# Using a Cell Log to measure voltage sag



## jondoh (Sep 8, 2008)

So, I've been wondering about my thundersky batteries for some time. I've been wondering if the slight loss of power was due to a weak cell or the colder weather we've been experiencing lately. I recently purchased a cell log to measure cell voltages under load. 

Cell log here:
http://hobbycity.com/hobbycity/stor..._Name=Cell-Log_Cell_Voltage_Monitor_2-8S_Lipo

I'm using just two leads since i've been too lazy to make a new cable. I could make a permanent harness attached to every battery and have 3 cell logs monitor every cell, but that's a project for another day. 

Here is some video I took of how I'm using the Cell log to test cells under load. I test two cells at a time since one is not enough to drive the cell log device. 











So far I've tested about 15 cells and they've all gone to about 2.8v per cell under load (after being fully charged or nearly fully charged). I take this to mean that there haven't been any weak cells yet. 

I've been a little concerned. I don't have an amp meter but the controller is rated for 450 amps and i frequently have the accelerator floored. I don't know if the batteries are frequently seeing 450 amps or not. 

Here is my EV conversion on EV album:
http://www.evalbum.com/2975

I've been quite proud. It's been simple, durable and reliable in operation for about 3 months and has about 1900 miles on it already-- despite having smallish batteries. It's only 72v and yet I can still go 53 mph. 

I feel if i test all cells and don't have a weak cell, I should be in the clear.


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## pm_dawn (Sep 14, 2009)

Hi !

I hope you know that your not supposed to mount the LFP batteries horizontal. They are supposed to be upright.

I dont know if that would affect your voltage drop but who knows ?

Nice setup though !!

Best Regards
/Per Eklund
Sweden


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## dimitri (May 16, 2008)

What was the temp when you done this test?

Funny that I watched your video right when I was sitting trying to figure out how much to compensate LVC against ambient temp 

Do you have LVC working on your VBs? What is the LVC value? Do you easily reach it when its cold?

Since I'm in Tampa, its difficult for me to test this myself  , but I need to figure out how much to lower LVC at freezing temp. Say for example your LVC=2.6V at 25C , what would be appropriate LVC at 0C ? My guess is about 2.2V-2.3V ? Any thoughts?

Thanks


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## jondoh (Sep 8, 2008)

Hi Pm dawn,

I actually check with evcomponents.com about mounting the batteries horizontally before I bought the batteries and he said it was fine. So far, I haven't had any issues with it. I've also checked out the video lecture at evtv.me where Jack cuts open a cell. It's not quite exactly like a gel cell. It's wet inside but the plates are very thin and packed in pretty tightly. It doesn't seem that there is any hydrostatic pressure to push any liquids out of the battery terminal or anything.


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## jondoh (Sep 8, 2008)

Hi Dimitri,

EV components actually recommends not going below 2.5v per cell. As I understand it, 2.5v per cell represents some kind of limit at which damage begins to occur. I try to stay as far from that as possible especially since I can draw 4.5C. With my 24 cells, that 60v. I've never seen the volt meter drop below 64 - 65 volts before.


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## dimitri (May 16, 2008)

jondoh said:


> Hi Dimitri,
> 
> EV components actually recommends not going below 2.5v per cell. As I understand it, 2.5v per cell represents some kind of limit at which damage begins to occur. I try to stay as far from that as possible especially since I can draw 4.5C. With my 24 cells, that 60v. I've never seen the volt meter drop below 64 - 65 volts before.


That's a common misunderstanding, 2.5V limit is at resting voltage, which you should never get to, however, under load its not uncommon to get to 2.5V, especially at 3C-4C loads, then as soon as you let go of the pedal, they bounce right over 3.0V. This sag is more apparent at colder temps, which is why I was interested in data from colder areas.

My LVC is set to 2.6V and I can reach it while pulling 3C towards the end of my range. The challenge in picking LVC level is to pick one that is not too high so it doesn't interfere with expected high loads, yet not too low, so you have time to get out of traffic before your pack is empty. Temp variations make this even more difficult, that is why I am trying to design a simple BMS which will compensate for temp variations and I need real world data to draw some conclusions.

What range have you gotten from your pack so far? Sounds like you are quite conservative if you don't see sags below 2.66V at 4C at low temps.

Thanks, BTW that cell log thingy is pretty neat, I should get one of those.


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## jondoh (Sep 8, 2008)

crap! I just fried my cell log.  

I accidentally touched on probe to the plates that keep the cells from puffing up. 

I didn't think the plate was grounded but apparently it's touching the chassis somewhere. 

So I'll buy them again when I have the time. 

I'd like to get 3 cell logs and monitor every cell, but I'm very hesitant to disassemble the pack since everything seems to be running fine. It seems to me that the more wires you have running to your pack and the more you mess with your pack, the more chances you have of something going wrong.


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## Dave Koller (Nov 15, 2008)

dimitri said:


> Thanks, BTW that cell log thingy is pretty neat, I should get one of those.


HAH I was about to get you over here and I see you made it! Nice logger!


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## jondoh (Sep 8, 2008)

Hey Dimitri,

I've driven about 25 miles on a charge. Thing is, 25 miles feels about 80% discharge. I've had a resting voltage (read off the charger) of about 77 volts at 80% DOD. I think I'd have a heart attack if it rested at 65v.


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## O'Zeeke (Mar 9, 2008)

jondoh said:


> I'd like to get 3 cell logs and monitor every cell, but I'm very hesitant to disassemble the pack since everything seems to be running fine. It seems to me that the more wires you have running to your pack and the more you mess with your pack, the more chances you have of something going wrong.


Hi Jon, thanks for the info. If you were to monitor all your cells (45 in my case) using multiple cell loggers would each measure its own 8 cell bank independently or would there be any effect on voltage readings from the first bank of 8 on the second bank and so on,? Sorry about your log frying . Thanks


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