Voltage sag graph & "ArduWattmeter"

[ccriss]

Hello guys,
I was not sure where to write this topic because it's not only referred to battery, but also DIY and E-Scooter..
Recently I did my first project using Arduino, and I was surprised by the fact it was so easy to implement that solution.

I made a sort of cycle analist where I can see by using a LCD: Voltage, Temperature, Battery Temperature, Ampere, Ampere Hour, but this is only the start :wink:

The image below show the LCD


Here you can find a video with Arduino in action !!
http://www.youtube.com/watch?v=zzJGZ5NogDs

Coming back to "Voltage sag graph" after checking the instantaneous value I wired my laptop to Arduino in order to log the value and I found the incredible sag during acceleration. Please note my scooter is 18 40AH TS cells bought 6 months ago.

Below you will find a full throttle accelleration sample of 26 sec


What is you feeling respect this value, I know Thundersky Cells are just 3C max continuous, but I was expecting something better, what are your feeling or real experiences?

 

[neptronix]

ha, congrats.. you've made a turnigy watt meter / cycle analyst

At 55-60v nominal, a 10v sag is fairly bad. I notice that you are pushing them to 2C occasionally.
It's my belief that the best performance out of lifepo4 is around 1C..

 

[ccriss]

And here to whom are interested attached the excel file containing the complete log (it last just 6min) coming out from Arduino and related graph
View attachment 3-12_giro intorno a quartiere.xls
The complete graph of my test is also below

 

[parabellum]

What wire gauge are you using(bat-controller)? Do this TS have full capacity as stated by manufacturer? Where are you measuring Voltage (bat side or controller side)?

 

[ccriss]

I'm not sure, but as far as I remeber it's a 8 or 9 AWG (I will check)
This TS stil have 40AH capacity
Voltage come from controller side

Please note the temp is around 10° here and when I get this graph the battery were near 70%SOC

 

[theRealFury]

Hello,

your ~10v of sag is not that bad really, during the summer with my old 36v LI-MN battery i used to get ~3V of sag @ 2C but in the winter that voltage sag was going up to 4-5v @2C. I would immagine what you are seeing is about normal for a TS pack that is run @~2C discharge. Unfortunately they have a reputation for being a saggy battery, hence the nickname "ThunderSag". But all in all, they are a reasonable battery as long as they are able to put out the rated cycle life.

 

[ccriss]

Thank you for your info

The sag of 10V is related to a near 3C discharge (2.75C to be precise -> 110A on a 40AH battery) at 10° temp, so we can say it's a normal sag for "Thundesag"?
I decided to buy Thundersky because the expected cycle life is 3000, but I hope it is true. I search real life experiences on the WEB, but none has still reached that number

 

[theRealFury]

Hey,

@ 2.75c discharge 10v of sag i would say is very normal. As for the cycle life. they are reated for 3000 cycles at 0.5c discharge.. or 20amps discharge. You are discharging 5x that fast at peak.. i would immagine therefore that your predicted cycle life would be somewhere in the 500-800 region, maybe a little more if your average discharge falls more into the 1-1.5C area. I'm no battery scientist but i would be highly surprised if your TS cells reach 1000 cycles at those discharge rates.

 

[texaspyro]

They don't call them ThunderSags for nothin'...