What is a "fully charged" battery?

Maine Sail · Feb 8, 2012

Charles Erwin said:
In order to have relaible information from any battery monitor, one needs to know when the battery is "fully charged."
AmplePower defines “fully charged” as battery voltage 14.4v., charging amps 2% of the 20hr rate, but they do not specify how long these values must exist.
Victron defines “fully charged” as battery voltage 13.2v., battery amps 4% of the 20hr rate, which values must exist over a 4 minutes time span.
I do not know how Xantrex products define fully charged.
So how does one realistically define “fully charged” thus to sensibly program/manually synchronize our battery monitors?

I almost always re-set my monitors down to my float voltage and 1% acceptance or less. A real fully charged battery, in good shape, will accept less than 1% current based on its 20 hour Ah capacity. I have a customers 105Ah battery on the bench right now for equalization. When full it is accepting 0.038A to maintain 13.6V... Of course your battery monitors shunt does not have the resolution to show a 0.04A charge so it shows zero and most folks think the charger just turned off...

Lifeline suggests .5% acceptance or less as full. Battery monitor manufacturers often "bump" the "full" acceptance % so your monitor "re-sets" to full more often. This avoids the tech support calls of "My battery monitor never reads full..?" but it does not mean the bank is actually "full" if set to 4% acceptance. I get away with going to 1% or less because we have solar but I still manually re-set it when I know it 's chuck full, this is a manual re-set about once per week... If you are charging via alternator only then leaving it at 2% and your float voltage is a better choice. If you don't have float you'll need to adjust your voltage and acceptance % for "full" re-set..

For basic accuracy 2% is often fine unless you can shore charge regularly. Victron's 4% is to high for me so I adjust it down to 2% or less.....

You can also check your banks SG, after resting for 24 hours, and compare that to the acceptance you saw. Sulfated batteries may never drop below the 2% in a time frame you'd expect so testing the SG and perhaps equalizing may help if you rarely see acceptance drop below 2%....

Maine Sail · Feb 8, 2012

Charles Erwin said:
Thanks a heap for the response . but I am still not completely clear on this, You mention a standard (not published) - ie synchronize when the battery is "chock-full". It seems clear that one needs to manually synchronize regularly no matter the charge source. However, my question is what values show us the battery is "chock-full"? Seems to me that "fully charged" ="chock-full". What are the values that tell one that ithe battery is both "chock full" or "fully charged?"

With the charge source running and in float an acceptance of less than 1 - 1.5% of the 20 hour capacity and you can safely manually re-set. For general use really anything below 2% and you're fine.

So a 100Ah bank would be manually re-set anytime you are seeing less than 1.5A of charge current flowing while in float.... ALL DEVICES MUST BE 100% off when judging your banks acceptance.

I usually get to the boat and see 1-1.5A flowing in float mode, on a 375Ah bank, and hit re-set before flipping on the battery switch. Usually it is blinking full already but I still re-set it manually.

Benny17441 · Feb 8, 2012

I was just watching "The Big Bang Theory" on TV...

Stu Jackson · Feb 8, 2012

Maine Sail is correct. A longer answer is the Gotcha Algorithm: http://c34.org/bbs/index.php/topic,4922.0.html

Pretty much the same result, while this tells you what you need to do with the defaults in your battery monitors.

monkeybars · Feb 9, 2012

all that discussion was a little to techy for us amatuers,i generally consider a fully charged battery to measure around 12.9 volts after resting 24hrs off the charger.

Maine Sail · Feb 9, 2012

monkeybars said:
all that discussion was a little to techy for us amatuers,i generally consider a fully charged battery to measure around 12.9 volts after resting 24hrs off the charger.

Of course this all depends upon your battery and what the manufacturer deems "full". The average wet cell will read an OCV of about 12.7V - 12.73V when full and rested. AGM & GEL batteries can take more than 24 hours at rest to reach "resting open circuit voltage" and some can be as high as 12.8V resting as "full". Also cold temps help slow the surface charge from dissipating. I can still see my wet cells at over 13V, when temps are below 32F, even after being disconnected from a charge source for a few days. Full for our batteries is 12.72V.....

