12V Sealed Lead Acid Discharge Limit - Page 1
Forum section: electronics / projects

author: juanfermed
author_rank: Regular Contributor
timestamp: December 16, 2014, 06:12:11 am
content: Hi everyone,I am working in an already designed charge controller for sealed Lead Acid batteries. The charge controller has as an input a solar panel that is always connected. The batteries I need to charge with the charge controller are 12V 12Ah up to 12V 80Ah. But there is one problem that I have been dealing with for 5 days now and I am in the same point as the beginning: nowhere.The problem is that I must decide what is the optimum battery voltage at which the charge controller should disconnect the battery so that it is never discharged to less than 50%, or at most, 45%. The reason for this is that a depth of discharge of 50% would give us the battery life we need to achieve. But I cannot select an appropiate voltage given the uses of our system. I have found in all literature I have reviewed that the relation between battery voltage and State of Charge (SoC) is very current dependent. I am using as reference the curves in page 68 of this document:http://www.zetatalk4.com/docs/Batteries/FAQ/State_Of_Charge_Ver_Voltage_2004+.pdf, because we dont have data of our own batteries. But besides that, it shows perfectly well how different are the 50% SoC voltage for a C/3 and C/5 discharge rates. Choosing one would disconnect the battery too soon (SoC of 75% ) or too later ( SoC 30%).The charge controller disconnect voltage threshold can be programmed to any value, but with a tolerance of +/- 0.2V. That is the best that the chinese manufacturer can guarantee, which is annoying.The discharge rates (output current) at which the batteries are subjected are the following:For the 12V 12Ah battery: C/6 (around 1.94A) for around 1 hour/day, C/13 (around 13A) for 4 hours/day maybe, and C/300 (around 40mA, which can be ignored because at this low current the panel would be effectively charging the battery) the rest of the day.For the 12V 80Ah battery: C/3 (around 24A) very ocassionaly (5min for 3 times/day) up to 1 hour/day, C/17 (around 17.2A) for around 2 hour/day, C/45 (around 1.8A) for around 4-5 hours/day and C/95 (around 0.84A which can again be ignored for the same reason above) the rest of the day.From various sources I have get to know that a discharge voltage of 1.85V per cell (6 cell in 12V), which corresponds to 11.1V, should not be reached, so at least now I know a lower boundary, but if, for example, I select the disconnection voltage threshold to be 11.3V +/-0.2V to get the lower 11.1V, this would only work for the higher discharge rates (C/3) and would be extremely to low for the other discharge rates, which account for most of the time. If in contrast, for example, I choose 12V +/-0.2V, this would more or less work for the medium discharge rates C/13 and C/17 but would not work at all for the higher discharge rates. In both cases, the tolerance of voltage can corresponde to +/- 20% of the SoC which is simply to much.At his point I have viewed so much information, some of them related, some not, but I am basically nowhere. Do you any have experience or ideas with this type of batteries? This is the first time I work with them, and I am not sure about what I find over the internet. Any help will be greatly appreciated and any doubt that arises I would be glad to answer.Sorry for the long post, but I wanted to share or the relevant information to not fall in the mistake I have fallen before of not providing enough information. Thanks a lot for all your help guys! Gates Energy Products, Rechargeable Batteries Applications Handbook, EDN series for design engineers. Page 175.https://books.google.com.gt/books?id=JWL-t4syQ_UC&printsec=frontcover&dq=rechargeable+batteries&hl=es&sa=X&ei=7MmPVLf4BrSOsQT494C4CA&redir_esc=y#v=onepage&q=rechargeable%20batteries&f=false

author: MK
author_rank: Regular Contributor
timestamp: December 16, 2014, 07:26:15 am
content: There are also different battery chemistries that are more or less resistant to deep discharge, calcium or antinomy can be used as an alloying metal. Changing to a deep discharge resistant battery might simplify the limit you are able to set

author: Jeroen3
author_rank: Super Contributor
timestamp: December 16, 2014, 08:33:17 am
content: Laptops keep track of battery capacity by measuring the amount of power.Batteries are specified by an amount of power. Or current per hour, which is equal to power at nominal voltage.A voltage/capacity curve is difficult. You'd need to take into account the actual current, age and temperature of the battery to get an accurate reading.For example, a 10 Ah battery is can give you 10 Amps for an hour, it'll be 50% after 30 min on 10 Amps. But it will not give you the nominal voltage when you do that.

