Battery State of Charge

Apr 3, 2009 02:40 pm
Battery State of Charge

Most of the 12volts inverter cutout the battery at 10.5volts to prevent over discharging.
 
Based on the battery stage of charge i got it shows the following;
% of Charge  12V Battery
 100           12.70
  95           12.64
  90           12.58
  85           12.52
  80           12.46
  75           12.40
  70           12.36
  65           12.32
  60           12.28
  55           12.24
  50           12.20
  45           12.16
  40           12.12
  35           12.08
  30           12.04
  25           12.00
  20           11.98
  15           11.96
  10           11.94
   5           11.92
Discharged     11.90
From the above scenario what could be the fate of the battery that reaches 10.5volts before cutoff? At what percentage does 10.5volts stand for?
Help me out
 
220 Posts
Apr 3, 2009 06:28 pm
Re: Battery State of Charge

 
there are quite a few battery state of charge charts floating around. and many of them fail to mention that they are for "at rest" battery voltages at normal temperatures and others still are just plain wrong..one of the best i have encountered was put together by one of our own forum regulars and posted on his ever growing blog. lots of good info there from solar to cornstoves to gardening.

http://solarjohn.blogspot.com/2007/03/measuring-battery-state-of-charge.html#links

 you may have seen in a previous post some folks were talking about checking the AH capacity of a battery and made reference to the 10.5 voltage? (a drawdown test)..this is a controlled test for capacity and when done we are ready with our charger to start the recovery of the battery as soon as the test is completed. we do not want to use that 10.5 volts as a cutoff on a regular basis (ever) and some inverters have a manual adjustment so we can set our LVD (low voltage disconnect) to some higher setting.

 your right of course that most inverters have it set at 10.5 volts, but don't forget that is under load and the actual value (at rest) will be higher.

 bottom line is 10.5 volts is a very bad place to be with a lead acid battery and should only be brought to that voltage under a watchful eye for testing or by blunder.. and a charge should begin ASAP if we ever get that low.

best regards, dave
 
Apr 4, 2009 10:46 am
Re: Battery State of Charge

Thanks Dave,
That link has immensely improved my knowledge indeed on the misery of battery stage of charge.
Once again, Thank you.

Usman Kazaure
 
Apr 5, 2009 05:46 am
Re: Battery State of Charge

Most of the 12volts inverter cutout the battery at 10.5volts to prevent over discharging.


It all starts with knowing in advance what the anticipated power requirements are. Not what the batteries bottom line is. A well thought out and sized system should never come to this.

Although it is recommended by some that, battery to inverter cables be sized according to the maximum sustained wattage output of the inverter at the lowest possible voltage. Example - 1500 watt continuous at 10.5 volts equals 143 amps. This would be the amperage to start all other wire sizing computations from, such as line loss, for battery to inverter cables.

More importantly, a battery bank should be sized by a factor of nothing less than 2 times the expected highest amphours draw between recharge cycles. Personally I prefer nothing less than a factor of 5. This improves the odds that the low voltage cutoff never comes into play.
 
Instead of focusing on a batteries absolute bare minimum, focus on keeping it within, at least, the top 20% state of charge between anticipated recharge times. In other words "aim high." This is where a battery state of charge monitor, placed where it can be readily seen by all who use the power, is essential to system well being and your state of mind.

Lets say for example that all of your computations have told you that a battery capacity of no less than 440 amphours is required to get you through one night. Simply multiply that 440 by a factor of 2 or 5 or more. Of course budget may dictate this, but absolutely no less than 2. This will help to keep it in the top 50% state of charge. A lot will depend on the means by which the battery or batteries are recharged. Choosing the means of and, sizing that correctly will have a significant impact as well.

Think about the performer who is spinning a plate on top of a stick, he or she is not thinking about that plate crashing on the floor, he or she is thinking about keeping it spinning on top of the stick.Smiley

 

 
184 Posts
Apr 5, 2009 11:02 pm
Re: Battery State of Charge

Bottom line is that you need another way to remove the load.  A VCS (voltage controlled switch) will meet your needs.  The VCS can be wired to a relay which turns off the inverter when the battery voltage reaches a specific state of discharge. (You set the voltage value).   The second setting on the VCS is the voltage at which the inverter is once again be reconnected.  I have mine set up to reconnect the inverter ONLY when the batteries are nearly fully charged.  By doing that, I am avoinding damage that would occur by chronically undercharging my battery bank.  You can find the details here: 

http://solarjohn.blogspot.com/2008/02/ive-automated-my-off-grid-pv-system.html

I hope that helps.  And, thank you Dave for the kind words.

