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post #29 of Old 12-05-2012
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Re: .

Originally Posted by Maine Sail View Post
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.

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...

To complete that good explanation one question: Assuming (like you did) a 100A alternator putting out about 80A with a 400Ah bank how much time would the engine have to be running to go from 50% to 80% and 85% charge on the Bank?

About one hour and a half for a 80% charge? And if so it is better to let it fall to 50% before charging an hour and a half or just let it go to 65% and charge it for 45 minutes? What is more efficient regarding charging and what is better for the battery life?



Last edited by PCP; 12-05-2012 at 11:57 AM.
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