author: juanfermed
author_rank: Regular Contributor
timestamp: December 18, 2014, 05:15:52 am
content: @evb149 Thanks for the welcomingYes, I think I will have to learn to live with the inaccuracy and uncertainity of any calculation I can make. And, well, nothing can be done to the current system to remedy the situation or make less worse: these things are already made and the price/schedules already been established. We are in the last situation you mentioned: different discharge rates so trying to have an "average" discharge rate just dont make sense and not a logical/good threshold value an be obtained.@MK We are using Sealed Lead Acid AGM deep cycle batteries, and we are aiming to get the best lifetime of the battery, i.e. around 2 years or more, and the best usage out of the battery, i.e. around 5 hours at the different discharge rates / currents I mentioned in the first post.@Jeroen3 Yes, exactly, what you are saying is correct and the cause of the problem. The charge controller we are using right now can only measure voltage of the battery, the voltage vs state of charge current is very discharge rate dependent. Even the capacity is very discharge rate dependent. You can take as example, the following technical manual:http://www.power-sonic.com/images/powersonic/technical/1277751263_20100627-TechManual-Lo.pdfin page 5 you can see the capacity vrs discharge rate variation.Maybe some more specific questions are:What are the physical quantities for a lead acid battery in cyclic use that can modify or increase battery life?What would be the relationship in numbers/graphs?How are the systems that use lead acid batteries designed to increase battery lifetime depending on the discharge rates at which it is exposed?Can anyone recommend a cut off voltage or give some highlights about how to estimate it?Any recommendation about what to do or how to carry out the calculations?Thanks everyone for your input.

author: mtdoc
author_rank: Super Contributor
timestamp: December 18, 2014, 05:38:35 am
content: You are asking the right questions. There is a lot to learn about deep cycle lead acid batteries.HEREis a good place to start.

author: MrAl
author_rank: Super Contributor
timestamp: December 18, 2014, 02:59:20 pm
content: Quote from: juanfermed on December 16, 2014, 06:12:11 amHello,It sounds like you are having a problem figuring out the capacity for a given load current so that you can estimate the state of charge.First, i dont think the state of charge can be calculated exactly from the voltage, but knowing the battery helps here too, and if the batteries are all the same or different. The main idea is to measure the voltage, then calculate the approximate state of charge knowing the voltage and what is called the Peukert factor (or "P" factor for short).The P factor tells you how the discharge rate affects the capacity, so you would get two different capacities at say 1 amp and 10 amps even though the battery is the same. Each battery has it's own P factor, so you'd have to either test each battery or get test data from the manufacturer. The test data from the manufacturer would have ampere hour capacities at different discharge rates from which you can compute the P factor for each battery model. Knowing the P factor, you can then estimate actual capacity. I think that's the best you can get unless you want to test each battery.

