# Thundersky / Winston Operating Voltage



## PTCruisin (Nov 19, 2009)

sourcefinder said:


> Hello!
> 
> I have a question regarding Thundersky 100ah cells.
> 
> ...


Cells will exhibit voltage sag under load. The amount of sag is highly dependent on temperature and charge level. That is normal. What is more important for LiFePO4 is the state-of-charge (SOC). If the cells are fully charged then the BMS should not react to the low voltage condition. If the cells have a low SOC then the BMS should react accordingly and reduce the power available to the controller/load.

My advice: 
1. Charge all cells to the same capacity (see posts on top balancing) but NOT to the 4V limit! Maybe 3.5V or 3.6V (you will only miss of few Amp-hours of capacity)
2. Invest in instrumentation that measures and records current in and out of the pack and reports that in Amp-hours.
3. Discharge to 80% or less and then stop! (That's with 20% or more of the capacity remaining)
4. Repeat step 1


If your are pulling 3C continuously then you will get maybe 20 minutes of discharge before damage could occur due to fully-discharged cells.


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## MN Driver (Sep 29, 2009)

Down to 2 volts? I wouldn't unless the duration was incredibly short. 2.5 volts will make some heat about 210 watts per cell in a 300 amp draw. 2.5 volts would be a 10 second on batteries that aren't past warm. 2 volts and your cells are 360 watt heaters but in a plastic case that doesn't exactly release the heat. Watch the temperature, max limit before you dip into life degradation is 140 degrees F internal, measuring directly on the terminal(don't aim an IR gun by its laser, better yet don't use an IR gun) for temperature is as close as you'll get to the internal temperature and there is some time lag even there.

How old are your Thundersky cells, I remember quite awhile ago seeing the charts between TS and Sky Energy and it sees TS didn't hold its voltage up under a 3C load too well so more people started going with SE which turned to CALB. Your cells seem to have high IR or at least they don't seem to be cut out for 3C continuous.


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## mora (Nov 11, 2009)

I have Winston cells too. 90Ah versions though. I also have 30 pieces of older (TS) 90Ah cells which seem better regarding voltage sag. 3C isn't meant to be continous on these cells, more likely a peak. I could draw 5C from older cells without voltage sagging under 2.8V. Winston cells sag to about those same levels at 3C. When voltage sags to 2.5V per cell at 1.5-2C I consider my cells near empty. If miniBMS gives low voltage alarm at 1C I'll stop driving. Charging to 4V per cell is safe, though I don't charge to those levels. I stop at 3.65 per cell and seem to have all the capacity.

I measured internal resistance from 15 older cells lately. They ranged from 0.8mOhm to 1.5mOhm. I haven't measured my Winston cells but I think I should do it some day.

Winston and TS both seem rugged. I have reversed polarity of one Winston cell once and it wasn't even hot to touch. It recovered to around 2V in 15 minutes and doesn't seem to sag more than other cells do. But I surely know which of my cells goes bad first, hehehe.


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## sourcefinder (Dec 17, 2010)

First of all thank you for your answers!

I found some tests at google where they tested 100ah cells till 5c down to 2 volt.
I set 2.2 as limit at my bms.

I have some minibms on the cells to stop charging at 3,7 v.
I also use a bms computer that shows each cell and communicates with charger.
This computer also calculates remaining capacitie by the current.

So I will charge all cells to 3.7v and set the capacity to 90%
so when the computer says 0 ah remaining I have 10% left to goback.

I will post the result.


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## sourcefinder (Dec 17, 2010)

It worked fine, we had 320a current and stopped at 10% cpacity remaining.
Outside temperature was 5c celsius, we had a power of 110kW.
Voltage goes down to 2.0v at this conditions, in idle back to over 3.something.

Looks like they get better since last try...

Best regards
Hermann


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## MN Driver (Sep 29, 2009)

It makes more sense when you know the cells were cold. The voltage will sag a bit and towards the middle to mid-end of the discharge the voltage usually goes up a little after it drops a bit in the beginning as they warm up.

This is less dangerous to the cell when it is cold because voltage sag creates heat, which is fine as long as too much heat isn't concentrated in hot spots in an extreme discharge or if the overal temperature of the cell doesn't get too hot.

In the case of cold temperatures, the voltage sag will warm them up so sagging them deeper will allow them to be more efficient and have less sag further on in the discharge. There are limits of course to this as the cell might not necessarily heat uniformally in the cell.


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## Planky (Aug 22, 2011)

hey sourcefinder, you said "I also use a bms computer that shows each cell and communicates with charger."

What kind of bms computer and charger do you use?

greets from Austria
Planky


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## DJBecker (Nov 3, 2010)

We are using CALB 100ah cells with our own STM32 based battery monitor system, logging the data to a laptop.

We were initially surprised to see significant sag with a fully charged pack, down to 2.7V under a 3C load at 15C. On re-reading the datasheets we concluded that this was expected and acceptable.

It points out the need to have the motor controller and BMS communicate. Or at least for the BMS to not alarm based solely on voltage. It needs to track state of charge and current draw to make a reasonable estimation of the alarm point.


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