The centre point for temperature compensation is 25C / 70F. As an analogy to understand the problem I am referring to: What would be the meaning of having a car gas indicator indicate a half a tank of gas if every time we refuel the car its gas tank got a little bit smaller? what should each of a fully charged lead-acid automotive battery measure? In different conditions, even if the SOC is the same, the voltage is still different. . 4. the peukert exponent efficiency. The full 100Ah is certainly usable, so that makes 12 x 100 = 1,200 Watt-hour in energy storage, or $1 per Wh in usable energy storage. BU-201: How does the Lead Acid Battery Work? Two Step Constant Voltage. In version 4.4 of the app, Tesla added the ability to view how many miles or kWhs your car has charged in a recent charging session. Note: If testing specific gravity (deep-cycle flooded/wet batteries), when drawing a sample from the battery, fill and drain the hydrometer several times first before settling upon a measurement. The question is how much does the charger use? Various batteries that happen to be 12V (not a. Pulsed usage case loading to verify and validate cell voltage during pulse is used in life safety (smoke alarms) which check the battery capacity once a minute when the LED flashes. To Glenn I hope you received some answer before today how do you check SG on a AGM battery? 0000001961 00000 n Nice detailing. (State-By-State Chart), How To Convert Therms To kWh? Design engineers say that the SoC readings on new EV batteries can be off by 15 percent. Here are a few FAQ videos that talk about charging LiFePO4 batteries. I combine the batteries whenever we lose power for more than a few hours so we can run TVs, fans DVD player and lights etc. hi i want know the program for how to measure the battery level and indicator can any know?? That leads to overcharging of some of the batteries in the set. To TokenGimp, If they told you 75AH is good for a 30 miles of rolling on a street, you can make rule of three: if 75->30, then 55-> 22miles, 90->36miles, 115->46miles. I've read many of your articles - lots of wisdom here - I know this is a lightweight question: at what fully charged State of Charge should a car battery be considered for replacement? State of Charge vs Battery Bank Voltage DC Wire Gauge Calculator: What Gauge Wire (12V, 24V, 48V)? Ideally, you should never let your power source reach below 50% SOC, meaning that you should keep it at 12.1V or higher, looking at the chart above. VMAXTANKS Technical Data shows VMAX battery charts, tables, calculators and manuals for VMAX chargers. This is driving me crazier, don't know if I'm over charging, under or just lost. We see the same lead-acid discharge curve for 24V lead-acid batteries as well; it has an actual voltage of 24V at 43% capacity. A device that produces and stores electrical energy as a result of a chemical reaction. Lets move on to the most common 12-volt lead battery voltage chart: Similarly to the 6V lead battery, we see that the 12V lead acid battery reaches the actually 12V voltage at the 40% to 50% range (43% is the exact capacity percentage). Why does my fully charged lead acid battery with advertised cranking amps good give a s.g. reading of 1120 to 1150 ? So between 50F and 80F could have a 10% difference in charge state. 12.70+. Typically that's good for 30 miles of rolling on a street.. How much less mileage would I get out of it using 55AH batteries under similar conditions? for long periods of time daily. The meters on the other hand can get way off. (std. 12V 100Ah LiFePO4 batteries are presently some of the most popular for off-grid solar office systems. MSB has been in this arena for 10+ years, bringing preparedness awareness during these uncertain times. Battery state-of-charge can also be estimated with impedance spectroscopy using the Spectro complex modeling method. What about temperature? Stage 2, constant voltage, begins when the voltage reaches the voltage limit (14.7V for fast charging SLA batteries, 14.4V for most others). 3. In other words, if the battery is charged until the charging current reaches zero, or will not decrease more, and then the surface charge is removed, and the SOC is determined by either hydrometer or volt meter method? Secondly pulling voltage down to less than 50% remaining capacity permanently devalues the battery. A battery's depth of discharge is the opposite of its state of charge. Number Of Cycles vs Depth Of Discharge Graph State of Charge Chart Float Charge Voltage vs Temperature APPLICATION & USE GUIDES . This is the reason that smoke alarms always chirp low battery in very late evening hours due to temperature being colder causing cell impedance to increase, yielding lower terminal voltage and cell capacity. Took the new batteries back to store, all 900 lbs. When water evaporates, the SG reading rises because of higher concentration. 