Fully charged 12 volt battery

Submitted: Wednesday, Mar 21, 2012 at 22:44
ThreadID: 92698 Views:18323 Replies:6 FollowUps:7
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I am wondering at what voltage is a 12 volt 120 a/h AGM battery fully charged?.
I have 3 x 120 ah AGM batteries in the c'van, 3 x 140w solar panels on the roof, a 40 amp C 40 xantrex solar regulator, a Projecta IC 5000 50 amp 7 stage battery charger and a xantrex linklite battery monitor.
When we are on 240 volt power, I set the IC 5000 to power supply mode as we are always drawing some power from the batteries (we have a vitifrigo DP 2600 compressor fridge that is wired direct to the batteries). This mode keeps the batteries at a fairly constant 13.8v. Amperage is variable depending on the state of charge of the batteries. Occasionally, I change the mode to AGM and this takes the voltage up to around 14.5v.
The linklite monitor can display, among other things, the battery voltage and percentage charge. When on solar only, the display will show 100% full but the voltage has never shown above 13.9 (to the best of my knowledge), and amperage has been as high as 24.
At 100%, I assume the batteries are fully charged?. At 100%, the voltage can be anywhere between 13.8 and 14.5v. The fully charged status is confirmed by the remote readout on the IC 5000 (when on 240v).
However, when the charger is tuned off or when we are camped at night (obviously no solar) and start using power, there is a very quick drop to approx 12.8v or thereabouts and then a slow progression downwards to our lower (self imposed) limit of 12.1 volts (or 55% usage as shown on the Linklite monitor).
Is the battery fully charged at around 12.8v but requires over 13.8v to get there?.
We are reasonably heavy users of battery power (had our 11 years of camper trailer and canvas, outside showers, shovel powered dunny, one battery and a 60 w solar panel and are over that) and am not too worried about the usage but just the technicality of the fully charged battery.
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Reply By: Battery Value Pty Ltd - Wednesday, Mar 21, 2012 at 23:46

Wednesday, Mar 21, 2012 at 23:46
Hi Ian,

..Is the battery fully charged at around 12.8v but requires over 13.8v to get there?.

Yes, that's correct. The lower voltage is measured in open circuit (no battery current), and the 13.8V when charging current flows through the battery.
The 14.5V is a necessity too, to counter any sulphation which otherwise could form. But the 14.5V have to be time limited to 1~2 hours per day.
You can skip the 14.5V stage if the state of charge wasn't allowed to drop much below 80% in the previous night.
At every 14.5V charging session, the batteries lose a little bit of electrolyte, but that's only a tiny loss, compared to sulphation which would eat up battery capacity a lot faster if it would be allowed to sneak in.

cheers, Peter
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Follow Up By: Member - DereelGirl - Wednesday, Mar 21, 2012 at 23:56

Wednesday, Mar 21, 2012 at 23:56
Thanks Peter, You've put it so much better than me. I know my regulator does just that .... it regulates the time and type of charging sequences to optimise the life of the battery.


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Follow Up By: Member - Ian G (NSW) - Monday, Mar 26, 2012 at 21:57

Monday, Mar 26, 2012 at 21:57

I guess the other important side of this discussion is when is the 12 volt battery 50% discharged. Is it at approx 12.0 volts or is it when the linklite meter shows I have used 180 amps ie 50% of the 360 amps available when meter shows batteries at 100% charge?
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Follow Up By: Battery Value Pty Ltd - Monday, Mar 26, 2012 at 23:31

Monday, Mar 26, 2012 at 23:31
Hi Ian,

is it 50% when the meter reads 50, or is it slightly more or less - ask yourself the question how important really is an accurate display of SOC?

It is actually possible to get a pretty accurate figure, but the calculations are quite extensive thus these SOC meters with their microprocessors don't come cheap.

As an aside: For any charging gear to do the right thing, it doesn't need to 'know' the SOC at all.
The battery just continues taking in charge which gets presented in the form of charging current. And when it's nearing full charge, it signals this to the controller by a diminished appetite for current. The charger then starts the final stage of charging (controlled over-charge) by waiting until the current has dropped below a certain level. Then the charging voltage is being lowered from the gassing region, to the sustainable float charging voltage level.

cheers, Peter

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Reply By: Member - DereelGirl - Wednesday, Mar 21, 2012 at 23:49

Wednesday, Mar 21, 2012 at 23:49
Hi Ian,

Now I don't profess to know everything about solar and batteries but I was wondering if the regulator is setup properly. I know you can set the max voltages on some of the better ones. Perhaps this needs to be changed, perhaps the batteries when on solar get up to the limit set by the regulator and stop charging. Just a thought. I know with my AGM 75A/H Thumper that 12.75V = 100% but while charging it can get up to 13.xxV and when using the solar panels the regulator can show the voltage at even 14.xxV.

