How long should engine run to charge back battery ?

Benny17441 · Aug 4, 2013

More than likely you are right. As a battery approaches full charge the rate of charge acceptance declines. It is the last 5% of charge that will take the longest. A 15 minute ride will not do it. Full time cruisers on the go will charge their batteries just to about 90% to avoid the hours that it takes to get them close to 100%. To satisfy yourself of the state of charge of the batteries go ahead and disconnect them and after 2 hours of rest take a voltage reading. 12.67V is near 100% and 12.58V is near 90% and 12.40V is 75%. I had a friend that would take a transient slip at a marina once a month so that he could carge his batteries full. He would take advantage of having water and do maintenance and any work that required power tools while the battries were charging.

Maine Sail · Aug 5, 2013

Delawaredave said:
Hunter 27 with Yanmar 1GM. 2 batteries, AB switch. Believe standard alternator (35 amp ?). Charging system seems OK, alternator belt tight.

We are on a mooring - so no 110V available.

After using battery to start engine, any idea how long the engine needs to run to "put back" energy drawn to start engine ?

Thanks !

Delawaredave said:
After using battery to start engine, any idea how long the engine needs to run to "put back" energy drawn to start engine ?

If it is just 15 minutes in/out and an engine start you should do fine. The starting draws perhaps .2 - .5 Ah's from the bank. However it is when you start drawing deeper into the battery for other items that you run into issues.

Delawaredave said:
When engine is running (about 2400 rpm), meter reads 13.5 volts charging the battery (I expected closer to 14 to 14.5 volts).

This could be indicative of many things:

#1 The bank is still in "bulk mode" and the alt has not yet been able to bring it into the 14's..

#2 The wiring has some pretty bad voltage drop between the back of the alternator and the battery bank.

#3 The alt has gotten hot and reduced voltage. Many Hitachi's do this.

#4 Your volt meter is measuring the voltage drop in the wiring..

Rx = Measure the voltage at the back of the alternator then at the battery bank with a digital volt meter to see if this is a voltage drop issue or a low state of charge issue. Voltage at the back of the alt and battery bank should be within +/- 0.1V.

Delawaredave said:
Usual day is start engine, motor 15 minutes, sail, run speakers and charge cell phone for 1-2 hours, start engine and motor 15 minutes back.

Doing this routine, it seems like it is a "net discharge" - I don't think the batteries are getting charged back to original state.

What about depth, wind, GPS, VHF, bilge pumps etc. etc..?? The bottom line is this:

#1 You have a 35A alt capable of about 20A when hot. This is 20Ah per hour of run time. This is a best case scenario, if the batteries will take it depending upon SOC. You are running for half that so figure on a max potential into the bank of 10Ah's..

#2 Current flowing into the bank is greatly dependent upon voltage at the battery terminals. At 13.5V you will be lucky to even get 10A into the bank. So in reality you could be seeing half of that or 5Ah's of recharge.

#3 The simple answer is that yes you could be seeing a net drain based on your run times.

The easiest fix for this is a solar panel and controller to charge the batteries when you are not there... The more in-depth fixes include correcting your voltage issue or simply running the motor a LOT longer..... This is the typical conundrum for mooring sailed boats and why batteries just don't last very long unless you take some measures to prevent net declines in state of charge...

How long should engine run to charge back battery ?

Delawaredave

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Benny17441

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Maine Sail

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