SJ
« Last Edit: Apr 7, 2009 10:11 am by John Dalhaus »
 
Apr 7, 2009 05:49 am
Re: Battery State of Charge

Sorry solarjohn but I have to respectfully disagree, adding another piece of electronic/electrical control equipment may be another option but "bottom line" it is not.

We don't have, nor have we every needed, an automated LVD on our inverter circuit, or any other circuit, and our batteries have never hit bottom! They have come close on a few occasional weeks of different years during winter months and, we didn't use a generator ether. (We live off grid and don't even have a generator.)
We did however make good use of our Bogart Tri-Metric monitor that is mounted in the living room wall by the front door light switches. When battery percent of full gets low (below 50%) we simply conserve more. Its become almost second nature and mind you, this is only during extreme weeks of some winters and we are not stumbling around in the dark or burning candles or kerosene lamps. Just a little less time on the internet, a little less time watching TV (stuff rots your brain anyway Smiley ), maybe all in one room reading instead of separate rooms, things of that nature.
I suppose, in all fairness, I should add that we had lived off grid without any type of electricity for over 25 years before we went PV. This could be a big difference also.
So far we have only had to replace a bank of Trojan T-105's but we got the better part of 8 years of household service from them. They are still in use, just not powering the home anymore. Time will tell how successful we are with the new bank of Surrette 530's. I hope to get at least 20 to 25 years of actual household service from them.

Living day to day in a household with electricity coming from battery power, is about, more than just battery voltage. One has to consider the number of amphours removed and how they are replaced as well as the number of cycles built into the battery and the relationship of shallow or deep cycling. Sure there is always going to be some gizmo or gadget for sale to do it for you or there is just plain good ole common sense. To me, saying "the misery of battery stage of charge." is like saying a cat is miserable with a mouse.
 
184 Posts
Apr 7, 2009 10:04 am
Re: Battery State of Charge

I respect your extensive off-grid experience Thomas, but the solution you suggest requires the user to constantly monitor battery voltage.  This may be a good strategy for those with big battery banks, but not so good for those of us with smaller ones.  I work away from home, and therefore don't monitor my batteries during the day.  I also needed to avoid sleep interruptions caused by Low-Voltage-Alarms.  For me, the voltage controlled switch (VCS) not only solved these problems, it allowed me to get more energy from my system, and it reduced my electric bill.  I suspect that Usman's system is also smaller than it should be, considering his loads, and that he might benefit from the solution I've suggested. He could, of course, buy bigger batteries, but not all of us can afford to do that.     
 
Apr 8, 2009 05:32 am
Re: Battery State of Charge

No, no your all wrong about us. We don't have to constantly monitor the battery state of charge. Sure, at first we did leave the battery monitor (in the wall by the front door light switches) set on "battery percent of full." But as time went by (maybe a year) we learned the "personality," for lack of a better word, of our system.
So now we, no more have to constantly monitor the battery state of charge than we have to constantly monitor a clock to know what time it is or a calender to know what season it is.
Now, if we want to pin point battery state of charge we can and, we do but thats no different that using a clock to pin point a time of day or a calender to pin point a day of the year.
We watch the weather like most folks I know. If its the middle of winter and the weather man is forecasting a week of overcast (sunless) weather, we check the battery state of charge and we just, know, how much or how little to start conserving power. Just like how you, knew, that overcast is synonymous with sunless. I didn't have to tell you that.

Can we agree that the battery monitor is a tool just like your VCS, each has its own unique purpose? By the way, what alarms? I didn't know we had a large battery bank. I don't know wether to feel special or feel like a glutton. Is 1600 amphours at 12 volts nominal, for a family of four; two adults and two teenage girls (who by the way have lived their entire life "off grid") living on a more or less self sufficient farmstead, considered large? Our amphour draw in a 24 hour period can flcuate from 100 to 400. Typically around 200 amphours. Which if you condiser a lighted billboard along the I-95 corridor that has; 2 - 400 watt MH lights that burn for 10 hours every night, thats 8 kWh's. In compaerison, we typically use 2.4 kWh's over a 24 hours period to power our entire place. I thought that would be considered small. Maybe not?
 
