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
FollowupID:
882789
Follow Up By: Allan B (Sunshine Coast) - Sunday, Jul 16, 2017 at 10:22
Sunday, Jul 16, 2017 at 10:22
.
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."
FollowupID:
882790
Follow Up By: Allan B (Sunshine Coast) - Sunday, Jul 16, 2017 at 11:00
Sunday, Jul 16, 2017 at 11:00
.
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.
FollowupID:
882792
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
FollowupID:
882797
Follow Up By: Allan B (Sunshine Coast) - Sunday, Jul 16, 2017 at 13:49
Sunday, Jul 16, 2017 at 13:49
.
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.
FollowupID:
882802
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
FollowupID:
882821