Not sure if I've helped any or just added confusion.


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Follow Up By: Sand Man (SA) - Thursday, Mar 22, 2012 at 07:23

Thursday, Mar 22, 2012 at 07:23

The voltage shown when charging from any source, is the output voltage of the charging system rather than the state of charge of the battery.
12.8v (2.14v per cell approx.) is fully charged.


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Reply By: zigdog - Thursday, Mar 22, 2012 at 07:52

Thursday, Mar 22, 2012 at 07:52
Hi Peter,

You say here that 14.5v should be limited to a few hours a day. There must be thousands of us who have aux batteries / campers / van batteries getting charged on the run from the vehicle battery & alternator. These are pushing out above 14v while the engine is running, often for many hours.

Are you suggesting that we are slowly killing our aux batteries by this method of charge on the run?

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Follow Up By: Battery Value Pty Ltd - Monday, Mar 26, 2012 at 22:51

Monday, Mar 26, 2012 at 22:51
Hello Zigdog,

batteries are like us mortals, ultimately they die.
Charging them in an uncontrolled way, is like munching any amount and type of food - watch your body go out of shape and your life expectancy drop.

Number one rule:
the battery needs a regular amount of controlled over-charge, especially if it's being discharged substantially on a regular basis.
Over-charging involves driving more amp hours through it than it can actually store.
This causes the voltage to rise, which in turn causes water molecules to decompose - the battery gases.
This isn't desirable per se, it just comes along with it.
You could also say, the battery needs to gas for good health (although the consequences of gassing aren't so healthy).
Because most chargers control the voltage and not the current, you can achieve over-charging by raising the charging voltage into the gassing region.
A significant increase in the rate of gassing happens at around 14.2V (25 degrees). At this voltage and temperature a typical 100Ah AGM battery may take in 100mA and more even when it's already fully charged.
1Ah in this region corresponds to about 1 gram of water loss.
Over its life time, the battery loses about 20% of its electrolyte, or about 300~500 grams.
So that's up to 500Ah of total over-charge, or 5000 hours of sitting on 14.2V.

But you also have to look at the temperatures when charging.
The gassing rate increases about 2~3 fold for every 10 degrees above 25.
So the 5000 hours can easily shrink to only several hundreds under typical under bonnet temps.

cheers, Peter

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Reply By: MEMBER - Darian, SA - Thursday, Mar 22, 2012 at 09:58

Thursday, Mar 22, 2012 at 09:58
I've got 3 x Ritar 100 Ah AGM's in my van (using a Prostar solar controller and Ctek mains charger). When new, the batteries often got to 14.8V on mains charging but would soon be back down to the high 13's when the charger was off. Now after 3 years, about 14.2 seems the max on charge and 13V comes quickly when turned off (as Sandy says though, the post-charging V is the one of interest - and as you say, the V on draw is the vital figure). My voltage levels are falling sooner now that the batteries are aging. My solar controller and Ctek charger are still configured as set by the installer at the van factory, but I've been over the manuals and the setup seems ok to me (but I am an electro-klutz of sorts). We have 2 x 130W Kyocera panels and a small gennie for top ups. When the service period becomes practically too short, I guess new batteries are indicated. I've been researching them lately and the prices for quality seem very scary.
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Follow Up By: Battery Value Pty Ltd - Monday, Mar 26, 2012 at 23:11

Monday, Mar 26, 2012 at 23:11
Hi Ian,

...about 14.2 seems the max on charge and 13V comes quickly when turned off ...

Please check your charging gear, including wiring to the batteries.

If your chargers were set to 14.8V charging end voltage, they should maintain this regardless of battery age.

Another possibility is that one of your 3 batteries has lost half a cell (most likely the charge in one of the negative electrodes has disappeared or it's gone into reverse).
This can be tested by disconnecting the batteries from each other and measuring their open cell votlages.
If one battery shows only 12V or lower shortly after charging, that's the one which drags everything down.