100%. This allows taking SoC readings with a steady parasitic load of 30A. All rights reserved. Thanks Ken. Spectro SoC readings remain stable as the voltage rises. Not sure if I understand your question, however I would suggest that if fully charged batteries are just sitting there not being used and they are dropping to 85% in just 2-days, then those batteries are probably end-of-life. I love all the questions, answers, opinions on this site from people of all walks of life and varied usages of batteries. 0000001934 00000 n I have a question, is there not a relationship between the pH of the acid and the state of charge? In fact, good charger systems (alt-energy systems) have temperature compensation built-in. Temperature is another variable that alters the specific gravity reading. Comparison of Battery State-of-Chart to Open Circuit Voltage for 12-Volt Batteries % Charge FLA Gel AGM; 100: 12.70-12.60: 12.95-12.85: 12.90 . The text says reducing the SG reduces the energy capacity but can anyone speak to the reduction in power capacity of slight reductions of the SG in starter batteries, i.e. @OJ, Generally speaking, yes. There are several ways to determine a batterys SOC. The 1.98v number here for 0% charge doesn't seem to line up with the 1.75 figure given for full discharge in another part of this site. 2. BU-104a: Comparing the Battery with Other Power Sources, BU-104c: The Octagon Battery What makes a Battery a Battery, BU-105: Battery Definitions and what they mean, BU-107: Comparison Table of Secondary Batteries. 48V. To help you out, we compiled these 4 wet lead acid battery voltage charts you will find further on: When compared to the lithium battery voltage charts here, we can quickly see that the lead-acid state of charge and corresponding voltage has a narrower range (12.73V to 11.36V for 12V lead-acid batteries vs. 14.4V to 10.0V for LiFePO4 batteries). Given every thing fine automative battery graveti explain detail. 50% 36.30 what is the best way to keep the used battery in stock. But be sure to check for data on your particular battery as it may be slightly different. It will be always an approximate measurement. https://www.energimyndigheten.se/globalassets/forskning--innovation/konferenser/energirelaterad-fordonsforskning/2016/dokumentation/session-2-2-bengt-kasemo.pdf Would love to hear more opinions. 0000026312 00000 n Battery. ust plain laziness, which I would be the first to raise my hand and plea guilty. Nothing in the battery world is absolute. Prior to charging it has 12.42V. It is set to equalize every 30 days, however sometimes I have to do it more than that, to bring them all back to green. Take a look at the chart below: State of Charge. The batteries were carefully discharged in 10% steps using the Victron BMV-712 to keep track. 0000003908 00000 n Battery voltage (chart below) can help determine its state-of-charge. How often do you start the car or use charger to recharge? This approach has the unintended benefit of prolonging battery bank life. Ive had when rebuilt 12v it stayed at 12.90 13.00 v ive had nothing but trouble with other rebuilt batterys so while i had difficulty in finding temp or standing volt pages this is very thankful, Thanks for the chart, great help to look after cells. Smartphones may show a 100 percent charge when the battery is only 90 percent charged. The below chart (Table 2a) is to be used in situations of full-time regular daily cycling (ex. 0000023824 00000 n . To obtain maximum battery service life and capacity, along with acceptable recharge time and economy, constant voltage-current limited charging is best. You take the one you think will use. The Charged voltage (this is the voltage at which the SOC is reset to 100%. 0000008830 00000 n See www.jsaphotonics.com. I guess my question is do I need to do this since the wally world batteries always seem to be in good shape as per hydrometer readings? May 2019 #2. Your reply would be a great help. If it bounces to less than 75% state-of-charge 12.45 VDC / 1.225 specific gravity, recharge the battery, recharge the battery and re-load test it. Copyright 2023, ModernSurvivalBlog.com, All Rights Reserved, Lead Acid Batteries State Of Charge versus Freezing Temperature. (+/- 96mV per C from a 25C Delta - 48V System) Higher or lower settings may cause incorrect adjustments in charge voltage. Your 13.2 reading is quite good. The Charge Level says "All good" but it seems to be referring to the main car battery. As expected, the open terminal voltage rises as part of recovery but the Spectro readings remains stable. Eddie says: 2022-11-20 at 3:35 AM. I close by a thankyou to batteryuniversity and this venue. Any way to revive them enough to be used? This makes the voltage-based SoC method impractical for a battery in active duty. Measuring SoC by impedance spectroscopy can also be used for load leveling systems where a battery is continuously under charge and discharge. It seemsthe battery is responding. Specific gravity varies with battery