Basic question about voltage

Dec 2, 2006 08:50 pm
Basic question about voltage

I known for awhile that to get the most life out of one's batteries, it is best not to discharge them below 12.5 to 12.4 volts (10 to 20%, respectively).

Okay, but is this "under load" or not? 

To clarify:  Say I have a 12 volt battery (it doesn't matter what size, but let's say it is 200 amp hour battery), and that it is connected to an inverter, and that I am powering a 200 watt load.   In addition, let's say it is not receiving any charge from the solar panels because it is night, and there is no other charging taking place (no hydro, no steam, no grid etc.).

Okay, so my voltmeter tells me that the battery voltage (while I am running this 200 watt load) has dropped down to 12.4 volts.  So I cut the inverter off (and thus disconnect all loads, including the inverter's), and the volts immediately jump up to 12.65 volts.

What percentage did I discharge the battery?  Was it 12.4 volts (20% discharge) or 12.65 volts (less than 10% discharge).

This is important, as I wonder if maybe I could get a bit more use out of my batteries without significantly shortening their lifespan.  The above "hypothetical" situation is something that often happens (though usually I don't get down to 12.4 volts under load, but something more like 12.5).

Would appreciate any answer!  Thanks!
 
163 Posts
Dec 2, 2006 09:21 pm
Re: Basic question about voltage

Lawerence,

You have to look at Power and not Voltage alone. A 200ah battery will give you 200 amps for 1 hour, or 20 amps for 10 hours, or 16.67 amps for 12 hours. Power = Volts X Amps.

A 200 watt load will draw 16.67 amps at 12 volts. Therefore a 200 watt load will discharge a 200ah battery by 50% after 6 hours.

While you may be able to draw some inference about the state of a battery by measuring its voltage, remember that an almost dead battery will still measure about 12 volts.

John
« Last Edit: Dec 2, 2006 10:53 pm by john bodden »
 
Dec 3, 2006 10:38 am
Re: Basic question about voltage

John, you state,

"While you may be able to draw some inference about the state of a battery by measuring its voltage, remember that an almost dead battery will still measure about 12 volts."

Well, I realize that an almost dead battery might measure 12 volts, but in my experience with batteries -- no matter what kind you're talking about (car batteries, flashlight batteries, ni-cads, lead acid etc. etc.)-- when they're dead or almost so, and there is no charging being attempted, their voltage never measures anything close to what it would be if it was fresh and able to hold a charge. 

While I understand what you're saying about a 200 amp hour battery being able to give 200 amps for one hour etc. , there is no practical way I know of to calculate (on an everyday basis) how much you're discharging a battery.  The only exception would be if you always use the same load (same amp draw), and keep a fairly close track of the time. In real life, I use different loads at different times (as I would think most users do), and there is no way I am going to start carrying a stopwatch around and/or a clipboard with all the loads I've used (and how long I've used each of them). 

While a voltmeter may not be as accurate, I would think it would give me a close enough estimate of the percentage of the discharge, so long as the battery(s) in question are not
being charged when the measurement is observed. 

I've had my solar system for around two+ years, and I've always gone by what the voltmeter says.  Furthermore, at night, the volt reading always drops predictably, depending on the amount of the draw from what I'm powering.  If it is a 100 watt load, then obviously the volts drop slower than if the load is higher.  If it is a 600 watt wet/dry vacuum, then the volts drop much quicker.  So to me, the voltmeter is definitely telling me how much I am "withdrawing from my account," and how quickly.  To say it is only a general inference would imply that sometimes it would drop significantly quicker or slower, but that this drop would have nothing to do with the load being put ont he battery, or for how long (which makes absolutely no sense). In other words, you seem to be saying that there is no correlation between the formula which you gave in your post for a 200 am p hour battery, and what a voltmeter would say.  This I can't believe.  I would think that if I discharged a "fresh, fully charged" 200 amp hour battery with a 200 watt load for 6 hours, that the voltmeter would consistently read near 12.00 volts, give or take a tiny amount.  That would be more than a mere inference to my mind. 

What I need is a practical answer to a practical question. And that question is still: "If one withdraws one's battery(s) down to 12.4 volts (measured with all loads off, and all charging off), has one discharged them (approximately) 20%? . . .  or is the correct measurement taken with the loads on?"

If voltage is so irrelevant, then why do so many people use it, and why do chargers often have voltmeters built in? 
« Last Edit: Dec 3, 2006 10:45 am by Lawrence Carroll »
 
351 Posts
Dec 3, 2006 04:23 pm
Re: Basic question about voltage

Here is a good article for you on detirming state of charge with a voltmeter.
www.homepower.com/files/battvoltandsoc.pdf

The answer is that your battery is about 10 percent discharged.
It can be detiremined by the resting voltage, or the voltage underload.
The 16.67 amp load on the 200 amphour battery would totally discharge the battery in about 12 hours. This is known as a C12 discharge rate.  If you use the chart on page 3 (also labled 68 at bottom of page) and follow the C10 curve, the 12.4 volts also indicates about a 10 percent state of discharge.