There are certain techniques to bring such a battery back to life, but you need a charger which controls the current, and it takes time.
And the outcome is unpredictable.

cheers, Peter
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Reply By: The Bantam - Saturday, Mar 24, 2012 at 18:07

Saturday, Mar 24, 2012 at 18:07
Lets clear up a few false notions.

A battery is not a bucket, it is a chemical system, and the notion of "FULLY CHARGED" in my opinion is a contrived one.

There are approximate voltage figures that indicate "state of charge".

These figures are only valid with the battery at a resting state, not being charged, not being discharged and having been that way for some time.

These figuers are temperature dependent and there are slight variations dependent on battery chemistry.
While more or less reliable thay are far from absolute.

There is no fixed and relaible measure of battery capacity and state of charge is likewise a fluid concept.

People want to think their battery is like a bucket and there are plenty out there that will sell to that desire.

Moving on.

To charge a battery we have to apply a voltage higher than its resting terminal voltage otherwise no current will flow and no charging will occur.....

We charge a battery with a higher volatge than its resting voltage..after we remove that charging voltage the battery will take some time to settle back to a resting state.

Now this is where some will argue about what is "Fully Charged"...if we charge a nomonal battery with a nominal voltage like 13.8 volts once universally considered the optimum long term charging voltage.
The battery will reach a state of "charge equalibrium" where the battery is accepting no more charge and any current flowing is wasted or lost ( no drama).
Your conventional nominal battery will not excessivly gass nor loose excessive water.
This battery would have once been considered "FULLY CHARGED" and would settle back to 12.5 ish volts, and if it is healthy it will hold that for weeks or months if left alone.

There have been some changes in battery chemistry and some changes in what people are saying.
many would now claim that that battery was not fully charged and recomend charging at a higher voltage to get that battery "FULLY CHARGED"

Now the terminal voltage may settle to 12.8 volts ( or whatever) after this....but in the long term over a couple of hours or so ( even a day) the battery will still settle to arround 12.5 volts and stay there if healty and left alone.

In my opinion it has been over charged, not in a problematic way, but over charged all the same, because it will settle over time.

with the addition of calcium to batteries and changes in acid chemistry, some of the charging voltages have changed..and lots of things are misunderstood.

Especially sulphation.
One of the chief causes of sulphation is overcharging.

I do not believe for a minute that charging your battery at a higher voltage each day will reduce sulphation....the opposite actually

Keeping your battery cool will reduce sulphation
Not allowing your battery to remain under charged will reduce sulphation.
Minimising deep cycling will reduce sulphation.
Charging your battery at the optimum and expedient rate will reduce sulphation.
Using a charger withn a pulsed antisulphation program will reduce sulphation
Not regularly over charging your battery will reduce sulphation.

As for various batteries being regularly charged with higher voltages reducing their life......damn tootin' it will.......but don't get bent out of shape over it...elivated underbonnet temperatures probably do more damage to car batteries.....especially AGM and other sealed batteries.

13.8 volts will still fully charge any of the lead acid battery family, but it may take some time.

The only time it is necessary to charge a battery at the higher voltages is to overcome charge resistance that some batteries can develop or to speed the charging rate.

If you have a good quality modern charger or regulator it should do all this for you.

Don't get bent all out of shape over all this, you could worry all you want about ya batteries an not add a single hour to their short lives.

As for what is optimum for an individual battery......there is a great variation in small things with what is being done with batteries....consult the manufactures specifications for that model of battery.

AnswerID: 481281

Reply By: GT Campers - Saturday, Mar 24, 2012 at 21:19

Saturday, Mar 24, 2012 at 21:19
your are right bantam, but the concept of the battery being a bucket, and voltage being water pressure, and wires being a hose, and amps being how much water is flowing... (you get me?)is one that gets the concept across to many people.

situations such as resting state, surface charge etc etc cause many eyes to glaze over!
AnswerID: 481294

Follow Up By: Allan B (Member, SunCoast) - Saturday, Mar 24, 2012 at 21:31

Saturday, Mar 24, 2012 at 21:31
Yes GT, water analogy can be useful for explaining electrical circuit behaviour but it sometimes falls down, especially so in respect to battery capacity and charging. The performance aspect of batteries that is so critical to longevity and so often debated, falls somewhat outside of the water analogy spectrum. In "getting the concept across" by this means can lead a babe in arms up the garden path, to thoroughly mix a metaphor. LOL


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