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Old 11-21-2012, 12:12 PM   #14
Delfin
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Join Date: Jan 2010
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Quote:
Originally Posted by Andy G View Post
sunchaser;"On many newer boats the alternators are 24V and 175 to 200 amps. This is great for house battery charging while underway. Many 24V boats have 12V electronics to avoid the 24V premium."

I always understood that batteries liked a 'gentler' charging rate than the 175/200amp units. However I happily admit to a high degree of ignorance when it comes to electrics.
Andy, the amount of current a battery can accept is a function of its type and size. AGM have a higher acceptance rate than wet cell, so can benefit from higher initial charge rates without damage. Some of the very new technology batteries that have corresponding price tags will accept a lot of current right up to the point where they are almost fully charged. And, the larger the bank, the more current can be accepted by whatever type battery you have. Large charging capacities are only useful for a relatively short period of time since for most batteries, after 30 minutes or so, the acceptance rate drops quite a bit, meaning the higher capacity of the charging system isn't needed. Modern 3 stage regulators use temperature sensing at the battery to detect when too much current is being supplied during the bulk charging phase.

The lesson is that simply putting a really big charging system online doesn't hurt the batteries if properly regulated, but may not be of much use most of the time as the acceptance rate of most types of batteries drops as the voltage of the bank goes up.

By way of reference, I put a 1380 amp hour 24v house bank on board and between the genset and the alternator can get the AGMs to accept around a peak of 200 amps, slowly tapering off after 45 minutes or so to below the capacity of just the alternator, so if I have also spun up the genset to accelerate charging, it goes off. The next 4 hours of run time on the alternator brings the batteries up to nearly 100%. In other words, I have greater capacity to charge than I can use for all but around 20% of the time.
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