# LiFePO4 pack voltage sag?



## dimitri (May 16, 2008)

This is from LFP docs. Its a true picture, I personally verified up to 3C  , I can't verify 5C because Warp9 would blow up at 5C, I have 160AH cells.


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## Number 6 (Feb 18, 2008)

dimitri said:


> This is from LFP docs. Its a true picture, I personally verified up to 3C  , I can't verify 5C because Warp9 would blow up at 5C, I have 160AH cells.
> 
> View attachment 4475


Dimitri, I'm still rather new to the terminology. By 5C of 160AH do you mean 800 Amps? If so, Bryan Belk has dyno tested a Warp9 motor serveral times at 800 Amps with his 144volt/800A controller (actually the test was for his controller). From what I understand, the Warp9 didn't blow up. I know he is using flooded cells and you're talking lithium, but what I'm talking about here is the motor capacity. Excuse my naivete and please explain if I'm not understanding what you mean or if his setup is totally different from your case. Thanks in advance (I appreciate any tips or help).

Here's a quote from his blog:
8/27 2 controllers dyno'd and 1 to go. Was tripping the Electronic Circuit Breaker (ECB) circuit which was set to 450A. I 
found the AC ripple current from the generator was adding to the current that I was pulling from the pack. I increased 
the speed of the dyno to lower the ripple current, and got to 800A with no problem. I'll likely have to build a cap bank 
to reduce the ripple current at the pack so the ESB sees DC current from the pack. Just a reminder, be sure to refresh 
your web browser when visiting web pages here to get the latest updates. Its getting late, more to do tomorrow!
http://www.belktronix.com/blog.html


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## dimitri (May 16, 2008)

As usual ( I don't mean you personally, but a very common mistake ) you confuse motor current with battery current.

I was referring to battery current limit which is imposed by the whole system, not any one component. Its a relationship between RPMs, voltage and duty cycle. In may case of 128V pack, there is only so much current I can push before BEMF starts to resist it and creates a peak.

If I had more volts, I could push more amps, but there wouild be a point at which Warp9 can't take any more power and ether melts or disintegrates from centrifugal force.


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## Bowser330 (Jun 15, 2008)

dimitri said:


> This is from LFP docs. Its a true picture, I personally verified up to 3C  , I can't verify 5C because Warp9 would blow up at 5C, I have 160AH cells.
> 
> View attachment 4475



wow so...3.2V nominal to 3.0V all the way up to 70% DoD @ 5C...

NICE


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## CroDriver (Jan 8, 2009)

Bowser330 said:


> wow so...3.2V nominal to 3.0V all the way up to 70% DoD @ 5C...
> 
> NICE


Yes, WOW.

So even 10C could be possible for a few seconds (maybe even 30 sec) since it looks like they pulled the 5C continuous all the way to 0% SOC in this graph 

Btw., if anyone hasn't seen this:



> - For how long can I discharge the TS-LFP 100Ah cells at 10C (1000Amp)?
> 
> Discharge at 10c,it release 1000A/3 second,and then stop 3 seconds,and then release 1000A ……
> 
> ...


...a mail I got from ThunderSky


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## dimitri (May 16, 2008)

Its possible, sure, but voltage sags much deeper during high C rates and terminals get hot. It also shortens life of the pack. But I guess none of this matters when you have to win the race, right? 

At rates over 3C you have to watch cables, crimps, bolts, every connection, put lots of noalox on each terminal. I recently discovered one of my cell terminals was getting warmer than others after 3C loads even though bolt was tight. I took off the copper link and put more noalox on the terminal and tightened it again, now it same temp as others. I imagine at 10C this terminal could have gotten too hot and damaged something.

You also notice that at higher rates Puekert effect raises its ugly head, even on LFP cells. 5C line gets to the bottom earlier than 1C line, that a pure loss of energy.


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## jondoh (Sep 8, 2008)

i'm using TS 100 ah x 24 pcs for approx 72v (more like 77v).

i get 66 v at full acceleration (450 amp alltrax controller) with a fully charged battery.

does this mean i have a few bad cells?

I tried to find a bad cell by checking cell voltage after driving a few miles but they're all the same.

It's getting cooler now-- about 60 deg F. could this have an effect?


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## dimitri (May 16, 2008)

jondoh said:


> i'm using TS 100 ah x 24 pcs for approx 72v (more like 77v).
> 
> i get 66 v at full acceleration (450 amp alltrax controller) with a fully charged battery.
> 
> ...


Check if any terminals get warm or hot, maybe you are dropping voltage on bad connections.

How's your range? Is it what you'd expect? If you had bad cells your range would probably be low.

Do you have cell level voltage monitor like PakTrakr? It would show immediately if some cells sag more than others, I highly recommend getting it if possible.

What is your cruising amp draw and voltage?


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## jondoh (Sep 8, 2008)

I'm the guy without a lot of instrumentation. I don't have a current meter or pak trakker-- just the volt meter. 

I appear to be cruising at around 72 ~ 75 volts.

Range appears to be in line with expectations about 25 miles at 250 wh/mile.

i did have a loose connection at the controller battery minus which got so hot that it deformed the bolt. I replaced it and haven't had problems yet. I will check all terminals now.

i guess i'm wondering:

a) how can you check for a bad cell?

b) is my sag reasonable in your estimation? What does thunder sky consider a normal temperature?


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## jondoh (Sep 8, 2008)

Dimitri, is your curve for a 160 ah cell? Since I'm using 100 ah cells, would the curves be shifted down?


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## neanderthal (Jul 24, 2008)

No the curves would be the same if you look at C ratings. C ratings are multiples of the batteries amp hour rating. So 3c for your 100ah batteries would be 300amps for a 160ah battery would be 480 amps. But if you look at total amperage you are exactly right, the curves would be shifted down. A 100 ah battery putting out 400a will sag to a lower voltage than a 160ah battery.

You can get the curves for you batteries on evcomponents.com


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## dimitri (May 16, 2008)

On my drive this morning I paid close attention to my voltage drop at 1C and 2C and I think that we should also take into consideration voltage drops on battery links and cables. 

I think it's fair to reduce total voltage by 0.05V per cell to accomodate for cable / link losses at 1C-2C rates, maybe even 0.1V at 3C-4C.

Of course this all depends on cable thickness, quality of crimps, cable lengths , etc etc.

jondoh, what AWG is your battery cable and do you have long runs? check all your crimps, make sure none are hot after heavy accelerations. Did you use noalox to fill your crimps and between posts and copper links?

How typical is it for you to draw 4.5C? Is it every acceleration or just when you are showing off? Maybe you need lower gears.... you are murdering your pack if you do it all the time.

Also, can you attach multimeter to one of the cells such that you can see display when you drive? This way you can get a voltage baseline on a cell level, then move voltmeter leads from cell to cell to see if some cells are worse than others, etc.

Brian did this trick to see if BMS was misreporting a bad cell, which it was as it turned out...


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