Battery voltage and dc dc charger

Submitted: Sunday, Jul 16, 2017 at 07:03
ThreadID: 135265 Views:2167 Replies:4 FollowUps:13
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I read the earlier post in which Bantam provided excellent and detailed information which made complete sense. However and this is purely subjective. I have had two vehicles with dual battery setups. The first with a single 105 ah battery the second with 3 120 ah full rivers. I used both for some time before adding dc dc chargers right at the batteries. The result was a noticeable but as I said subjective improvement in SOC under similar conditions.
I also have a query. Cool weather at the moment so the fridge doesn't work much overnight but we use lights and pump. Quite often before dawn ie after many hours of rest voltage shows 13v. Why is this so?
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Reply By: Allan B (Sunshine Coast) - Sunday, Jul 16, 2017 at 08:43

Sunday, Jul 16, 2017 at 08:43
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Hi Terry,

Possibly because your meter calibration is reading high. And I am not being facetious.
What sort of meter is it?
What does it read at the beginning of the night?
What does it read with no input voltage?

In my experience, many meters are described as having better accuracy than they actually deliver. Rather like tyre pressure gauges.
Cheers
Allan

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AnswerID: 612543

Reply By: Battery Value - Sunday, Jul 16, 2017 at 09:06

Sunday, Jul 16, 2017 at 09:06
G'day,

as Allan pointed out, meter accuracy should be checked first.
Multimeters usually are of sufficient accuracy, but small digital voltmeters including plug in types and hard wired can easily be out by 0.2V.

The other thing, Chinese made AGM batteries often have resting voltages of 13.1V to 13.2V when fully charged.
Looking at the combined 360Ah?, a few Ah of discharge overnight won't bring the SOC down sufficiently for a more noticeable drop in the resting voltage.
AnswerID: 612545

Follow Up By: terryt - Sunday, Jul 16, 2017 at 09:19

Sunday, Jul 16, 2017 at 09:19
Thanks for that. The reading comes from a dingo 20 amp solar controller. If resting voltage can be 13.1v what does this mean with the 12.8 is fully charged which I have always considered a reasonable guide.
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Follow Up By: Battery Value - Sunday, Jul 16, 2017 at 12:17

Sunday, Jul 16, 2017 at 12:17
No worries, the Dingo seems to be a well designed unit with an accurate internal voltage reference.

According to the Nernst equation, the resting voltage mainly depends on the acid concentration (specific gravity of the electrolyte) and to a lesser degree on cell temperature.
Many Asian sourced AGM batteries these days come with very high specific gravities, like 1.3+.
In the diagram below you can see 2.15Vpc (12.9V) pertains to a SG of 1.3.
Since you experienced 13.0V in the morning, your batteries apparently come with SG of ~1.32 which is similar to what I experience on mine.
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Reply By: The Bantam - Sunday, Jul 16, 2017 at 09:58

Sunday, Jul 16, 2017 at 09:58
#1 and above all, as has been mentioned previoulsy, you need to be sure of the accuracy of your voltmeter.

In this day and age, very high accuracy can come realy cheap, but that does not mean ever meter is accurate or will stay accurate.

Because I am in the electronics business, I have several meters I can compare to get a reasonable impression of accuracy.

Most digital meters I encounter are spot on ... well at least to 1 decimal point., BUT I do occasionally encounter meters that are one or two decimal points off on a 15 volt scale.

It is so very important that you meter is accurate or you at least know what the error is when dealing with batteries where one whole volt can mean the difference between a battery being pretty damn fine and dead as a maggot.

#2 Some batteries take quite some time to return to what could be considered their real resting voltage ...... this is good it is an indicator of battery health and quality ...... a battery that takes a long time to reach its real resting voltage will have low self discharge .. this is a good thing.

If you are getting pedantic and want to know the real resting voltage of a battery, charge it well, disconnect it from everything and leave it 24 hours before calling it "rested".

Some batteries will remain at a higher voltage and at an above normal resting state of charge for days or weeks.

#3 the generic state of charge voltages are fine and beaut and do work reasonably well for everyday batteries.

BUT if you want realy meaningfull figures you need to get state of charge graphs for that specific battery AND there will be multiple temperature corrected curves.

