Cruisers Forum
 

Go Back   Cruisers & Sailing Forums > Engineering & Systems > Electrical: Batteries, Generators & Solar
Cruiser Wiki Click Here to Login
Register Vendors FAQ Community Calendar Today's Posts Log in

Reply
  This discussion is proudly sponsored by:
Please support our sponsors and let them know you heard about their products on Cruisers Forums. Advertise Here
 
Thread Tools Search this Thread Rate Thread Display Modes
Old 08-07-2018, 05:08   #1
Registered User
 
rgleason's Avatar

Join Date: Mar 2012
Location: Boston, MA
Boat: 1981 Bristol 32 Sloop
Posts: 17,778
Images: 2
Measure present Full Battery Capacity (as it declines)

How to Determine the Current Capacity (Size) of Batteries in AH or %Capacity
Why is this needed? For the Ah Counter parameter for battery capacity.

From an earlier thread (involving John, I think):

Put a 9 amp load for one hour, so 9 amp-hrs consumed. Read a voltage at the end of that period of say 12.63, which the Battery State of charge chart says is a state of charge of 95%, then C=9/(1-0.95) = 180 amp-hour capacity at full charge. In this case the battery is still as new.

Now use this battery for 3 years (or 3 months) and after 9 amp-hours, you read 12.42 volts, which state of charge chart says 80%. C=9/(1-0.80) = 45 amp-hour capacity at full charge.

The other alternative is a 20 hour Discharge Test down to 10.5 volts by first charging up to Full then discharging it at a specified rate. Few people do this.

The 9 amps out of this old battery is much greater than the c20 rate, so redo the test 45/20=2.25 amps and you will get a better capacity.


Are there other methods?
rgleason is offline   Reply With Quote
Old 08-07-2018, 10:44   #2
cruiser

Join Date: Jan 2017
Boat: Retired from CF
Posts: 13,317
Re: Measure present Full Battery Capacity (as it declines)

Only the 20-hour test is accurate if the goal is to compare with the published when-new rating

Maine Sail gives a poor-man version on that BM howto page, if you set a benchmark at commissioning time, can then compare later long as its variables are repeated identically, give a rough idea.

But forget the voltage charts, bank SoC relative to voltage varies by

Each mfg and model, size

true resting V (48-72 hours) vs relative C rate of dis / charge

and the relationship itself changes over time as the batt ages.
john61ct is offline   Reply With Quote
Old 14-07-2018, 10:54   #3
Registered User
 
rgleason's Avatar

Join Date: Mar 2012
Location: Boston, MA
Boat: 1981 Bristol 32 Sloop
Posts: 17,778
Images: 2
Re: Measure present Full Battery Capacity (as it declines)

If the only way to measure the battery capacity is by 20hr test down to 10.5v, then how could Smartgauge possibly be accurate at this task?

What is it's methodology such that it is accurate?
rgleason is offline   Reply With Quote
Old 14-07-2018, 11:15   #4
Registered User
 
rgleason's Avatar

Join Date: Mar 2012
Location: Boston, MA
Boat: 1981 Bristol 32 Sloop
Posts: 17,778
Images: 2
Re: Measure present Full Battery Capacity (as it declines)

Isn't the partial discharge technique better than nothing? I need a practical way of resetting my Link10 capacity setting.
rgleason is offline   Reply With Quote
Old 14-07-2018, 11:18   #5
Registered User
 
rgleason's Avatar

Join Date: Mar 2012
Location: Boston, MA
Boat: 1981 Bristol 32 Sloop
Posts: 17,778
Images: 2
Re: Measure present Full Battery Capacity (as it declines)

Maybe the most practical approach is to purchase a smartgauge.
rgleason is offline   Reply With Quote
Old 14-07-2018, 13:13   #6
֍֎֍֎֍֎֍֎֍֎

Join Date: Apr 2006
Posts: 15,136
Re: Measure present Full Battery Capacity (as it declines)

There are battery testers that use the internal AC impedance of the battery to determine the actual remaining state of capacity.

