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Voltage puzzle

3K views 17 replies 9 participants last post by  hellosailor 
#1 ·
I put in two of these batteries brand new.
Buy Autocraft Marine 12-Volt Marine/RV Deep-Cycle Service Battery 24DC-1 at Advance Auto Parts

Boat is a Bene 32s6 with Volvo Penta engine.
The owner left the ignition switch on for about 24 hours and it drained the battery to about 60 percent.
I trickle charged it at 2 amps with a shore charger for maybe 4 hours max.
Then motored about an hour and a half to a mooring.
The next day I 24 hours later I check the voltage.
I was expecting maybe 80 percent charge 90 at best.
The charge was 12.9 volts.

Any explanation for this?
 
#2 ·
The no-load voltage may well be 12.9, but it's unlikely the battery is recharged in terms of stored amp hours... put a few lights on and I'd not be surprised to see the volts drop quickly.

btw how did you determine the '60%' drawdown?
 
#3 ·
btw how did you determine the '60%' drawdown?
I glued a sticker on the back of my meter with the voltages and percentages as I can never remember them.
12.6 is 100% 12.2 is 60% if I remember correctly.
 
#4 ·
Unfortunately voltage alone doesn't tell you much - a rough indication at best, esp with no load.

With your shore charger you only "put in" 8 amps, and while motoring only 15 to 20 I'd guess.... So how much charge you got to depends on how run down the battery actually was vs its amphours capacity.
 
#6 ·
Unfortunately voltage alone doesn't tell you much - a rough indication at best, esp with no load.

With your shore charger you only "put in" 8 amps, and while motoring only 15 to 20 I'd guess.... So how much charge you got to depends on how run down the battery actually was vs its amphours capacity.
Right. The unloaded voltage of a lead-acid battery tells absolutely nothing on the state of charge (well, nearly nothing, you can tell that the battery is fully discharged).

You'd need a battery monitor, which continuously senses current flowing in and out of the battery to have a more or less exact measurement on the level of charge.

Measuring voltage with load can give an estimation of charge, but this would usually be only enough to differentiate a depleted battery from a partially or fully charged. It won't tell you whether it's 75% or 90%.
 
#5 · (Edited)
Like the others said, you really can't tell much from the voltage, but at the same time it isn't completely useless. Any example is my own bank, while I'm charging it, it can, of course, go up to 14.2 volts or even higher if I let the unregulated KISS wind generator run it up to 16 volts or higher (I'm working on putting a divert load in now). But as soon as you take that power away, the voltage then drops from 14.2, or 13.2, back down to sometimes 12.9 or in that neighboorhood.

Then go the other way, without any charging, put a load on it, from 12.9 my bank can be at 12.4 under load in a matter of minutes, and if the load is harsh enough down to 12.2. Let that go for hours and hours and I can be down at 11.9, but even that isn't the whole story because if I then remove the load and let it "rest", the voltage will climb back up to 12.3, 12.4, etc ...

You just really can't depend on the voltage to tell you state of charge. I used to think it was more useful than it really is but I have learned through experience that it is a very course measurement at best. That doesn't mean it isn't useful at all, if my bank is under a light load and it starts showing something around 12.1, 12.0, then I know I need to put a charge on it, even though the rest voltage is higher than that.
 
#7 · (Edited)
Actually, the "resting" voltage of a lead-acid battery is quite a good indicator of its state of charge. It needs to be completely disconnected from any charging or discharging source for at least 12 hours, though, and 24 hours is better. Most people, however, check the voltage far too soon after charging or discharging to get useful information.

If you are saying, davidpm, that after 24 hours with absolutely no charging or discharging, the battery voltage was 12.9, then it is either very nearly fully charged, or your voltage meter is not reading accurately.
 
#8 ·
David-
What d0n said. You don't need a sticker on the back of your meter is you just remember that 12.6 is the normal full charge, and one volt equals 100% of the useful charge. So every tenth of a volt that you are down, is a 10% loss. A simple 1:1 relationship.
For batteries that have rested overnight, or had the surface charge pulled off by a few minutes of a decent load, this is perfectly good enough. Even the battery makers are getting away from hydrometers and nasty acid whenever they can. A hydrometer will spot a bad cell in a wet acid battery, but a voltmeter is a good enough first step.
You can use one of the electronic testers that measure internal resistance (instead of the old carbon pile load testers) for another "point of view".

I question whether those are true deep cycle batteries since no amp-hour capacity is given for them. If they are typical, i.e. 80AH each, you'd have 160AH when new. At 60%, you'd be down to 96AH. After an hour and a half of motoring, maybe with a 50A alternator putting out 35A average, you might be back close to 140-150AH capacity in a perfect world.

