# When to measure static voltage?



## alvin (Jul 26, 2008)

Hi 

I have some beginner questions that I have not found answers. Please answer any or all.

Static means without any load ,right? So how long after the load is removed?

If 3.4 volts is full , what would the voltage at 80% DOD? Or can you really know from a voltage ?

How far down do others discharge their cells , to be gentle with them?

When figuring watts per mile, I have seen a calculator thats says amps times volts divided by MPH. Is it motor amps or battery amps? Is it nominal volts?


Thanks
Alvin


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## Ziggythewiz (May 16, 2010)

Yes, no load. How long you wait is up to you, it won't fully settle for a couple days, and who wants to wait that long? Just be consistent.

You can guess at SOC based on voltage, but you can only know using an AH counter. I plan to try to use mine between 90-30% SOC.

watt hours per mile (wH/m) is AH used * voltage / miles traveled

Only the motor and controller care about motor amps. Usage is based on the batteries.


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## alvin (Jul 26, 2008)

Ziggythewiz

Thanks for the reply.
I am in the process fo bottom balancing now.

That watts per mile formula works out about the same as the one I was using. 

Alvin


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## dougingraham (Jul 26, 2011)

alvin said:


> Static means without any load ,right? So how long after the load is removed?
> 
> If 3.4 volts is full , what would the voltage at 80% DOD? Or can you really know from a voltage ?
> 
> How far down do others discharge their cells , to be gentle with them?


As was previously stated it takes a day or more before they stop drifting but a few hours gives a pretty good reading.

The voltage at a 20% SOC would be around 3.2704 volts depending on temperature after settling for a week or so. (With GBS 100AH cells, could be a little different with other brands. It also depends on what point you consider 0% SOC and 100% SOC.)

Here is a link to a spreadsheet with a scatter plot which can be used to tell the state of charge from the unloaded resting voltage. You need a sensitive and accurate meter. The chart does not incorporate temperature compensation which would help. Accuracy appears to be about 1.5% on the GBS batteries which I used to generate the data. I will fill in the areas that have no data later this year.

Link to my google docs spreadsheet.

On all of the LiFePo4 types there is essentially no charge left when you get much below 3 volts so you can pick an arbitrary value. I used 2.5v for my own testing although early on I used 2.0v. Under heavy loads you can let the cells drop a lot below this without issues. They spring back up when you release the load. It is this characteristic that makes BMS low voltage cutoff pretty much without value. If you let a BMS control the discharge you could be limiting performance.


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## alvin (Jul 26, 2008)

dougingraham

Thanks for your post. That is what I was looking for. Just some kind of reference .

Alvin


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## tomofreno (Mar 3, 2009)

dougingraham said:


> ... Under heavy loads you can let the cells drop a lot below this without issues. They spring back up when you release the load. It is this characteristic that makes BMS low voltage cutoff pretty much without value. If you let a BMS control the discharge you could be limiting performance.


 Yes, the voltages rebound when the cells are not being discharged, but the idea of low voltage cutoff is to set it so that the cells normally will only sag that low in voltage under heavy discharge when down below 25% SOC or so. Then the bms gives a warning that one or more cells are getting low. This will generally happen under heavy acceleration and discharge current, so you can still drive a bit at moderate current without over-discharging the cell. Every once in a while I drive my pack down to about 28% SOC and then floor it to draw peak current and see if the BMS warning goes off. It hasn't over the last 3 years. I also disconnect a connection between two cells and insert a jumper with a momentary switch so I can set it off while driving slowly to see if the buzzer, warning light on the dash, and half throttle circuit are still working properly.


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## Ziggythewiz (May 16, 2010)

What cells do you have? You'll want numbers from your brand, or maybe even build your own charge curve using a few of your cells. Doug's 20% SOC voltage is what I've seen for calbs or TS around 50% SOC.


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## dougingraham (Jul 26, 2011)

Ziggythewiz said:


> Doug's 20% SOC voltage is what I've seen for calbs or TS around 50% SOC.


49% SOC with the GBS cells is around 3.3076 volts. 53% is around 3.3089 volts. This is corrected for cell capacity. Near the center we are talking about several percent per thousandth of a volt. My Fluke has not been calibrated since it was new and almost nobody gets their calibration checked but these voltage differences are tiny so a really good test would require temperature control as well as calibrated instruments. I would like to see numbers for other brands. The other cells I have are the A123 26650 size cells and I plan to map those out as well to see how they compare.


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## alvin (Jul 26, 2008)

Ziggythewiz said:


> What cells do you have? You'll want numbers from your brand, or maybe even build your own charge curve using a few of your cells. Doug's 20% SOC voltage is what I've seen for calbs or TS around 50% SOC.


 
Mine are Winstons. I ran mine down over the weekend so I could bottom balance. It is 5 miles around my block and I would do a lap wait about 10 minutes then check voltage.

I think mine may be closer to 3.2 volts at 80% DOD. This is at a very low discharge rate about .3C

I am still working on the bottom balace. When get done I'll try again.

Alvin


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## Ziggythewiz (May 16, 2010)

dougingraham said:


> I would like to see numbers for other brands. The other cells I have are the A123 26650 size cells and I plan to map those out as well to see how they compare.


The only numbers I've seen are from EVTV, but most of them are charge/discharge, not static.




alvin said:


> It is 5 miles around my block and I would do a lap wait about 10 minutes then check voltage.


Do you have an AH counter on there? That should give a pretty good curve with your method.


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## alvin (Jul 26, 2008)

Ziggythewiz said:


> Do you have an AH counter on there? That should give a pretty good curve with your method.


 
I don't have an AH counter. What I have is a display that shows Miles To Empty. It's counting amps I guess but I dont't see it. When I put the display together I set it for lead and a certain range. I'll have to change that.


Alvin


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