# Battery voltage and discharging



## illuminateddan (Dec 19, 2009)

Hi, 
I am trying to judge state of charge for a my lifepo(TS90Ah,38 cells, ~120vpack) batteries and looking at the graphs from thundersky I am having issues....
I basically want to know the top voltage (100% charged) and the bottom voltage (70% discharge) range for the 90Ah units.

The Thundersky docs show that these voltages (working off a 3C discharge) are from 3.2v (100% charged) to 3.1v (30% charged). This sounds really quite wrong. Am I being a graph retard???
I thought that after a full charge the cells should start at around 4.2v ish and then slowly drop down to 2.5v (around 70-80% discharge).

If the voltage curve changes for different 'C' discharge rates does anyone have use a formula to calculate more accurate pack charge percentages?

Can anyone provide a graph of a cells discharge please?

Thanks!


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## favguy (May 2, 2008)

Under discharge conditions, you have no chance of measuring state of charge by voltage, the thundersky graphs should also be taken with a pinch of salt!

You can measure state of charge at rest, as the drop in voltage is quite linear between around 95% charged and 30%, below 30%, you'll struggle to measure it accurately using voltage as it's falling so fast.


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## Travdude (May 11, 2009)

This graph might help.

As you can see there is a huge difference between 3.1 and 3.2 volts! 

Favguy is right, it is difficult to tell SOC by voltage alone.

I am waiting for someone to make a good battery monitoring system.


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## illuminateddan (Dec 19, 2009)

So what I really need to do is measure the voltage drop under multiple load conditions and then chart all this data to then calculate a formula for a discharge curve, and then use this formula with the packs 'trip discharge history' to calculate what I have used and what I have left based on my recent driving style. Oh. 

How long do the batteries take to spring back to 'no load' voltage conditions?

How have other people estimated battery percentages on their BMS systems?


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## frodus (Apr 12, 2008)

a better way would be to measure voltage AND Amps, calculate watts and then subtract that from a full charge. Then you'd know SOC.

Voltage alone is not a good measurement of SOC. If you measure energy in and out, it can provide a far better idea on how much energy is left.


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## illuminateddan (Dec 19, 2009)

So current BMS's also measure current use, or do they just look after the charging?


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## favguy (May 2, 2008)

You could use an Ah counting meter to show energy used. This will measure your voltage and current and calculate it for you.

I'm personally using a bargraph style meter built and calibrated to show state of charge between 95% and 30% at rest (30% will show empty) one for each pair of cells, in an array. It's still not ideal, joe public would no doubt still manage to wreck the battery, but it'll be ok for me along with some form of BMS yet to be decided.


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## CroDriver (Jan 8, 2009)

frodus said:


> a better way would be to measure voltage AND Amps, calculate watts and then subtract that from a full charge. Then you'd know SOC.
> 
> Voltage alone is not a good measurement of SOC. If you measure energy in and out, it can provide a far better idea on how much energy is left.


How to count in looses like heat? Since we're pulling high amounts of power from our pack the heat looses are quite high and can be over 10% of the overall stored energy. This could cause some problems white trying to measure the SOC.

I wonder how Tesla's BMS manages that problem.


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## peggus (Feb 18, 2008)

Just integrate the current, not the power! The energy charge efficiency and discharge efficiency are not the same and varies with temperature, amp draw, SOC, phases of the moon, etc.

Coulombic charge efficiency (Ah in / Ah out) is much more reliable and consistent.


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## MN Driver (Sep 29, 2009)

illuminateddan said:


> So current BMS's also measure current use, or do they just look after the charging?


Most BMS's do not measure Ah consumption but there are some that do, if you are willing to build your own based on instructions and can know electronics decently, can solder well, and have the patience you can put together a very nice BMS with a small display that indicates Ah consumption, amps, voltage, voltage deviation between cells, temperature, etc using a programmable PICAXE setup. Likely overkill for most and pointless for some but I'll probably go with that sort of system.


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## glaurung (Nov 11, 2009)

Blue Window dashboard computer has a"fuel gauge". User manual might give a hint how it calculates available energy.Any user have feedback on how accurate it is? Might want one
Harri


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## pm_dawn (Sep 14, 2009)

Hello Harri !

Have you taken a look on the Swedish Forum ?
If you can read Swedish there is a lot of info there for you.
One of the builds are using the Blue Window.
This Thread:
http://www.elbil.forum24.se/elbil-about1773-80.html

I guess you have already seen the Cycle analyst. it does SOC calculations also. And reasonable in price.

Best Regards
/Per
Östersund Sweden


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