For most batteries 12.9V is considered "more than full", and that can't happen, so that tells me the meter is off or the batteries are possibly not sufficiently rested. There may be some batteries out there where 12.9V OCV is full...???

Trojan Battery Rested Open Circuit Voltage (OCV)
100% = 12.73

90% = 12.62

80% = 12.50

70% = 12.37

60% = 12.24

50% = 12.10

40% = 11.96

30% = 11.81

20% = 11.66

10% = 11.51

Benny17441 · Feb 9, 2012

It seems to be a moving target. I look at full time cruisers that only seem to get concerned when their battery banks get below 75% and wonder how practical is it to worry about that seemingly elusive 100%. I go by the "departure rule"; when I'm ready to cast off and disconnect the shorepower cable I officially decree that my batteries are full at 100% charge. I got to say it has not failed me yet so it works.

GorillaToast · Feb 9, 2012

Thanks for this thread.

After reading two large books on marine electrical systems, I still was confused about what voltage my battery bank should be at when fully charged. I'd somehow gotten the impression that 14.4 v was the goal, although now I see that that is a number that is used Ina circuit involving an alternator. I don't have one, and for now am charging my batteries with an automotive charger. After resting, I'm seeing 12.65 v, and thought I wasn't getting nearly a full charge. Once I get the boat back in the water, and install a 40 w solar panel and mppt controller, I should be good.

Maine Sail · Feb 9, 2012

Charles Erwin said:
Many thanks to all.
Many people - including me - just charge, launch and while cruising hope I don't overtax the batteries. So far, for many lustrums, that has worked fine. As for 12.65 v. = full I am not so sure that will work.
In either case, some folks (me) charge, launch, and cruise for eight weeks time. Seems to me one will need to know a bit more information sometime during those eight weeks particularly where all one has is a dumb alternator. Hence one acquires a Victron.
Mr. Jackson's solution may not work with Victron. Seems to me one needs to consider volts and amps exclucsivley since Victron publications do not show an ability to "count down." Personally, I avoid the whole "Jackson gotcha" by turning off refrigeration, etc, when charging. Thus one cannot accidentally fool the monitor's logic.
So put most directly -- does one set amps at 1% and 13.6 v. (shore charger float) most the time, and then - when one is off for eight weeks - just leave it be? Or does one change to 2% and 14.4 to accomodate an extended cruise?

Charles

Charles,

On an extended cruise unless you're running the alternator for 12-16+ hours don't expect to ever get to "full". Full is elusive whether you have smart or dumb regulation and most cruiser only get to top up with wind or solar or gen set via shore charger.... Start off with it calibrated and full and use it until you know for sure you are full then do a manual re-set. I would set it at your particular chargers float voltage and 1% - 2% will be more than adequate for automatic re-set to full.

Bob S · Mar 1, 2012

I'm getting a little anxious for spring. I have my (2) group 31's Sportsman batteries I bought from Defender in 09 on a storage shelf at work. I put one on a charger last week. It's been sitting 3-4 days now since being charged and I just checked and it's reading 12.7v The second has been sitting for an hour and reads 13.1v. Can I assume these are healthy batteries? How do you know when you'll need to equalize them?

Stu Jackson · Mar 1, 2012

Yes, Bob, sounds good. Have you ever equalized them?

Stu Jackson · Mar 1, 2012

Charles Erwin said:
Mr. Jackson's solution may not work with Victron. Seems to me one needs to consider volts and amps exclucsivley since Victron publications do not show an ability to "count down." Personally, I avoid the whole "Jackson gotcha" by turning off refrigeration, etc, when charging. Thus one cannot accidentally fool the monitor's logic.
So put most directly -- does one set amps at 1% and 13.6 v. (shore charger float) most the time, and then - when one is off for eight weeks - just leave it be? Or does one change to 2% and 14.4 to accomodate an extended cruise?

Charles

The algorithms are exactly the same. The monitors read NET current. So changing the defaults, whether fridge running or not, will still work. No reason to change back, even when cruising, since you can read SOC, amp hours or net charge. You should also know whether or not your fridge is running, and so decide what gross charge is going in.

Your boat, your choice.

Bob S · Mar 1, 2012

Stu,

They've never been equalized. Can't find a plug in charger that equalizes? Any suggestions?