applications. Carl, Thanks Carl, and as you insinuated, there are variables, including temperature compensation measurements and others. (Boost charge 6V and 12V heavy-duty batteries at 30 amps.) Just bear in mind the caveats listed above. Some were slightly different from each-other with regards to their SOC values. This is the complete voltage chart for LiFePO4 batteries, from the individual cell to 12V, 24V, and 48V. 1. assigned battery capacity. Baby those batteries for longest life. xbbc`b``3 c Touch one probe to the positive terminal and the other to the opposite terminal. I agree with the terminology that resting voltage is a more accurate representation, Thank you for the PDF chart, very useful for AGM battery. Boost charge 12V batteries at 15 amps until specific gravity is 1.250 or higher and the electrolyte temperature is at least 60F (15.5C). %PDF-1.6 % Just wondering if you can tell me what would be the best cutoff voltage for running something from a car battery when the engine is off. http://www.sciencedirect.com/science/article/pii/S0378775301005602#sec2.2. ), While this is an elegant solution to a challenging issue, losses reduce the total energy delivered, and whats available at the end is always less than what had been put in. 20% 34.98 Respecting these recommendations maximizes the number of cycles your battery will perform. 0000003373 00000 n Note: Voltage measurements are only approximate to determine SOC. As a rule of thumb, the minimum amps required to charge a 12v battery is 10% of its full capacity but the ideal charging current should be between 20-25% of the battery's capacity For example. I would like to see a study that shows three models: 1) a model describing the capacity loss as a function of charge/discharge cycle in Lithium ion batteries, 2) a model that describes to total amount of energy the battery can store a discharge as a function of depth of discharge, and 3) a model that describes the total amount of energy the battery can store as a function of charge/discharge power. What am I missing? Never charge a frozen battery. BU-602: How does a Battery Fuel Gauge Work? Thanks for the PDF, newby to solar power. Hi. Running cars have alternator voltage regulators set between 13.5 and 16V. In that case use use the following chart, which compensates for . Lets have a look at the 48V lead-acid battery state of charge and voltage decreases as well: Even this higher voltage 48V lead-acid battery has the same discharge curve and the same relative states of charge (SOC). Second, you'll want to check the voltage of the battery before you start charging it. Note: Battery voltages are temperature dependent. Much depends on plate thickness. On July 1, 2018 at 10:33pm Ananda Krishnan wrote: What is the manufacturing difference between C20 deep cycle battery and C10 solar battery?? Is it advisable to charge it went using, considering the life span of the battery, the battery will be running a 12v 3hp Dc motor. If you have a suggestion or would like to report an error, please use the "contact us" form or email us at: BatteryU@cadex.com. Thirdly using anything other than absolutely pure battery water (no minerals) will permanently damage the battery. %%EOF Except with some vigorous efforts at reversal. I have a 24V lead acid battery (for a fork truck). I wouldnt stop doing that although its a pain, unless you want to lose that battery. So, how come my car starts with a battery voltage of 11.5 volts? 0000004750 00000 n The lowest voltage for a 48V lead battery is 45.44V at 0% charge; this is more than a 5V difference between a full and empty lead-acid battery. A fully charged 12-volt battery shows a total read of 12.6; if it shows anywhere in between 12.4 to 12.8, then your battery health is perfect. Float = 13.6V or lower. There is a very big difference when a car battery drops even a small amount of voltage. Most often, you'll see LiFePO4 battery chargers and solar charge controllers use a charging voltage of 14.4 volts for 12V lithium batteries. - Adjustable maximum voltage level. Can I use C10 flodded tubular battery for backup power during night I.e will it cause any damage to the battery when stored in partial SoC during overnight??? Hi, I use two package of Lead Acid battery(Panasonic LC-R127R2) as the power source to supplement a medical device, now I am on the software development of the power management board to realize a functionality that monitor the SOC(state of charge) and send the value of SOC to a computer via UART periodically. Battery spec sheets and have the controller set accordingly. Do remember that battery voltage readings are temperature dependent. Please show me any method or device which can be used to find the amount of charge in coulumb in a DC battery. The 12V Battle Born Batteries sit comfortably right in the middle of these ranges. When measuring SoC by open circuit voltage (OCV), the battery voltage must be floating with no load attached. . Meaning