Battery State of Charge Chart

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Battery State of Charge Chart

Postby bdosborn » Sat Sep 09, 2006 9:13 pm

Here's a chart to show your battery condition by voltage. I like this one because it has multiple temperatures shown:

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Postby GHOSTSSX » Sun Sep 10, 2006 12:00 am

Sticky??
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Postby mikeschn » Sun Sep 10, 2006 12:50 am

If someone adds the red, yellow and green chart, this will become a sticky! ;)

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Postby 48Rob » Sun Sep 10, 2006 7:34 am

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Postby mikeschn » Sun Sep 10, 2006 8:48 am

Thanks Rob...

It's sticky now!

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Postby angib » Sun Sep 10, 2006 10:27 am

My experience and understanding of using battery voltage to estimate state of charge is:
- different battery types give different results;
- if the battery is being used (eg, discharged), the voltage changes (drops);
- most voltmeters are of limited accuracy.

I think the charts above are for traditional flooded lead-acid batteries - so they don't apply to gel/AGM or higher tech batteries.

My conclusion is that you need to calibrate your voltmeter and your battery so that you can work out that, for you, an indicated 12.4v means 'fully charged' and 11.8v means 'pretty empty', or whatever.

I don't believe you can be more accurate than this.

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Postby bdosborn » Sun Sep 10, 2006 4:29 pm

Andrew,

Good point on the accuracy of the chart, it's at best a "wet finger in the wind" method of estimating remaining capacity. A digital voltmeter should have sufficient accuracy, it's the uncertainty introduced by the voltage versus load characteristics you mentioned that makes it difficult to read the battery's charge state from a voltmeter as well as differences from battery to battery. Ideally, the battery should be measured after 24 hours of no load, which isn't very practicle when its 11:00 p.m. on the third night of camping and you're wondering if you should turn the lights off. However, measuring after 5 minutes of no load will give you a good idea of where you're at with the chart being a general guide. I did a brief internet search on the chart and I believe it's applicable to all battery types including AGM and gel but I've been wrong before...
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Postby asianflava » Sun Sep 10, 2006 4:35 pm

angib wrote:I think the charts above are for traditional flooded lead-acid batteries - so they don't apply to gel/AGM or higher tech batteries.


Oh great, doesn't work for AGMs. Good thing I didn't bother getting one of those "state of charge" meters.
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Postby mikeschn » Sun Sep 10, 2006 4:41 pm

Don't they make some kind of a computerized gage that tracks your usage, and lets you know remaining battery life based on usage? And if I recall it wasn't cheap. $200 to $300 for one silly looking little round gage. :shock:

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Postby bdosborn » Sun Sep 10, 2006 4:55 pm

Mike,
I think this is what you're talking about.

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It tracks amp-hours so its supposed to be pretty accurate. But yeah, its spendy.
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Postby angib » Sun Sep 10, 2006 6:06 pm

I'm not sure how 'different' the battery has to be before its state-of-charge/voltage differs from flooded lead-acid - AGMs may be similar enough. I can be wronger than Bruce without difficulty.......

The one point to make about digital voltmeters for the novice is that there is a big difference between resolution and accuracy. Cheap meters may read in steps of 0.01v, but may only be accurate to +/-0.1v - some people assume that because the display looks accurate, it is accurate, but we are into free lunch territory here!

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Postby cracker39 » Sun Nov 05, 2006 9:39 am

I know that this has been discussed before, but my memory's not the best. Are the readings on these charts for under load or no load? My battery is a standard deep cycle from WM. Right now, it is about 70 degrees outside and the temp in the battery box could be between 65 and 70 degrees. My digital battery meter I have plugged into a socket inside the camper is reading 13.6 volts. I've had the shore cable plugged into a 30 amp 110VAC outlet for a couple of weeks.

My no load reading is ABOUT 1.0 volt below the number on the chart for 70 degrees and 100% charge (i.e., 12.562). Nearly every auto or deep cycle battery I've owned neasured over 13 volts when fully charged.

Regardless of the answer, I'd use this chart to measure an at-rest battery's state of charge. It's better to err on the side of caution rather than let it get too low before re-charging.
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Postby Alphacarina » Mon Jul 23, 2007 6:56 pm

cracker39 wrote:I know that this has been discussed before, but my memory's not the best. Are the readings on these charts for under load or no load?
No load - In fact, for best accuracy, the battery needs to be 'at rest' for a couple of hours before you take the readings

Any load depresses the readings and the higher the load, the lower the voltage will measure. Once you remove the load, it takes quite a while for the battery to rise back to it's true 'resting' voltage

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Postby hurricaner » Mon Oct 12, 2009 2:08 am

That first chart is not correct. The voltages do not vary any where near that much. Here is a correct one .
http://jgdarden.com/batteryfaq/carfaq4.htm
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Battery State of Charge Chart

Postby KimCaples » Wed Aug 18, 2010 7:01 am

Battery management systems are simple in theory. Using a current monitor, only put as much current back into the battery as you take out. Instead of maintaining a constant 13.8 volts (V) or thereabouts, a battery management system checks the battery's state-of-charge (SOC) when the engine is off and when current flow is essentially zero. From then on it will target a SOC of 100 percent or less in order to save fuel, avoid overcharging and extend the life of the battery.