Even then, voltage is only a sign of state of charge not a hard and fast actual measure .... the only truly reliable measure of state of charge is specific gravity of the electrolite which we simply can't measure in sealed batteries.

#4 an important thing to understand is that a single voltage reading gives us only a fleeting glimpse of what is going on .... if we want to know what realy goes on with a battery, we need to take multiple readings over time and thru state of charge

It must be understood that battery behaviour is far from linear and far less consistent than many would like to believe.

There are so many factors that influence battery behaviour ...... anybody that things they have a realy accurate measure of what is happening with the battery is dreaming.

cheers
AnswerID: 612546

Follow Up By: The Bantam - Sunday, Jul 16, 2017 at 10:18

Sunday, Jul 16, 2017 at 10:18
Another matter is

What is Fully charged.

Back in the day of open top batteries we would judge state of charge by two things ...... #1 that the battery was bubbling and "gassing freely" and #2 that it's electrolite was a particular specific gravity. .... these both indicated that the required chemical transformation was largely complete.

This was a time of crude unregulated battery chargers where there was a very real prospect of overcharging a battery and " boiling it dry".

we typically charged 12 volt batteries with 15 to 16ish volts for cycle charging or out of vehicle recharge, and used 13.8 volts for continuous or float charge applications.
13.8 volts was chosen because it would charge the battery in a reasonably expedient manner and not result is excessive fluid loss and thus not require excessive topping up and maintenance.

Those generic state of charge figures come from that time and so does a particular notion of "fully charged"

These days both batteries and chargers have changed ...... the old school batteries and chargers still exist ...but there is a variety of new technology in both batteries and chargers.

Many of the modern chargers will take a battery safely to a higher state of charge than would previously been considered safe ....... some batteries will hold this higher state of charge for quite some time, old school batteries won't.

This higher state of charge is not worth much in terms of battery capacity held, because as soon as a load is connected the curve will flatten out rapidly.

What is the benefit of this higher state of charge ....... yeh well that is a whole multi-page argument in it's self

cheers
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Follow Up By: Allan B (Sunshine Coast) - Sunday, Jul 16, 2017 at 10:22

Sunday, Jul 16, 2017 at 10:22
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I agree absolutely with all that bantam has said in his Reply 612546 above.
In particular his final sentence. Many people put great importance on the exact voltage reading of a battery as an absolute indication of the battery State of Charge or its health when there can be so many influencing factors. It is a guide, nothing more.
Relative comparisons of voltage on any one particular battery can be useful but even then there would be differences caused by temperature and true balanced or 'resting time' that will confuse the comparison. As you say Bantam.... "Tell 'em they're Dreamin."
Cheers
Allan

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Follow Up By: Allan B (Sunshine Coast) - Sunday, Jul 16, 2017 at 11:00

Sunday, Jul 16, 2017 at 11:00
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Bantam, you posted before I got in my response. Damn, I am such a slow typer!!!!

Your account of charging "open-top batteries" brings back memories of Woomera many years ago.
We had several hundred flooded lead-acid batteries used for camera supplies, firing circuits etc. Without local charging facilities, these were rotated from duty to a charging shop where they were lined up in series circuits and given a "good boilup". Of course those batteries starting from a higher SOC would really get a good dose of gassing! Some science may well have gone into the missiles, but not a lot went into battery charging!

Incidentally, as a leftover from my instrumentation days, I have several electrical calibration devices, the most useful being a 'standard' voltage reference. It produces 20v, 10v, and 5v in sine-wave, square-wave, and DC and also 200mV, 100mV, and 50mV in DC to an accuracy of better than 0.25% and contains a precision voltage reference for self-calibration. (No, not a Weston Cell. Did you ever encounter those?) I find digital panel meters to vary somewhat but my multimeters have been set and maintain calibration reliably, only getting checked now if I have some misgivings of a reading. Most times, accuracy of 1 or 2 percent would be all I am looking for anyway.
Cheers
Allan

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Follow Up By: The Bantam - Sunday, Jul 16, 2017 at 12:17

Sunday, Jul 16, 2017 at 12:17
Woomera ... hey Allan ..... do you still glow in the dark?