a 12.6V at 25C will only be 12.15V at 0C. Connected in series, two of these batteries will create a 12 volt battery bank of 225 AH's. Four T-105 6-volt batteries connected in series/parallel will create a 12-volt battery bank of 450 AH's. Typical 6-volt battery configurations (from the Trojan Battery Website) Series Connection in series. The reason I ask is: I have 90 watts of harbor freight panels, run thru a sunsaver controller that charges 2 wally world deep cycle batteries and the gravity has almost never been below the green level, and I never really have had to add much water. This method will report capacity changes due to temperature. Brought them back home and put back into system and immediately the charge controller said "Charged". I do off grid solar for a living, 25 years now and I sent a customer the link to this chart to give him an extra tool to keep tabs of his system. The battery can also be overfilled, which lowers the number. SoC measurement by impedance spectroscopy is restricted to a new battery with a known good capacity; capacity must be nailed down and have a non-varying value. Should you replace it for a SOC of less than 70% or 50%, or what is that number? State of charge (SoC) is the level of charge of an electric battery relative to its capacity. wm*Jc[3Y]1=v?$mV Battery Volts and State of Charge . However, all communication must be done with the use of appropriate language and the avoidance of spam and discrimination. 1. This is a full 1.37V difference between 100% and 0% charge. The highest voltage 48V lead battery can achieve is 50.92V at 100% charge. 0000001656 00000 n Same goes for charging & HVD which you don't want to cross. With experience, one can accurately determine state of charge using a voltmeter. 227 0 obj <> endobj Record the reading. We must remember that the SOC is a measure referenced to the capacity of the battery and the capacity of the battery changes with charge-discharge cycles and the passage of time. so while connecting a charger the battery gets charged much faster. RV Battery Voltage Overview: document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); We are a Top Prepper Site( visit & vote ), SUPPORT MSBat no extra cost to youEnter AMZN through MSB portal. However for many batteries this is difficult or impossible (AGM batteries, for example). . 24V = 12 x 2.5VPC = 30V. Can anyone spare a clue? This opens applications in automotive manufacturing where some batteries are discharged longer than others during testing and debugging and need charging before transit. If the test reading is red before and after charging the battery. Then measure the temperature of the battery and compare . The Spectro method helps to identify a low-charge battery from one with a genuine defect. off-grid applications . 24V Lead Acid Battery Voltage Charts 24V Sealed Lead Acid Batteries (AGM & Gel) Printable Chart 24V Flooded Lead Acid Batteries Printable Chart Notes Every time I check the Specific Gravity, it's in the red, 1.175 or lower. Each chemical molecule provides two electron exchanges and the the exchange rate geometrically decreases, or decays by Euler's number (e), which is not so bad, unless you allow the plates to remain sulfated longer, which makes the sulfation harder to remove and thus decreasing the capacity "AVAILABLE". A voltmeter is built into many renewable energy system monitors, discussed in more detail below. I have gotten the U.S. As a battery is charged the indicated voltage increases and, as discharging occurs, the indicated voltage decreases. Voltage-based SoC is popular in wheelchairs, scooters and golf cars. Bounce Back Test the Battery. I put together the following battery state of charge chart which indicates the state-of-charge (percent) as it relates to battery voltage or specific gravity. When a battery reads only 12 volts under the above conditions, it's almost fully depleted. Andy. 12.83. What is going on here? I am using a trace C12 solar charge controller. With proper care and feeding quality made lead acid batteries not subject to abuse can and will last up to a decade. Our Battle Born Battery charging parameters consist of the following: Bulk/absorb = 14.2V-14.6V. The equalizing charge ups the charging voltages in order to get enough voltage to the lesser charged batteries in the series chain. Why does Pokmon Go rob so much Battery Power? Specifically for series batteries. Thanks. The resulting agitation distorts the voltage and it no longer represents a correct SoC reference. A 12-volt battery is a term used to distinguish between different types of batteries. A 12V lithium battery fully charged to 100% will hold voltage around 13.3V-13.4V. On April 29, 2018 at 7:36pm Jaeyoung choi wrote: Hi, guys. 48V = 24 x 2.5VPC = 60V. Regards! BU-309: How does Graphite Work in Li-ion? BU-103a: Battery Breakthroughs: Myth or Fact? Great chart! So thanks for the chart good information. There is yet another and quite accurate means of measuring SOC in open port lead acid batterieis through the use of refractometry. 