SailNet Community

SailNet Community (
-   Gear & Maintenance (
-   -   Voltage puzzle (

davidpm 05-28-2012 10:41 PM

Voltage puzzle
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?

Faster 05-28-2012 10:57 PM

Re: Voltage puzzle
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?

davidpm 05-30-2012 10:33 PM

Re: Voltage puzzle

Originally Posted by Faster (Post 876929)

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.

Faster 05-30-2012 10:48 PM

Re: Voltage puzzle
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.

wind_magic 05-31-2012 04:44 AM

Re: Voltage puzzle
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.

szigi 05-31-2012 07:16 AM

Re: Voltage puzzle

Originally Posted by Faster (Post 878036)
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%.

denverd0n 05-31-2012 09:04 AM

Re: Voltage puzzle
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.

hellosailor 05-31-2012 09:46 AM

Re: Voltage puzzle
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.

travlin-easy 05-31-2012 09:50 AM

Re: Voltage puzzle
The only true method of determining the charge state is to use a hydrometer. If it reads 1.275 specific gravity the battery is fully charged. It's that easy.

Gary :cool:

Maine Sail 05-31-2012 10:40 AM

Re: Voltage puzzle

Originally Posted by travlineasy (Post 878162)
The only true method of determining the charge state is to use a hydrometer. If it reads 1.275 specific gravity the battery is fully charged. It's that easy.

Gary :cool:

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

All times are GMT -4. The time now is 09:22 AM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
User Alert System provided by Advanced User Tagging v3.1.0 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
(c) LLC 2000-2012

For the best viewing experience please update your browser to Google Chrome