184 Posts
Apr 8, 2009 10:29 am
Re: Battery State of Charge

Thanks Thomas, your posts have helped me understand some of the fine points of off-grid living.  However, my situation is much different, and I believe that is why we have different priorities where equipment is concerned.  Unlike you, I am grid-connected.  My goals are to get as much from my system as possible, reducing my carbon footprint and my electric bill, and to have a backup source of electricity to serve in the event of a grid failure. 

My VCS helps me to accomplish these goals by cutting and reconnecting the load at precise setpoints.  Before I installed the VCS, I had to disconnect and reconnect loads manually.  In doing so, I was not using my system to its full potential.  On several occasions (when I was away from home or sleeping), battery voltage dropped to unacceptable levels.  To avoid low battery conditions late at night, I found myself disconnecting loads at bedtime.  According to my calculations, I am getting about 50% more energy from my system than I did before installing the VCS.

How much PV do you have?  What do you use for refrigeration, heat, and cooling?  You must be doing a lot of things right to get by on 1600 amphours of battery capacity.

John
 
Apr 9, 2009 06:36 am
Re: Battery State of Charge

1K, LP, wood, fans.
I wonder if Usman Kazaure Abdullahi is even reading these posts? I would have thought he would have chimed in by now. This is all for his benefit. Right?
His question...

From the above scenario what could be the fate of the battery that reaches 10.5volts before cutoff? At what percentage does 10.5volts stand for?

At the risk of sounding like a jerk, its dead. However it can be revivied and equalized again but the more this happens, the shorter its overall life span will be.
Batteries are built to have just so many "cycles."
The abstract is: 1 cycle = from 100% charged down to 0 and back to 100% - but if you take it down from 100% to only 50% and back up to 100% then your only using half a cycle; 100 down to 80 = 1/5th of a cycle. By increasing a battery banks amphour capcity by several multiples, over what is expected to be drawn from it in between recharges, recharge time is decreased and it lives a longer life.

Example - Lets say a battery is built with 1800 cycles and the demand on it is, every night its discharged down to 0 and every day its recharged back to 100%, in theory one would get 1800 nights of use from it or about 5 years. In the real world thought it would not last that long under those conditions. If the battery and load had been up sized by a factor of 2 then only half of the 1800 cycles will have been used in that same 5 years. By a factor of 5 and only one fifth of the 1800 cycles. So on and so forth and it lives a long healthy life. Hopefully.

Usman?
 
Apr 13, 2009 06:12 pm
Re: Battery State of Charge

I'm reading the posts for sure. i do not chimed in because im still learnig from your experiences, for you as an off gridder while John as on grid person like my self.
You need to understand that i fully relyed on utility power to charge my battery. therefore, John idea for CVS is also very imfortant cos i may not available always to monitor the battery myself.
 
Apr 17, 2009 03:09 am
Re: Battery State of Charge

Now this is where I need educating.
Let me see if I understand this correctly.

We are talking about living with electric utility power and utilizing photovoltaic power with minimal battery energy storage. Evidently there is a specific, dedicated load of some sort.
This load has the ability to completely drawn down the battery energy storage at anytime, even when there is no human presences. So this load while preforming some worthwhile task is also used as a means to maximize the PV gain each day but the CVS is utilized to keep the battery from being depleted entirely on an otherwise regular schedule, which we all know to be detrimental to a battery's overall life span.
Help me out here guys. Is any of this correct?


I sure will be glad when home scale, PV powered Hydrogen production becomes as much of an everyday household item as LP or Nat. gas. With of course the appliances to boot. But when a simple "tutorial" kit runs for almost 300 bucks! Maybe one day before this body is used up I'll be able to tell the LP man what he can do with his tank. Politely, of course. http://www.fuelcellstore.com/
http://www.icestuff.com/~energy21/electrolysis.htm
 
184 Posts
Apr 17, 2009 10:46 am
Re: Battery State of Charge

My dedicated loads are my refrigerator and a chest freezer.  I've been adding solar panels, and I've upgraded my battery bank a couple of times, as my budget permits.  I look forward to the day when I'll have enough capacity to run these loads entirely off of solar power. 