So by the numbers, it should be possible that they recharged. But even then, a reading of 12.9 on a battery that has been standing overnight? Should not be possible unless the meter is off. I gave up on meters, unless you've got a Fluke or similar sold pre-calibrated and still IN calibration, you need a calibrated voltage source to see if the meter is lying.

If the battery is at 12.6 and the meter is off by 1/2%, the disaply could be off by 1/10th of a volt. If the meter is off by 1%, the reading could be off by 2/10ths of a volt. Then it gets worse, since the analog-to-digital converter circuit in most meters will also be off, causing the rightmost digit to float by 2-3 places. So the voltage could well be 12.6 on the battery, showing as 12.9 (or 12.3) on a meter.

You can't simply trust your meter, just because it shows those nice clear digits. They're good for relative readings (today versus yesterday) but not absolute ones, unless you can check the calibration.
 
#10 · (Edited)
As written that is not even close to the reality. Contrary to popular misconceptions many people use hydrometers incorrectly too, no make that most people.:)

The electrolyte still needs to settle out over time just like any OCV reading does. 24 hours rest, no loads, no charging is what should be the ideal minimum for an accurate SG reading. You then need to drain and fill three times per cell to get a clean cell reading and correct for temperature. Testing a battery shortly after charging will yield false SG readings. SG reading also tell you nothing about the batteries actual capacity at a given point in its life-cycle.

SG is best used to determine cell imbalances.. It is generally far safer to conduct an OCV test rather than risk acid spillage, burned clothes or a spill than to conduct regular SG readings. Once or bi-yearly is often a good protocol for wets to determine internal cell balance and to determine when to equalize the batteries. In a healthy battery all cells will have a nearly identical SG reading. Also "cheap" hydrometers can be VERY, VERY misleading. I personally use a far more accurate sight refractometer and when compared to a cheap EZ Red hydrometer it shows just how inaccurate they really are. I have seen people throw away perfectly good batteries because of erroneous errors from cheap hydrometers. The floating ball version is a complete and utter joke, the EZ Red style not much better.

The 12.9v reading after 24 hours may not have been enough "rest" time or your meters resolution is bad or it's just a "new battery".. Some battery types can take well over 24 hours to fully settle out, especially when new.

With many wet cell deep cycle batteries 100% state of charge corresponds to 12.7-12.74V not necessarily 12.6. 12.6V is only 90% SOC for many wet cells.. The batteries that DavidPM bought are Deka Deep Cycle batteries, same as Sam's Club, West Marine, NAPA or O'Reilly Auto sells. Deka says any voltage above 12.6 can be considered full. Kind of non-committal..

I have however tested MANY Deka/East Penn batteries and found a resting voltage for "full" to be 12.7-12.73V.... With a new battery 12.9V after 24 hours is not out of the question. Apply a decent load for 5 minutes and then test and you'll be closer to reality or, just let it sit for a few more days..

These are Trojan's OCV numbers and I find they match up darn close to the DEka/East Penn deep cycle batteries: Note the 10% capacity difference between 12.6V and 12.7V!! On a 400 Ah bank that is 40 Ah's or a full day or more of capacity for many boaters....

Battery SOC at Resting Voltage and 80f (Source Trojan Battery)
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
 
#11 ·
"The only true method of determining the charge state is to use a hydrometer."
Consider this, Gary. If that were so, then there would be no way to determine the charge state of AGM, gel, and sealed wet cells.
So, how do you think battery makers, distributors, and warranty services are done on all these other batteries? Right, with digital voltmeters, and internal resistance testers, aka electronic "load" testers.
The main purpose of a hydrometer is to burn acid holes in fabric, so folks can sell you new clothing and upholstery. It has some other minor uses, but that's the main one these days.
The electronic tools are easiily misused, but you can't blame the tools for poor workers.
 
#12 · (Edited)
denverd0n, Hellosailor, and MaineSail are absolutely correct.

Accurate open circuit voltage measurement, done correctly, is an accurate method of determining the state of charge (SOC) of all lead-acid batteries....flooded, AGMs, gelled, etc. Much, much better than attempting to use a hydrometer, for the reasons cited by MaineSail.

For the OP: 12.9 or even higher OC resting voltage is very common with AGMs and gels, but not with flooded batteries. Your battery appears to be a flooded "no maintence" type, so I'd guess that your voltage measurement device is inaccurate. Or, just maybe, there's some charging device still connected, like a solar panel?