https://smile.amazon.com/Clore-Autom...ery+tester+ba9

That's one of the more common cheap models. A battery distributor will have a much better one, and there are cheaper models out there as well. One alternative to trying actual discharge cycles.
hellosailor is offline   Reply With Quote
Old 14-07-2018, 13:57   #7
Registered User
 
Stu Jackson's Avatar

Join Date: Jan 2011
Location: Cowichan Bay, BC (Maple Bay Marina)
Posts: 9,737
Re: Measure present Full Battery Capacity (as it declines)

Quote:
Originally Posted by rgleason View Post
Isn't the partial discharge technique better than nothing? I need a practical way of resetting my Link10 capacity setting.

rg,


Sometimes we can tend to overthink theses issues, I know I have. This is primarily because the equipment, like my Link 2000, gives us the "cool" numerical readouts to 85 decimals places!


Either do a 20 hour test correctly, or take a "deduction" in how you operate your electrical system. Either reduce the capacity input to the Link, or just start recharging at an earlier time. One can do this [a tad more] yearly.
__________________
Stu Jackson
Catalina 34 #224 (1986) C34IA Secretary
Cowichan Bay, BC, SR/FK, M25, Rocna 10 (22#) (NZ model)
Stu Jackson is offline   Reply With Quote
Old 14-07-2018, 14:30   #8
cruiser

Join Date: Jan 2017
Boat: Retired from CF
Posts: 13,317
Re: Measure present Full Battery Capacity (as it declines)

Quote:
Originally Posted by rgleason View Post
If the only way to measure the battery capacity is by 20hr test down to 10.5v, then how could Smartgauge possibly be accurate at this task?

What is it's methodology such that it is accurate?
It is not **that** precisely accurate.

Just more so than the rest.

Gibbo's sensor readings + database + algorithms are all proprietary, I've seen high-level screeds full of speculation how it may work but all gobbledygook to me.
john61ct is offline   Reply With Quote
Old 14-07-2018, 14:33   #9
cruiser

Join Date: Jan 2017
Boat: Retired from CF
Posts: 13,317
Re: Measure present Full Battery Capacity (as it declines)

Quote:
Originally Posted by rgleason View Post
Isn't the partial discharge technique better than nothing? I need a practical way of resetting my Link10 capacity setting.
If you start with a standardized benchmark when new you can guesstimate.

Otherwise just make sure it is set lower than reality, it will catch up soon enough.
john61ct is offline   Reply With Quote
Old 14-07-2018, 14:36   #10
cruiser

Join Date: Jan 2017
Boat: Retired from CF
Posts: 13,317
Re: Measure present Full Battery Capacity (as it declines)

Quote:
Originally Posted by rgleason View Post
Maybe the most practical approach is to purchase a smartgauge.
Yes, if the cost seems worth it **to you**, once it has settled down finished its learning, use that to calibrate the Link.

But with anything use endAmps to set a benchmark for 100% SoC, whatever you choose, but be consistent across the board.
john61ct is offline   Reply With Quote
Old 14-07-2018, 14:40   #11
Registered User

Join Date: Jun 2009
Location: BVI
Boat: Leopard 40 (new model)
Posts: 1,386
Re: Measure present Full Battery Capacity (as it declines)

Quote:
Originally Posted by hellosailor View Post
There are battery testers that use the internal AC impedance of the battery to determine the actual remaining state of capacity.

https://smile.amazon.com/Clore-Autom...ery+tester+ba9

That's one of the more common cheap models. A battery distributor will have a much better one, and there are cheaper models out there as well. One alternative to trying actual discharge cycles.
Those gizmos are good to measure CCA (cranking amps) to start a big engine; they are noy very helpful to measure Ah capacity at slow (say 20h) discharge rates, which is the "capacity" measure that matters for a house bank. It is very very easy to do a (roughly) 20 hour test provided you have batteries in parallel so that you can use one while you test the other.
svlamorocha is offline   Reply With Quote
Old 14-07-2018, 14:43   #12
cruiser

Join Date: Jan 2017
Boat: Retired from CF
Posts: 13,317
Re: Measure present Full Battery Capacity (as it declines)

Quote:
Originally Posted by hellosailor View Post
There are battery testers that use the internal AC impedance of the battery to determine the actual remaining state of capacity.

https://smile.amazon.com/Clore-Autom...ery+tester+ba9

That's one of the more common cheap models. A battery distributor will have a much better one, and there are cheaper models out there as well. One alternative to trying actual discharge cycles.
Yes that is part of what SG may be doing.