 
Dec 4, 2006 11:00 am
Re: Basic question about voltage

Thanks, Ken (and John).

I may be getting a slightly better grasp now. 

From the chart, it would appear that if the load is less (a higher C-number), the voltage reading falls more slowly in comparison to the actual State of Charge (SOC) decline. On the C/100 curve, even when the battery has reached an 80% SOC, the voltage reading is still the same (12.6 volts) as it was at 100% SOC!  And the voltage doesn't actually drop down to 12.5 volts until the battery SOC is 50%!! Yikes!

A voltage reading on the higher load rates, like the C/10, on the other hand, would be more in line with what one would expect.  A 12.0 volt reading would mean a 50% SOC, according to the chart.  And a 12.3 volt reading on the C/10 is about a 70% SOC etc. etc.

What I don't understand now, though, is how people talk so casually about their 10-20% (or whatever) SOC that they allegedly keep their batteries in.  Surely they aren't keeping up an endlessly detailed record of their loads and then looking at this chart?  That would be ridiculous to me.

I think I'll just "pretend" that the simplified chart that  I've seen bandied about on the net now and again (12.5 volts =90% SOC, 12.4 volts =80% SOC, 12.3 volts=70% SOC, 12.2=60% SOC etc. etc.) is actually absolute, even though it really isn't, as the chart that Richard Perez uses in his article demonstrates. I have to have simple way to measuring my usuage limits or I'll go batty. 



« Last Edit: Dec 4, 2006 11:03 am by Lawrence Carroll »
 
26 Posts
Dec 4, 2006 01:17 pm
Re: Basic question about voltage

I used to watch the voltage closely and I think you are taking it too seriously...especially under a large load. I have watched our battery bank drop to 11.7 when the vacuum cleaner is running or the grain mill (a suprisingly heavy 100 amp @ 12v load). Then it bounces back once the load is off (especially the grain mill, which only runs for a couple minutes). But to *really * know what is going on a good investment is a system monitor. Now I watch our Trimetric with the same intensity, but with much more information, that I used to give the volt meter.
 
351 Posts
Dec 4, 2006 06:16 pm
Re: Basic question about voltage

Lawrence:

Jim is correct that you can take the numbers too seriously, especially the momentary ones. The battery monitor is the best solution. But if you are not ready for one yet, there are other ways.

The simplified voltage method is OK, but the fine print on it says that there should be no charging or discharging for two hours before taking the readings. Particularly if your battery has been charging (or a high discharge rate), the readings can be thrown way off, if taken too soon.

Most systems have either an amp meter(bi-directional) or a watt meter in them. If not, you can add one, or use a clamp-on.

Make a simple table for the C rates showing both amps and watts for your battery. For a 200 ah 12V battery, C5 is 40amps or 480 watts. C10 is 20 amps or 240 watts, etc.  Put this Table with a copy of the curves from the article, near your system. 

You walk in and determine whether the system is charging or discharging. Read the watts or amps, (and the voltage) and look up the C-rate on the table. Then on the appropriate set of curves (charging or discharging), follow the appropriate C-rate curve to determine the state of charge by the voltage reading.  If your amp or watt reading is in between the C-10 and C-20 rate, just do a little interpolation between the two curves. After you have done it a couple of times, the whole process takes maybe 30 seconds.

To work around a low load, say C100, you can wire up a 100 watt bulb as a test load. (Larger systems, just use more light bulbs) If you come in with a 2 amp (24 watt load) on the system, flip the switch and add the 100 watts to it. Give it a few minutes, and then take your readings. In this example, you will be almost exactly on the C20 curve (for a 200ah battery).

Your objective is to know that you are somewhere around 90% or 95% state of charge. Trying to figure out “I am exactly at 92 percent” is beyond the accuracy of the method, any way.

You could also go to hygrometer readings. Temperature corrected, they are more accurate than voltage readings.  However they are sometimes messy, and take a little more time.

Many of the people that talk about state of charge (SOC) have some sort of state of charge meter or battery monitoring system. Some of these are as simple as the manual method outlined above, others use more complex algorithms and/or other sensor inputs, such as battery temp. Simple SOC meters used to start at about $50 and very complex monitoring systems can run well above $500.

If you are handy with a soldering iron, Richard Perez did an article for Homepower that had a design and parts list for a SOC meter that could be built for about $10 (early 90’s prices). 

Large systems with consistent loads and generation that operate in the 80% plus range are easy to build, particularly on an unlimited budget. Smaller systems, with inconsistent loads or generation (or both), will take larger swings in their state of charge.

And finally, some of the people who maintain that their system “stays above 80 percent” just don’t monitor it often enough, to see the swings happen.