The ECM and/or BCM remain the brains of most charging systems, but there are standalone systems as well, notably GM's Stand Alone Regulated Voltage Control (SARVC). In either case, a small current monitor built into the battery terminal or placed around the cable provides information regarding current flow in or out of the battery. By adding up all the current flowing in and out of the battery, the ECM or a standalone module can determine any net gain or loss to SOC.

Sensing Current, Voltage and Temperature


Bosch battery current sensor monitors current, voltage and temperature.
Current sensors built directly into the battery terminal from Bosch, Hella, Delphi, and others provide current, voltage, and temperature sensing. GM uses a Hall effect sensor placed around the battery cable for current monitoring and monitors temperature and voltage elsewhere in the system. Non-Hall effect designs typically utilize micro-ohm shunts capable of measuring large amperage flows during cranking all the way down to parasitic draws of 30mA (milliamps) or less. Hybrids such as the Prius extensively use current monitoring for both main battery and motor monitoring functions. Because of their widespread adoption, techs will need to become familiar with current sensors. Most will display amperage data as a Parameter Identifier (PID) on a scan tool providing a value which can be compared to a reading taken from a clamp-on ammeter.

A shunt-style current sensor is merely a very low value resistor, in the range of 50 to 100 micro-ohms (millionths of an ohm). This provides enough resistance to create a measurable voltage drop but not enough to interfere with cranking. A 200 amp starting current passing through a 50 micro-ohm resistance will only produce a voltage drop of 0.01volt.


GM Stand Alone Regulated Voltage Control has six different modes listed on an attached chart.
Current sensing is only one piece of battery management. Battery temperature sensing is already in widespread use to vary the charging rate on vehicles. Traditional charging systems adjust the alternator setpoint (12.8-14.5V) depending on the temperature of the regulator. Certain Chrysler vehicles use temperature sensors plugged into the battery tray. When a battery is cold, it can be charged harder and faster than when it's hot. Thermistors incorporated onto certain older GM vehicles' battery cables perform a similar function.

When temperature sensors fail, get damaged or unplugged, "strange" charging behaviors can result. For example, a "failed high" sensor on a Chrysler can cause the alternator to refuse to put out sufficient charging voltage. It can also prevent certain emissions monitors from running, an interesting "gotcha!" A Battery Volts Out of Range DTC P1630 may accompany a failed GM thermistor. Typically a battery temperature sensor is a simple thermistor and can be checked against specs with a Digital Multimeter (DMM). Normally DTCs will set for a shorted or an unplugged connector — but not always, as was recently reported on iATN (http://www.iatn.net) in a techhelp posting on a 2007 Caddy with an underseat battery.

Sending Sensing Information

A current sensor may transmit information to the ECM directly or via a low-cost LIN bus, Flexray or CAN bus connection. A databus-attached sensor that fails to respond to a query may set a DTC. In the case of GM, the Hall effect sensor utilizes a direct connection to the ECM or battery control module. Generator performance is monitored through the generator field duty cycle signal circuit. This signal is a 5 volt PWM (pulse width modulated) signal of 128 Hz. PWM ranges between 0 and 5 percent and 95 and 100 percent are used for diagnostic purposes.

Sensors built directly into the battery terminal or tray are located in the worst possible place for corrosion and damage from battery acid and fumes. When a sensor fails to provide correct current flow information or loses its connection with the ECM, erratic charging behavior is often the symptom. But be sure you actually have a problem.

Seemingly "unusual" charging behavior such as no alternator output at idle can trip up techs. It may actually be the work of a system working as intended. Take for instance GM Regulated Voltage Control, whether in its regular or "standalone" version. This charging system has six different modes listed in the attached chart. One even attempts to reduce sulfation of the battery. Someone accustomed to seeing the voltage gauge on the instrument cluster pegged at 13.8V is likely to wonder when he or she first encounters a GM vehicle where the voltage varies anywhere from 12.4 to 14.8 during normal operation. Hundreds of Internet postings attest to owners bewildered by this phenomena. A GM TSB attempts to alleviate customer concerns.
:shock: :x
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