Basic Battery Question - measuring capacity - Page 2 - SailNet Community
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post #11 of 16 Old 06-16-2009
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Here's a quote from the original post...."can I accurately measure how many amps a battery has left?"

This is a question of charge state. You need a battery monitor to determine how many amps a battery has left.
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post #12 of 16 Old 06-17-2009 Thread Starter
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Is there a way to figure out how many amps the 35 amp alternator is putting back in when the engine is running?? Typically you get less than the rated amps right? I'm likely getting 25 amps? Does this mean that one hour of engine running puts 25 amps back in?

I'd like to compare the benefits of running the motor against a small solar panel.

Would I gain more amps for my money by upgrading my alternator to a 70 amp model? I've got a 3 cylinder 22 HP Vetus diesel. I bet the belt is just 3/8 so I've got limited possibilities in alternator sizes.

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post #13 of 16 Old 06-17-2009
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Please don't confuse AMPS with AMP-HOURS.

Battery capacity is generally measured in AMP-HOURS—ie, how many amps * hours it has left. AMPS is a unit of current and does nothing to tell you about battery capacity. However, the actual number of amp-hours it has left is very dependent on the rate at which you draw the amps from it, due to things like the Peukert factor. For example, a battery that has 200 amp-hours of capacity might run for an hour with an 100 amp load, or 100 hours under a load of TWO amps.


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Here's a quote from the original post...."can I accurately measure how many amps a battery has left?"

This is a question of charge state. You need a battery monitor to determine how many amps a battery has left.

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post #14 of 16 Old 06-17-2009
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You will only get the full 35 amps under optimal conditions. You could measure how many amps the alternator is putting out using a clamp on true-RMS ammeter. The problem is there are so many variables here that it would take a lot of work to determine what was happening over time. RPMs, temperature, charge state, other power uses are going to affect what happening so just taking a few readings will have very limited usefulness. A battery monitoring system does measure amps, but does so continuously. It gives you a result over time creating a total amps in vs total amps out summary for you. The systems work very well. I assume they must use a model to determine loss since not everything you put into a battery comes back out. The loss is mostly due to heat. If you can measure all this and account for loss and still come up with a number that means anything you are better man than I.
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post #15 of 16 Old 06-17-2009
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Of course. You need to account for current and time (amp hours) to describe a "volume" of electricity. I was focusing on the concepts preservedkillick was asking about without getting bogged down in terminology too much. I was going along with his concept of "how many amps a battery had left". Another way to answer his question is that measuring amps (instantaneous current) is of very limited use. You really need to measure amp hours, analogous to the electrical meter on your home, to get useful information.


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Please don't confuse AMPS with AMP-HOURS.

Battery capacity is generally measured in AMP-HOURS—ie, how many amps * hours it has left. AMPS is a unit of current and does nothing to tell you about battery capacity. However, the actual number of amp-hours it has left is very dependent on the rate at which you draw the amps from it, due to things like the Peukert factor. For example, a battery that has 200 amp-hours of capacity might run for an hour with an 100 amp load, or 100 hours under a load of TWO amps.
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post #16 of 16 Old 06-17-2009
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I understand amps hours in is "close enough" to amps hours out to not worry about it.

This is not true of power in vs power out since the amp hours in are generally at a higher voltage than the amp hours out.

(edit - the "close enough" doesnt take into account the rate of charging - ie, Puekert stuff - just a general reason why the battery will disipate some heat when being charged but that the amp hours out can be roughly compared to amp hours in)

My wet cell battery bank is in it third year and I think its dying and Im wondering how others have determined when its time to declare them dead. I have a charge meter and using a three stage charger, it will say the battery is full (ie, the charger resets to 100% full). But after using less then 10 amphour (200 amp hour battery), and with say a 1 amp load, the DC voltage will drop to say 12.0 volts. Seem way too low.

Ive never equalized the batteries, I wonder how much they would recover from this??.

Last edited by waltsn; 06-17-2009 at 04:30 PM.
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