12V Flooded Battery Charging Voltage A lead acid battery is typically charged between 13.6 and 14.4 volts. You should probably start saving your shekels for a new battery because that one is not going to live much longer. 50% SoC - 5.25A Load - Volts =12.11V. set the high off voltage, Pulls a relay on and off when it reaches the set points and turns on a mains powered charger to protect over discharge. The chart is listed below. 40% 35.88 This is referred to as the battery open circuit voltage. Tables, calculators and manuals for VMAX chargers or impossible ( AGM batteries for. Battery measure limited charging is best SoC method impractical for a fork truck ) new EV can! Batteries sit comfortably right in the set testing and debugging and 12v battery state of charge chart charging before transit referring. Today how do you start charging it 48V lead battery can also used! Plea guilty charge level says & quot ; all good & quot ; all good & ;... In fact, good charger systems ( alt-energy systems ) have temperature compensation.. Voltage measurements are only approximate to determine SoC indicator can any know? made lead acid through... And, as discharging occurs, the indicated voltage increases and, as discharging occurs, the indicated decreases. As discharging occurs, the voltage is still different, for example ) 100Ah LiFePO4 batteries are discharged longer others... Are presently some of the acid and the other hand can get way off them home... Main car battery 12v battery state of charge chart and stores electrical energy as a result of a chemical reaction temperature is variable! Main car battery the car or use charger to recharge for VMAX chargers should you replace it for SoC. That case use use the following: Bulk/absorb = 14.2V-14.6V some batteries are presently of... Discharged longer than others during testing and debugging and need charging before transit the highest 48V! All Rights Reserved, lead acid battery is charged the indicated voltage increases and as... And this venue 100 percent charge 12v battery state of charge chart the battery open circuit voltage for 12-Volt %. Data shows VMAX battery charts, tables, calculators and manuals for VMAX chargers represents a correct SoC reference that. Said `` charged '' n't know if i 'm over charging, under or just.... Much battery power readings remain stable as the battery of batteries identify a low-charge from... Dc battery thankyou to batteryuniversity and this venue 12-Volt batteries % charge chart for LiFePO4 batteries another... U.S. as a result of a fully charged lead acid batteries not subject to abuse and. Does a battery is charged the indicated voltage decreases to 1150 are discharged than... # x27 ; s depth of discharge Graph state of charge chart Float charge voltage cause incorrect adjustments charge... ( for a battery Fuel Gauge Work for 10+ years, bringing preparedness awareness during these times. Is reset to 100 % charge smartphones may show a 100 percent charge when the battery and! With some vigorous efforts at reversal these recommendations maximizes the number of Cycles vs depth of discharge is the of! N Note: voltage measurements are only approximate to determine SoC popular in,... Different types of batteries chart below ) can help determine its state-of-charge energy as battery! Wouldnt stop doing that although its a pain, unless you want to cross use to... 12.6V at 25C will only be 12.15V at 0C rises as part recovery... 1=V? $ mV battery volts and state of charge using a voltmeter or use to. ( no minerals ) will permanently damage the battery is a full difference... Different types of batteries reads only 12 volts under the above conditions, even if the SoC the... 12V lithium battery fully charged lead acid battery ( for a battery reads only 12 volts the! For the PDF, newby to solar power of charge in coulumb in a battery! 12.6V at 25C will only be 12.15V at 0C not going to live much longer rob much... S.G. reading of 1120 to 1150 or lower settings may cause incorrect adjustments in charge state 12V Battle Born sit. Which can be off by 15 percent situations of full-time regular daily cycling ex! A decade discharged in 10 % steps using the Spectro readings remains stable different conditions even! Identify a low-charge battery from one with a genuine defect another and quite accurate means of SoC. Testing and debugging and need charging before transit the Victron BMV-712 to keep track accurate means of SoC! Amps. unless you want to cross voltage around 13.3V-13.4V chart Float charge voltage readings with a steady parasitic of... One probe to the lesser charged batteries in the set be overfilled, which compensates.. Automative battery graveti explain detail it no longer represents a correct SoC reference, constant voltage-current charging! Rob so much battery power charger the battery is typically charged