When I need to conserve, as in the case of a grid power failure, I don't run the refrigerator.  I keep the freezer going in order to protect frozen food, and I use an ice-chest for items needing refrigeration.  By doing this, I have enough capacity to run a few lights, and other household items.

I like PV because it's reliable, and requires little maintenance.  I would like to eventually eliminate my use of all fossil-fuels, but that is a long-range goal.  I've already installed a bio-fueled stove and made many energy-efficiency improvements.  Check my Gallery on this website, and my Blog for more info.  http://solarjohn.blogspot.com

John
 
15 Posts
Apr 17, 2009 09:20 pm
Re: Battery State of Charge

Hi Thomas Allen Schmidt,
 I really enjoy your blog posts and admire the fact that you have been off grid for as long as you have. I also have a scenario where I have a subpanel that I have wired to my inverter (Xantrex SW 4024). Currently I have the outlets in my home office (Computer) my freezer, refrigerator,lights and heating system (Radiant floor with solar hot water assist) tied to the PV system. See Dennis M Gallery. I am in Wyoming so we have good sun most of the time. I think that I save more energy staying tied to the grid but I have no way of knowing for sure. I think cycling the batteries consumes more power than staying grid tied. Any feedback would be appreciated.
Regards,
Dennis
 
Apr 19, 2009 03:18 pm
Re: Battery State of Charge

Oh, diffidently.
Battery energy storage is a luxury and not an all together inexpensive one, especially with todays metal prices. But then, to me, electricity is a luxury that I learned to live without. During that time I learned to live without electricity, there was time to reflect on how the human race as a whole lived without electricity. For thousands of years. Time to reflect on how we, the human race (having utilized electricity for only about the past one hundred years) could now, it would seem, not live without it. Very strange from my perspective.

Obviously the entire 6.5 billion people of the planet Earth cannot live as I do. Just look at the history of the East coast of the North American continent from the time of the first settlers up to now. Much has been lost forever in the way of flora and fauna from those earlier times. Did you know that the entire east coast was once blanketed with White Pines? Some that could rival the Giant Sequoias of California. Today there are only very small patch's of what could be considered virgin White Pine forrest left anywhere on the East coast. Most are just old growth that folks believe to be virgin growth from pre-colonial times.
 Anyway, Elk was once found in vast herds nearly as large as a county all up and down the East coast in pre-colonial times. The only Elk found here today were brought in by truck and are repopulating in restricted areas.

Progress, it would seem, can be measured in many different ways. There was a bit fiction I read once. Fantasy fiction actually. In these stories were a people who had evolved to learn the "magic" of the Land. There were those that could "forge" stone to their will and those that could "forge" wood and so on, the "magic" was limitless. There were the wise ones of old that also knew that if this "magic" was used wrongly it could destroy he Land forever. So, because of this, a University of sorts was created so that as these "magics" were discovered they were kept put away and closely scrutinized for hundreds if not thousands of years before they were allowed if ever to be utilized.

One could take this and apply it to mankind and what has been happening right here on the planet Earth. The human race has, over the centuries discovered new "energies" and have utilized them with blind abandoned and with devastating results. I wanted to believe that RE was going to create a new beginning out of that, sort of Phoenix from the Ashes as it were but there is something elusive about it all now. Something about RE that reminds of the same people that would go about it all with blind abandoned, all for the sake of money, bottom line. Not all people in the RE industry mind you but enough have been corrupted by greed as to yield to the will of the usurpers and they would blind all they could to all but their truths. They are after all, promoting the use of "energy." A rather broad term, don't you think? Kind of like "magics."

Oh! In case you were wondering the stories I refer to are of Stepphen R. Donaldsons, The Chronicles of Thomas Covenant the Unbeliever. A bit reminiscent of The Lord of the Rings but far enough away as to be entertaining in its own right.
« Last Edit: Apr 19, 2009 03:25 pm by Thomas Allen Schmidt »
 

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!!