# Re: [EVDL] SOC calculation for LiFePO4 cells



## EVDL List (Jul 27, 2007)

*Re: [EVDL] SOC calculation for LiFePO4 cells*

Just integrating current over time will provide a better measure than
integrating power over time. Ah will be 99%+ conservative, while the battery
internal resistance will create a difference between energy out and energy
in.

I think that voltage and amp-hours will provide sufficient measure of state
of charge.

As long as you have decent quality cells and perform occasional equalization
charges to 100%, you should be fine. The equalization charge must give each
cell/parallel group of cells a constant voltage finish, which means that
you'll have different amounts of current in different cells.

As the cells start to fall out of equalization, your monitoring system will
notice the voltage changes and mark that the next charge should be a full
equalization charge.

The more evenly matched the cells are, the less often you will have to do an
equalization charge.

-Morgan LaMoore



> Roger Heuckeroth <[email protected]>wrote:
> 
> > jd,
> >
> ...


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## EVDL List (Jul 27, 2007)

*Re: [EVDL] SOC calculation for LiFePO4 cells*

I might be misunderstanding you. Why would you need a lookup table of 
internal resistances? I thought with lithium the internal resistance 
would stay pretty much constant over SOC.

Instead, why don't you start with an estimated capacity (let's say 20 
kWh.)

As you discharge you constantly measure voltage and current, and 
integrate to get WH consumed.

So, if you've consumed 10kWH, it will report 50% SOC.

Then a while later, you've consumed 19.8kWh so it reports 1% 
remaining, when your LVC notified the SOC meter that the worst case 
cell just hit the cutoff ( obviously you should leave some buffer 
here ). It then updates the estimated capacity to 19.8kWh.

As you said, as you drive more it will become more accurate.

Is this the same thing you were suggesting just explained differently? 
Or is your method different?

I don't think I understood how you were using a lookup table and index.

- Nate

On Fri, Sep 12, 2008 at 6:44 PM, Jeff Shanab <[email protected]> 


> wrote:
> I have though about this alot.
> I was thinking we start with a guestimated internal resistance
> lookup table and apply that based on amps along with the amps x volts
> ...


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## EVDL List (Jul 27, 2007)

*Re: [EVDL] SOC calculation for LiFePO4 cells*

>
> I might be misunderstanding you. Why would you need a lookup table of 
> internal resistances? I thought with lithium the internal resistance 
> would stay pretty much constant over SOC.
Well I am guessing. I Think it stays realativly constant compared to 
lead acid but the small changes and the non linearity make this a simple 
way of implementing the calculation quickly. It may also be amp related. 
The lookup table allows me to put in a internal resistance "curve" If I 
learn it is flat, then the table still works, but if I learn it is a 
complex function, I don't have to re-code.
>
> Instead, why don't you start with an estimated capacity (let's say 20 
> kWh.)
>
> As you discharge you constantly measure voltage and current, and 
> integrate to get WH consumed.
Because differnt usage styles will still effect the capacity. I know 
people love to say they have no pukerts effect, but there is still a little.
The internal resistance is temperature related for sure(maybe that is 
another index into the internal resistance map table) and the more amps 
you pull the hotter they get. The balance between heating and the 
Cooling you have then plays a role in how much to subtract in each 
integration interval.
>
> So, if you've consumed 10kWH, it will report 50% SOC.
>
> Then a while later, you've consumed 19.8kWh so it reports 1% 
> remaining, when your LVC notified the SOC meter that the worst case 
> cell just hit the cutoff ( obviously you should leave some buffer here 
> ). It then updates the estimated capacity to 19.8kWh.
>
> As you said, as you drive more it will become more accurate.
>
> Is this the same thing you were suggesting just explained differently? 
> Or is your method different?
Primarily the same, integrating the WH, just trying to get more accurate.
>
> I don't think I understood how you were using a lookup table and index.
It is an implementation technique. When you want to approximate a 
complex function generically and quickly. A micro doing a lot of 
calculations per second for a lot of batteries would be taxed if you 
were actually calcultateing complex functions every iteration.

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## EVDL List (Jul 27, 2007)

*Re: [EVDL] SOC calculation for LiFePO4 cells*

My setup is like a serial hybrid. A generator can always charge the battery
if neccessary. So I am able to keep the SOC always between 20 and 80%, which
makes the SOC calculation more difficult. But does it really make sense
charging only to 80%.

Does limiting to 80%SOC helps to prolong the cycle life?

I still have the rumor in mind that it does. ('charging batteries to 100%
cuts the cycle life of batteries'). But I do not have any official source to
prove it. I only find documents about SOC to cycles with 100%DOD at
different temperatures. But nothing that depends on different levels of SOC.

Do you know something that can help me with that topic?

-- 
View this message in context: http://www.nabble.com/SOC-calculation-for-LiFePO4-cells-tp19466653p19489907.html
Sent from the Electric Vehicle Discussion List mailing list archive at Nabble.com.

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## EVDL List (Jul 27, 2007)

*Re: [EVDL] SOC calculation for LiFePO4 cells*

On Mon, Sep 15, 2008 at 12:43 PM, jd.loewenherz


> <[email protected]> wrote:
> >
> > My setup is like a serial hybrid. A generator can always charge the battery
> > if neccessary. So I am able to keep the SOC always between 20 and 80%, which
> ...


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