I think you are a bit older than me ...... I vaguely remember Weston Cells, but only in reading ..... we had LEDs and other semiconductor voltage references when I was an apprentice.

AND big battery charging panels with large selenium rectifiers and manually adjusted multi tapped transformers .... big 6 inch meters and large knife switches

What is stunning is how cheap accuracy is these days ... my best meter these days is all singing all dancing, true RMS 4 digit accuracy, IP and mains rated AND cost me about $100.

I just baught a new Pro'skit analoge and it is surprising me ... especially on the low ohms range.

cheers
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Follow Up By: Allan B (Sunshine Coast) - Sunday, Jul 16, 2017 at 13:49

Sunday, Jul 16, 2017 at 13:49
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Nah, I was at Woomera, not Maralinga. Rockets, not bombs! Although I did spend half an hour at Emu en flight to Adelaide once. Standing waiting in the shade of a DC3. That was enough!

Yes, I would be a bit older than you. I'm a bit older than everyone, but still active.

My v/source gadget has a semiconductor reference, don't remember the type.
Western cells were commonly used in laboratories but also in many early 'mV pontentiometer' process instruments reading thermocouple temperatures. Fragile, touchy things and useless for diesel cranking. lol

The Woomera chargers were not even selenium! They were mercury arc rectifiers with, as you say, multi-tapped transformers. Lovely violet glow. Supposed to ramp the current down before switch-off but blew up a pair of rectifier tubes once when I killed the mains in a hurry!
We used to import distilled water in huge basket-clad glass bottles then later manufactured it in large laboratory-type stills.

Yes, there are some very acceptable test gear around now at attractive prices. Fluke and such are no longer the only choice.



Cheers
Allan

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Follow Up By: The Bantam - Sunday, Jul 16, 2017 at 22:52

Sunday, Jul 16, 2017 at 22:52
Yeh 3 phase mercury rectifiers ...... looked like the udder of an electric cow .... seen one handled one but never dealt with one in use ...... real mad scientist stuff.

cheers
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Reply By: swampy - Sunday, Jul 16, 2017 at 11:48

Sunday, Jul 16, 2017 at 11:48
hi
agm typically have a fully charged volts of 13.0 or greater
flooded/wet typically have fully charged volts 12.7 or greater

If u can get volt chart from the batt maker would be ideal.

Although batt volts are used even by battery chargers .
Measuring amp in/out is the most accurate .
AnswerID: 612548

Follow Up By: The Bantam - Sunday, Jul 16, 2017 at 12:06

Sunday, Jul 16, 2017 at 12:06
Actually amps in and amps out is the least accurate.
Because depending on how much current is being drawn and deposited the battery capacity and charge behaviour will vary, some batteries a great deal.

The charge and discharge losses will also change a great deal as the battery ages.

as for the fully charge voltage figure ... yeh well that varies depending on the metaurgy and chemistry of the battery.

Generalisations get less and less helpfull.

cheers

The only thing that can be remotely relied upon is the indiviudual battery specifcations.

cheers

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Follow Up By: Allan B (Sunshine Coast) - Sunday, Jul 16, 2017 at 13:25

Sunday, Jul 16, 2017 at 13:25
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There is that "typically" again.

It just does not apply to batteries.
Cheers
Allan

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Follow Up By: Member - Blue M - Sunday, Jul 16, 2017 at 18:51

Sunday, Jul 16, 2017 at 18:51
I liked my first setup the best, when the fridge stopped working and the lights were very dull, I knew the battery was flat.

Now I have all the gauges and gadgets these days and all I seem to do is check them, and sometimes worry a little.

Back then I had a job and to buy another good battery was half a days wages, now I have retired it equates to a third of my pension, that I am not yet old enough to collect.
Cheers
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Follow Up By: Gronk - Sunday, Jul 16, 2017 at 21:23

Sunday, Jul 16, 2017 at 21:23
I liked my first setup the best, when the fridge stopped working and the lights were very dull, I knew the battery was flat.


And people still use their battery the same......and complain when they only get 6mths life out of it !!
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Follow Up By: The Bantam - Sunday, Jul 16, 2017 at 22:55

Sunday, Jul 16, 2017 at 22:55
What ^ he said
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