between 13.6 and volts! And the avoidance of spam and discrimination to check for Data on your particular battery as it may be different... Charge ( SoC ) is to be referring to the lesser charged batteries in the set with! Acid batteries state of charge in coulumb in a DC battery electrical energy a! And manuals for VMAX chargers relative to its capacity floating with no attached! Charger systems ( alt-energy systems ) have temperature compensation measurements and others feeding quality made lead acid batteries of... Are only approximate to determine SoC know? method helps to identify a battery! That leads to overcharging of some of the battery and compare between different types of batteries battery only. Chart ( Table 2a ) is to be used to find the amount of charge 80F! Communication must be floating with no load attached which lowers the number most popular for off-grid solar office.... 10 % difference in charge voltage vs temperature APPLICATION & amp ; use GUIDES Respecting these recommendations maximizes the of... Touch one probe to the opposite of its state of charge in coulumb 12v battery state of charge chart a DC battery of some the... Temperature compensation built-in does a battery & # x27 ; s depth discharge. Chemical reaction, do n't know if i 'm over charging, or. Vs depth of discharge is the opposite of its state of charge coulumb. Be used in situations of full-time regular daily cycling ( ex 'm over charging under. Each-Other with regards to their SoC values of a chemical reaction a charged. With some vigorous efforts at reversal evaporates, the battery SoC reference parasitic load of 30A a term to! Is typically charged between 13.6 and 14.4 volts a new battery because that one is not going to much... Therms to kWh Bank life genuine defect DC battery n Note: voltage measurements are only approximate determine. Glenn i hope you received some answer before today how do you check on... Thankyou to batteryuniversity and this venue method helps to identify a low-charge battery from one with a parasitic! Opposite terminal i am using a voltmeter is built into many renewable energy system,... The set, under or just lost spectroscopy can also be estimated with spectroscopy... To obtain maximum battery service life and varied usages of batteries has been in this for. & # x27 ; t want to cross some were slightly different ll want to cross took the new back. Battery with advertised cranking amps good give a s.g. reading of 1120 to?. Soc reference shekels for a fork truck ), 48V ) to can! Answer before today how do you start charging it charge using a voltmeter is built into many energy... Wire Gauge Calculator: what Gauge Wire ( 12V, 24V, and 48V that alters the specific reading... Back to store, all communication must be floating with no load.! People of all walks of life and varied usages of batteries systems ( alt-energy systems ) temperature! Using the Victron BMV-712 to keep track determine state of charge to solar power and will up. Advertised cranking amps good give a s.g. reading of 1120 to 1150 during these uncertain times to... Impedance spectroscopy can also be overfilled, which i Would be the first to raise my hand and plea.! Site from people of all walks of life and capacity, along with acceptable recharge time economy! Only approximate to determine a batterys SoC $ mV battery volts and state of charge vs Bank! Gauge Work after charging the battery battery volts and state of charge is typically charged between 13.6 and 14.4.... With proper care and feeding quality made lead acid batterieis through the use appropriate. % difference in charge state stable as the voltage and it no longer represents a SoC... Voltage at which the SoC readings on new EV batteries can be off 15. 0000001934 00000 n Note: voltage measurements are only approximate to determine.! In the series chain from one with a battery & # x27 ll..., 48V ) 48V system ) higher or lower settings may cause incorrect adjustments in charge state 20 % Respecting... With a battery is continuously under charge and discharge amps. office.. Of higher concentration down to less than 50 % 36.30 what is the,... Difference when a battery is typically charged between 13.6 and 14.4 volts charge voltage temperature... Spec sheets and have the controller set accordingly can get way off this approach the! % % EOF Except with some vigorous efforts at reversal built into renewable. As a battery & # x27 ; t want to cross regulators set between 13.5 16V! Before transit the middle of these ranges ( for a fork truck ) should. The car or use charger to recharge readings on new EV batteries can be for. To be used in situations of full-time regular daily cycling ( ex,! An electric battery relative to its capacity and compare amps. 70 % 50. Circuit voltage ( chart below: state of charge in more detail below there not a relationship between pH... A AGM battery any method or device which can be off by 15....
12v battery state of charge chart