author: juanfermed
author_rank: Regular Contributor
timestamp: December 22, 2014, 02:32:52 am
content: @mtdoc Thanks for the link, I will go through it.@MrAl Hi, thanks for the insight about the Peukert factor, I will investigate about that and try to figure what can be done. Yes, knowing even a good approximation about the capacity of a battery only knowing the voltage seems not possible. Its voltage- capacity relationship is very dependent on the discharge rate, and also non linear, which introduces more complication when trying to select a voltage that, for example, corresponds to 50% of charge for any discharge rate. Add to that this relationship varies also non linearly with discharge rate. It is simply not possible.The thing is that without access to the discharge rate it seems there is no good answer. And because the so different discharge rates that the battery will experience, any threshol voltage that was selected presented a tradeoff between disconnecting at too low State of Charge or too soon for the user. And the optimum point was very bad: disconnect at ~20% of charge at the lowest discharge rates, and disconnect at still 70% of charge at the highest discharge rates. The final answer, which was 11.9V (for the 12V 12Ah battery) came from assuming the worst case discharge rate that would be the more common use, so the more probable discharge rate that will make the battery reach finally 50%. Of course, this presented the problems about too late or too soon disconnection for other discharge rates.An approximation I think can help and comes from the analysis of several graphs of batteries and trying to understand their general behavior, and is to see how the how battery behaves under different constant discharge rates while measuring its voltage in the vertical axis and the time in the horizontal axis. This curve is presented as "Discharge characteristics" in many datasheets. It is possible to relate in such a graph the voltage- usage time-current and state of charge (via constant current -amperes- * time -hours-). See, for example . Also important is to have how the capacity of the battery changes with respect to discharge current (page 5). The procedure can be:1) Obtain the current - capacity data from the manufacturer for your battery (its Ah)(of course, this was not available in my case so I took the information of Power Sonic technical manual ).- In this case I will use the 12V 12Ah from powersonic PS-12120L 12 Volt 12.0 AH.2) Plot the data. It is fairly linear, so a good linear equation to approximate the capacity for other discharge currents can be obtained , i.e. the discharge currents that apply in your application.My equation is: C = -0.684*I -12.232 where C: capacity {Ah}, I: discharge current {A}.3) Approximate the capacity of the battery for each of the discharge currents you devices work at with the equation you got in step 2.For example, one of my discharge currents is 1.94 amps: C = -0.684*(1.94) +12.232 = 10.91Ah4) Decide what percentage of the battery you want to use. In my case it was 50% (I dont know the numbers or equations, but higher capacity discharge, lower life of battery).5) Multiply the capacity you got in step 4 by the percent you want to discharge, this will give the total capacity you will use.In this case: C_usable = 10.91Ah*0.5 = 5.45Ah6) Divide the number you got in step 5 (C_usable) by the discharge current, and this will give the total hours you can use your battery with that discharge current to drain only 50% of the charge:In this case: Use_time: C_usable/DischargeCurrent = 5.45Ah/1.94A = 2.81h7) Take that number and match it to a voltage for the voltage-time graph in the "datasheet" of your battery for the discharge current used. See "Discharge Characteristics" curve in . Of course a graph for 1.94A is not there, but using your imagination, you can try to see what voltage corresponds to 2.81h in an good approximated curve for 1.94h.In this case: the number is around 12V and 11.9VRepeating the steps described before for all the discharge currents you need, will give you several values, and from there on you can try different things like making an average, deciding which case is the most important or something better. In my case I decided which discharge current was more important or "probable to be good".Another approach:Another easier way to look at this might be to see the "Discharge Characteristics" curves and, taking into account that most of them are in logarithmic scale, see discharged capacity for each curve of interest by multiplying the constant current of interest by the time in the horizontal axis, and then getting to the discharged capacity of interest (50%, 30% any number your are interested) and looking at the voltage of interest. This might be more helpful for cases like mine, in which you have to consider several different discharge rates.Does any one have a comment or sees a fault or problem with these approaches? Any other recommendation?Thank you guys.http://www.power-sonic.com/images/powersonic/sla_batteries/ps_psg_series/12volt/PS-12120L_11_Feb_21.pdfhttp://www.power-sonic.com/images/powersonic/technical/1277751263_20100627-TechManual-Lo.pdf

author: 3roomlab
author_rank: Frequent Contributor
timestamp: December 22, 2014, 04:13:23 am
content: maybe use some kind of battery capacity minder that "remembers" the charge/discharge over time instead of just voltage/current. isolate the batteries and build a profile of each 1 using the " battery minders", set each battery to its own minder. (sounds like something arduino-ish lol, maybe a spin off from this version?http://www.instructables.com/id/Arduino-True-Battery-Capacity-Tester-Li-IonNiMH/)ultimately, voltage/current levels are affected by active loads in an always connected solar dc system , it may or may not trigger premature cycles of charging when loaded heavily due to voltage sags (but then again, it could be increased internal resistance due to aging).

author: theoldwizard1
author_rank: Regular Contributor
timestamp: December 22, 2014, 04:39:52 am
content: If you are this concerned about maximizing battery life by minimizing the depth of discharge, buy a commercial product !Make sure that is can be programmed for battery chemistry, battery type (flooded, valve regulated, AGM, etc), battery construction (standard vs heavy duty plates) and all of the other parameters you have researched. They do exist. They are expensive because (hopefully) they have been tested.

author: Psi
author_rank: Super Contributor
timestamp: December 22, 2014, 04:48:05 am
content: I would throw a coulomb counter battery gauge IC at the problem and be done with it.When the IC says you've used 50% of the capacity, trigger the cutoff.You can even do some intelligent monitoring of the battery volts with respect to battery capacity left and try to estimate the batteries true "full" capacity (since it will reduce over time).That way your not cutting off at a hard limit of "official capacity / 2" but rather the real capacity of the battery / 2.(Some of the more advanced coulomb counters may do this themselves)