But Maine Sail couldn't catch it on his scopes.

The consensus is none of the sub-lab-quality gear is worth messing with for residual capacity.

20-hour test or guess.

With an AH counter *and* a SG, working backward from 100%, with patience could maybe work out a reasonable estimate.

IMO setting good benchmarks from new, when you see performance declining, time to replace next time it's convenient if you're cruising to underserviced locations.

Pushing past 75-80% SoH, the odds of "unexpected failures" gets too high for my taste, better to be proactive.
john61ct is offline   Reply With Quote
Old 14-07-2018, 19:04   #13
Moderator Emeritus
 
a64pilot's Avatar

Cruisers Forum Supporter

Join Date: Oct 2013
Location: Jacksonville/ out cruising
Boat: Island Packet 38
Posts: 31,351
Measure present Full Battery Capacity (as it declines)

I think you can get a reasonable idea with an amp counter and a Smart Gauge, the longer and deeper You discharge likely the more accurate the guess will be. Ideal of course is to discharge over 20 hours to a Smart Gauge reading of 50%,then look at how many amp hours were used, double that.
I end up more in the overnight discharge to 75% number and multiply by four of course.
I think it’s good enough, not as accurate as a real 20 cap check, but good enough.

Lifeline publishes an excellent chart of SOC and discharge rate, plus of course the open circuit resting voltage. However for whatever reason my bank is much higher voltage than the chart says it should be at a particular SOC, so it’s not real applicable for me, for some reason.
I find my Smart Gauge is only even close to accurate in the morning, for the first several hours of discharge, it reads way higher than my amp counter does, however in the morning, they agree very close to each other, within a percent or two.
a64pilot is offline   Reply With Quote
Old 14-07-2018, 19:44   #14
cruiser

Join Date: Jan 2017
Boat: Retired from CF
Posts: 13,317
Re: Measure present Full Battery Capacity (as it declines)

Quote:
Originally Posted by a64pilot View Post
discharge over 20 hours to a Smart Gauge reading of 50%,then look at how many amp hours were used, double that.
Best to stick to the 20-hour rate e.g. 500AH bank would be a 25A load.

So if 50% is the target, should with a new batt take around ten hours.
john61ct is offline   Reply With Quote
Old 14-07-2018, 19:47   #15
Moderator Emeritus
 
a64pilot's Avatar

Cruisers Forum Supporter

Join Date: Oct 2013
Location: Jacksonville/ out cruising
Boat: Island Packet 38
Posts: 31,351
Measure present Full Battery Capacity (as it declines)

Your right, just a habit to think of 50% as being fully discharged, it’s not of course, just I have never gone below 50%.

However the point is with a Smart Gauge and an amp counter, I think you don’t necessarily have to do a real cap check, just pay attention to see if 25% or whatever number your usually at in the morning starts being a smaller number of amps, then maybe equalize if you can.
a64pilot is offline   Reply With Quote
Reply

Tags
battery


Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
How to measure the voltage of a battery? gauvins Electrical: Batteries, Generators & Solar 13 11-06-2018 17:07
Where to measure battery temperature zboss Electrical: Batteries, Generators & Solar 5 06-08-2014 08:59
Link 10 Battery Monitor -Setting Battery Capacity hugh c Electrical: Batteries, Generators & Solar 3 02-05-2012 11:57
Battery Capacity-What is Too Big? Pelagic Electrical: Batteries, Generators & Solar 30 06-10-2008 03:12
Battery Capacity Advise needed rleslie Electrical: Batteries, Generators & Solar 2 22-05-2005 22:56

Advertise Here
  Vendor Spotlight
No Threads to Display.


All times are GMT -7. The time now is 17:16.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2024, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.