Note that the SOC is not the same as capacity. Batteries typically begin to lose their rated capacity practically from the date of manufacture, and they continue to lose capacity throughout their useful lifetimes.

The best way to measure capacity is with a controlled, purely restive 20-hour load test, with the load being 1/20th the ampere-hour (AH) rating of the battery and the load test continuing until the battery reaches 10.5VDC. This is not possible for most users, lacking the time and/or equipment necessary.

The industry standard for measuring battery capacity these days is with a device which measures internal battery conductance and resistance, such as one of the Midtronics series. These cost upwards of $600 and more, so are not available to most users.

For measurement of SOC, however, IMHO you'd be hard-pressed to find a better way than an OC voltage measurement directly at the battery terminals with a calibrated voltmeter after a suitably long resting period.

BTW, this includes the fashionable-these-days "battery monitors", which are only accurate if they have been properly installed and calibrated -- and recalibrated as the battery ages.

Bill
 
#13 ·
bill, my impression was that the $50 load testers (electronic, not pile, and typically good for 1000-1200 cca ratings at that price) wre also a good way to measure real capacity. While they are "just" in theory measuring how much of the batteries rated CCA capacity is left, isn't that the same thing? If you're expecting a battery to put out xx amps into a starting load, and it only has half that capacity let...? Same same?

I thought they were measuring internal conductance and the price had just ramped down because they were a little less sophisticated, lower power rated, and some electronics had dropped in price.
 
#14 ·
Not sure which new meters you're referring to. Can you be specific about makes/models? Have you tested or used any?

The good ones still cost a bundle. My MDX-650 still costs $650-$750 new. The cheaper ones that I've seen all have some limitations.

But, hey...it's always possible. The Chinese have a "conductance tester" that sells for $10 :)

Bill
 
#15 ·
Thanks Maine?
I was hoping you would weigh in on this one.
Yes these are brand new batteries.
I can't think of anything to put real load on them. The boat has very little to turn on.
Maybe all the cabin lights together would do something.
The engine panel on for 24 hours brought it down to 60%
I will test it every time I get on the boat just to learn what to expect.
 
#16 ·
david-
If you have no heavy loads to use, disable the engine and crank the starter a bit, use the starter as a load. It should be good for a 15 second run, ten minutes of rest, another 15 second run, that way there's no danger of overheating it but you'll certainly knock off any surface charge on the batteries. And of course, make Real Damn Sure the raw water inlet is closed when you do that, so you can't hydrolock the engine.
If for some reason that's impractical, get any high-wattage auto headlamp or bulb i.e. 65-100 watts, and some jumper wires or alligators. Connect to batteries and leave it on for 5 minutes, check again at 10 minutes. If you don't mind dumpster diving, you can often find a dual-beam headlight behind any auto parts store, with the low beam burned out and the high beam (what you want) perfectly good.
 
#17 ·
I thought the OP was talking about standard, lead-acid, marine batteries. I have a very expensive battery monitor on the boat. And, when I had a totally dead house battery (1 of 4, 6-volt golf-cart batteries in a series parallel rig) that battery continued to provide readings that would have been consistent with a fully charged battery. Even with a light load, it still provided good readings. The hydrometer, however, quickly revealed one cell was sub-par and failed when under a heavy load.

Granted, you cannot test gel batteries with a hydrometer, but the vast majority of folks that I know don't use them. I guess some folks like sealed wet-cell batteries, but I'm not among them. Kinda' like the "no maintenance" car batteries that tend to die much faster than the traditional batteries that allow you to add water. As for getting battery acid on your clothing--I think I may have done that about 10 years ago, but it may have been longer than that.

I also have a very high-quality digital multi-meter here in the office, one that I use for measuring minute changes in line outputs of audio systems. It was very expensive when I purchased it a decade ago, and I don't take it near saltwater--too darned expensive. I also have an old-fashioned load tester from back in the 70s when I owned a service station. It still works, but I rarely use it anymore.

I guess I'm a bit old fashioned--I'll stick with the hydrometer. I've never had one that failed, and you cannot beat the price.

Cheers,

Gary :cool:
 
#18 ·
Gary, if it is ten years old, guaranteed it has drited out of calibration. As the Mad hatter said "But it was the best of butter!" and even the best components shift as they age.

If you had one bad cell, that should have been reflected in a significant change in the voltage measured from that battery, and a calibrated meter should have picked it up.

I know, "should". If the turkey baster works for you, great. I found that calibrating my meters was cheaper than replacing anything the acid splashed onto, acid loves me but I have different feelings about it.
 
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