Basic Battery Question - measuring capacity - Page 2 - SailNet Community
Old 06-16-2009
Senior Member

Join Date: May 2007
Posts: 558
Thanks: 3
Thanked 3 Times in 3 Posts
Rep Power: 10

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.
SteveInMD is offline

Senior Member

Join Date: Feb 2008
Posts: 164
Thanks: 1
Thanked 0 Times in 0 Posts
Rep Power: 10

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.

Preserved Killick

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

Seagrass
Boothbay Harbor, ME
Alberg 30:
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
preservedkillick is offline
Old 06-17-2009
Telstar 28

Join Date: Mar 2006
Location: New England
Posts: 43,290
Thanks: 0
Thanked 18 Times in 14 Posts
Rep Power: 15

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.

Quote:
Originally Posted by SteveInMD View Post
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.

Sailingdog

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

Telstar 28
New England

You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
.

sailingdog is offline

Old 06-17-2009
Senior Member

Join Date: May 2007
Posts: 558
Thanks: 3
Thanked 3 Times in 3 Posts
Rep Power: 10

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.
SteveInMD is offline
Old 06-17-2009
Senior Member

Join Date: May 2007
Posts: 558
Thanks: 3
Thanked 3 Times in 3 Posts
Rep Power: 10

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.

Quote:
Originally Posted by sailingdog View Post
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.
SteveInMD is offline
Old 06-17-2009
Member

Join Date: Jul 2008
Posts: 45
Thanks: 0
Thanked 0 Times in 0 Posts
Rep Power: 0

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 05:30 PM.
waltsn is offline

Message:
Options

By choosing to post the reply above you agree to the rules you agreed to when joining Sailnet.

## Register Now

In order to be able to post messages on the SailNet Community forums, you must first register.
Please note: After entering 3 characters a list of Usernames already in use will appear and the list will disappear once a valid Username is entered.

User Name:
OR

## Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

 Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)