amp-hours

This a pretty much a bare minimum while sitting at anchor, and so far we are at 106.2 amp-hours.  Let’s assume you do some sailing during the day.  You only run the motor for a few minutes to get out of the anchorage and back, sailing for 6 hours.

  • Depth sounder … 0.2 Amps … 6 hours per day … 1.2 Daily amp-hours
  • GPS … 0.5 Amps … 6 hours per day … 3 Daily amp-hours
  • Chart Plotter … 1 Amps … 6 hours per day … 6 Daily amp-hours
  • Radar … 4 Amps … 6 hours per day … 24 Daily amp-hours
  • Speed sensor … 0.1 Amps … 6 hours per day … 0.6 Daily amp-hours
  • Wind indicator … 0.8 Amps … 6 hours per day … 4.8 Daily amp-hours

This is another 39.6 amp-hours, making our total so far 145.8.

Now let’s add in a few luxury items.

  • TV … 3.5 Amps … 2 hours per day … 7 Daily amp-hours
  • Electric Head … 40 Amps … .3 hours per day … 12 Daily amp-hours
  • Microwave … 100 Amps … 6 minutes per day … 10 Daily amp-hours
  • Cell Phone … 2 Amps … 4 hours per day … 8 Daily amp-hours

Another 37 amp-hours, for a total of 182.8 amp-hours consumed over a 24 hour period.  This is well within our usable 225 Amp battery capacity!

Now all we have to do is charge the battery back up.  Based on our scenario, you are at about a little over 50% charge.  If you have an alternator rated at 120 Amp, that is the most output you are going to get.  With an alternator the maximum output is only achieved when the alternator is turning about 4,000 to 6,000 PPM.  If you are idling the motor at 700 RPM, the alternator is probably turning about 1,400 RPM, so you aren’t getting the full 120 amps.  At idle you might only be getting 40 to 80 Amps output from the alternator.  Let’s assume 60 amps.  You will get that output until the battery is about 75% charged.  After using 182.0 Amps of 450 total, it you will need to “add” back about 70 Amps to get to 75%.  Charging at 60 Amps, 70 minutes will get you there.  Now for the challenge.  At 75% capacity, the regulator is going to choke back the flow to the battery to the point it will take about 3 more hours to reach a full charge.  I think we have an answer.  To replace the 182 Amps used over 24 hours is going to require about 4 hours of charging.

Looking back to Part I.  Remember to add 4 hours of engine time to your fuel calculations.  At .55 GPH, that’s 2.2 gallons of fuel per day.

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