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post #14 of Old 12-02-2012
Maine Sail
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Re: .

Originally Posted by floridajaxsailor View Post
it would seem silly to run the motor more than say 10 minutes- I find mine charge in far less time but 80 percent of the people you talk to will tell you to keep the engine running for half an hour which is ridiculous
& a waste of fuel

Originally Posted by 73Morgan View Post
10 minutes will give the batteries a decent charge. However, there are 2 things you should consider, battery health and engine health. Batteries last longer when maintained at full charge. Cylinder walls glaze over on short run times, which results in loss of compression. The engine needs to reach full operating temperatures for a decent amount of time for best health.

I would definitely agree that using the boat engine to charge your batteries is not a good idea.
Wow, this tread is a prime example of why people like me replace sooooo many batteries on boats.

10 minutes does squat for charging a house bank of batteries.... This is simple math & physics.. Ten minutes run time won't even burn off start up condensation in the motor. Here in the cold waters of Maine 10 minutes won't even bring most diesels up to operating temp.

Flooded batteries take 10+ hours to charge to full even with a large charge source because of battery acceptance rates as the bank hits absorption voltage.

A 400Ah bank at 50% state of charge needs roughly 240Ah's returned to it to be full due to charge inefficiency.

With a 100A alternator it will really put out about 80A so roughly 1.3Ah per minute at 80A. Easy math shows that 1.3Ah X 10 minutes is about 13.3Ah returned to the battery or just 5.5% of the energy need to charge this bank to full....

5.5% is a long way from a "decent charge", in 10 minutes....10 minutes of run with a 100A alt on a 400Ah bank barely gets you from 50% SOC to 53% SOC when charge inefficiencies are taken into account..

Also keep in mind that just because a "charger" has gone through bulk, absorption and is into float in 2.5 - 3 hours also means little other than the charger is poorly programmed for the bank you are charging. When a charger comes out of bulk quickly all it usually means is that your batteries are already toast and sulfated enough to build a surface charge. Sulfated batteries can take double to triple the time it takes healthy batteries to charge because the current can't get into the plates. Many "smart chargers" need to be reset or re-started in order to re-enter absorption because the "absorption" stage is "timed", but not to match your bank. Many of these chargers are actually pretty stupid and chronically under charge batteries. Some of them have absorption voltage times of just 1 hour before entering float... Ridiculous really but it is what it is..

Frankly if I am sailing and need to charge the batts I will almost always put her into gear and get a little boost.

On our own boat we have around 3500 hours on the motor and the boat was a 24/7/365 "on the hook" cruiser for 5 straight years. The boat spent perhaps 1500 of those hours charging batteries and running the engine driven refrigeration at anchor. The cylinder walls are NOT glazed, still perfectly cross hatched and she burns no oil and runs as good as new. As long as you run the engine properly when not charging at anchor or under sail these engines can be run like this without much issue at all.

When charging via alternator it is wishful thinking that you'll ever get anywhere close to full unless you spend 10+ hours running the motor like a trawler would.

This is why experienced cruisers use the "cruisers rule" and cycle the bank between 50% SOC and 80-85% SOC. Trying to charge much above 85% SOC is simply a waste of fuel due to declining battery acceptance...

I see no problem with the Hinckley running the motor in that manner. If it were me I would have thrown it into gear and pushed her up her bow wave a tad bit more to keep the prop loaded and keep the engine up on operating temp...

-Maine Sail / CS-36T

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Last edited by Maine Sail; 12-02-2012 at 05:49 PM.
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