Ken
« Last Edit: Dec 5, 2006 01:32 am by ken hall »
 
Dec 5, 2006 12:15 am
Re: Basic question about voltage

Ken, your statement,

"The simplified voltage method is OK, but the fine print on it says that there should be no charging or discharging for two hours before taking the readings. Particularly if your battery has been charging (or a high discharge rate), the readings can be thrown way off, if taken too soon."

pretty much answers completely my query in my first posting.  I had wondered about the "under load" versus the "not under load" values -- which one was more accurate.  You not only answered it, you pointed out that one should wait two hours before taking a reading (and of course, without any charging going on either in those two hours).  I wasn't aware of the need for such a long wait, but am grateful to find that out!

Likewise, Jim's statement,

"I have watched our battery bank drop to 11.7 when the vacuum cleaner is running or the grain mill (a suprisingly heavy 100 amp @ 12v load). Then it bounces back once the load is off (especially the grain mill, which only runs for a couple minutes)."

also answers my question.

I think both you have pointed out quite well that to really get the best grasp about what's happening, though, one should buy some type of system monitor, whether expensive or not. 

For myself, I probably won't do that in the next year or two, and might even wait until I my batteries eventually die, which could be a long time (or not. . . !) .  I feel fairly comfortable using the simplified method, and am a little curious as to how long my batteries are going to last without going to the length of using a system monitor. 

Nonetheless, you have really piqued my interest in those gadgets, so it may be somewhat sooner . . . :-)  If so, like Jim, I will then be looking at my monitor "with the same intensity . . . that I used to give the volt meter."

Have a good one! 
 
Jan 14, 2007 07:27 am
Re: Basic question about voltage

Do great minds think alike? Or do they think for themselves?

You know how sometimes when you are pondering a thing you might hear someone say something or you may just have an epiphany? Either way, everything becomes as clear as a bell.
I had pondered the flooded cell, lead acid, deep cycle battery for years. I understood all of its abilities, all of the math concerning sizing, soc, dod, longevity, just what a cycle was. Anyway to make a long story short. There was still a cloud of mystery about it all. Even after having a battery bank in my home with a PV source/battery monitor to gawk at there was still something missing in knowledge. Then I found it. In the pages of a high school chemistry text book. These batteries do not store electricity. They convert electric energy into chemical energy and store that. Then when electricity is needed from them they convert it back from the stored chemical energy.
At a price!
 Next time you get the gumption to learn something, might I suggest research on Micheal Faraday and overlapping principles?
So much emphasis has been placed on Thomas Edison here in America. Where its true he is credited with inventing the light bulb, he actually "won a race" to invent it! To my mind his greatest accomplishment was putting together several would be inventors in one lab and providing them with the means to experiment and create inventions which Edison took credit for. Nikola Tesla was one of them, but Edison scoffed at Tesla's ideas using alternating currents. The history books all say that Ben Franklin discovered electricity. I would say, what he discovered is that electricity can be conducted. A good place to start.

http://micro.magnet.fsu.edu/electromag/java/index.html
 
3 Posts
Jan 14, 2007 10:45 pm
Re: Basic question about voltage

watthour load/ watthour capacity (batts) = DOD%

it's that simple.

thus(under your stated conditions)
load=
200w x 1hr is 200wh

capacity=
200ah x 12v nom is 2400wh

200wh/2400wh = 8.3% DOD

now rough-in add'l 10% powerloss through inversion
about  9% DOD.

that said, congratulations on your ultra-efficient lifestyle of 200 watts! in corcord with other posts: voltage is only an indication of battery health. spend $20 on chinese load tester for an additional indicator.
 
3 Posts
Jan 14, 2007 10:54 pm
dear flood electrochem researcher

you sound like a guy (read guru)to ask:

is plate thickness a function of floodacid longevity?
is there a point of diminishing return?
a sweet spot thickness?


please see my $/cycle post

and THANKS!
 

Disclaimer and Disclosure

The Alternative Energy Store, Inc reserves the right, within its sole discretion, to refuse or delete any posting or portion thereof, or terminate or block the access to this forum.

The opinions and statements posted on this forum are the opinions and statements of the person posting same, and do not constitute the opinion or act of the Alternative Energy Store, Inc (AltE). The Alternative Energy Store, Inc does not endorse or subscribe to any particular posting. No posting shall be construed as the act or opinion of the Alternative Energy Store, Inc.

Click here for BBB Business Review

McAfee SECURE sites help keep you safe from identity theft, credit card fraud, spyware, spam, viruses and online scams
Desktop Website | Mobile Website

Share

Click on an icon to share! If you don't see the method you want, hover over the orange "+".

Feedback

What can we do to help you?

Please enter a summary
Sorry, the copyright must be in the template.
Please notify this forum's administrator that this site is using an ILLEGAL copy of SMF!
Copyright removed!!