# State of Charge Meter vs. Voltmeter?



## ithinkidontknow (May 14, 2009)

I am trying to order some of the gauges for my small EV project and I am wondering whether or not it is important to have a state of charge meter or if I should just tell the charge of my pack based on the voltage drop. Also, if someone could explain to me exactly how the state of charge is measuring the charge that would be appreciated. The reason why I ask is that state of charge meters are fairly expensive from what I can tell while voltmeters are very run of the mill and cheap. 

Thanks


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## PatricioIN (Jun 13, 2008)

the voltmeter will be all over the place when you drive.. and only settle to a true readable level after a few seconds of no load. A state of charge is more like a fuel gauge, giving you a good general idea of how much you've used and how much more you have. Personally, I prefer to have both, but if watch your voltmeter, you can figure how much you have left (once you've really gotten to know your EV and it's limiits!)


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## neanderthal (Jul 24, 2008)

cheaper soc gauges guess your state of charge from the voltage, which is not linear, so it is not super accurate. It is even harder to guess for lithium based batteries, bc the voltage stays more steady as it discharges and plummets near 100% empty. Which makes it tougher to guess if you have a volt meter and lithium based batteries. more advanced soc meter will keep track of the voltage and the amp hours used to give a better estimate


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## IamIan (Mar 29, 2009)

True SoC... is almost never worth the effort to try to determine... most of the time a good idea of the SoC is enough.

Batteries get complicated fast when you want to try and figure out the 'true' SoC... too many variables can influence things ... temperature , pressure , age , cell to cell differences , variation in internal resistance as the SoC changes, variation in the Ah of usable capacity at different usage rates , etc...etc...

Its a mess.

So getting a good enough idea that your are comfortable with is by far the easier and more popular option.

So get as much instrumentation as you are comfortable with ... but after it is all said and done ... get out there and learn the finer details or your finished ride.


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## ithinkidontknow (May 14, 2009)

I think I will just keep searching ebay for deals on a SoC meter but otherwise I will just use my voltmeter for now. 

Thanks for all the responses. I will follow all of your advice and just go for what feels right.


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## cruisin (Jun 3, 2009)

ithinkidontknow said:


> I think I will just keep searching ebay for deals on a SoC meter but otherwise I will just use my voltmeter for now.
> 
> Thanks for all the responses. I will follow all of your advice and just go for what feels right.


 
You may want to consider the EV Display here http://minibms.mybigcommerce.com/products/EV-Display.html

which will be more accurate.


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## Sunking (Aug 10, 2009)

With respect to lead acid batteries State of Charge voltages are pretty much meaningless. State of Charge voltages are only somewhat accurate after the battery has been rested for several hours which is not practical for an operating system.


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## GizmoEV (Nov 28, 2009)

cruisin said:


> You may want to consider the EV Display here http://minibms.mybigcommerce.com/products/EV-Display.html
> 
> which will be more accurate.


Don't forget the CycleAnalyst at ebikes.ca. This one does have the benefit of not needing an external 12V source of power.


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## dtbaker (Jan 5, 2008)

ithinkidontknow said:


> I am trying to order some of the gauges for my small EV project and I am wondering whether or not it is important to have a state of charge meter or if I should just tell the charge of my pack based on the voltage drop.



If you are running lead-acid batteries you can use a relatively cheap meter based on voltage drop. 

If you are using Li, you MUST count ah to be anywhere near accurate... The Cycle analyst from ebikes.ca is a simple shunt-based technology that is cheap, and not bad, but barely up to EV voltage and amp needs. the evDisplay is Hall-effect based and more sophisticated with temp correction, etc, and a LOT less expensive at $200 than the high-end alternatives.


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## GizmoEV (Nov 28, 2009)

dtbaker said:


> If you are using Li, you MUST count ah to be anywhere near accurate... The Cycle analyst from ebikes.ca is a simple shunt-based technology that is cheap, and not bad, but barely up to EV voltage and amp needs.


The CA will work with any shunt so current limits are moot and as long as your pack voltage is not over 350V they work just fine. FWIW, Bill Dube' uses them on his EVs.


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