# Voltage Sag can kill lifepo4 cells. Yes or No?



## Amberwolf (May 29, 2009)

If you're pulling so much current from them that they sag that much, either they're not fully charged or you are probably exceeding their safe discharge ratings. 

*that* will kill them, eventually, even if nothing else does.
________
Zuzana


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## MN Driver (Sep 29, 2009)

Durr said:


> Say you bought a lifepo4 battery pack. Upon using the pack, you notice that it makes the cells sag down to 1.8v per cell and you can see this on a cell monitor. Ok, but you've only used a few AH from the pack. It's nowhere near completely used yet, would that still kill the cells?
> 
> Is it voltage sag that kills cells or is it amount of depletion?


1.8 volts per cell? That is very low. You aren't giving us very much information that would help us determine why they are so low.

Helpful information such as battery brand, Ah capacity, number of cells, does this pack have an active integrated BMS(like e-bike packs have), what load are you imposing on them? Have you checked to make sure there are no dead cells, or at least tried to look at the cell voltages immediately after the 1.8 volt sag before charging to get a better idea of what's happening?

If you have a fully charged pack and you are pulling a loads heavier than its load rating, expect problems, it seems that the pack you have might have some issues or perhaps is overrated. ...we won't know this and can't advise with the little information you provided.

If they are all roughly the same voltage under load and in the same state of charge and you are pulling amperage at a rate to pull them down to 1.8 volts, I would expect damage. This sort of sag will likely overheat the cells or cause internal damage(some cylindrical cells have thin bus strips that might burn with too much draw) and I don't know of any LiFePO4 cell manufacture that would tell you that they would last long with this type of abuse. If they don't die right away, their cycle life will likely take the hit. There is an article that FMA put out on A123 and the impact of high amperage draws at constant rates and the impact the internal temperature causes them against cycle life. It's a good read and pretty easy to search for online.

Answer some of the questions above, along with what application you are using the batteries with, some of the cells ratings provided to you, and how you are using them when getting this sort of sag.


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## Guest (Apr 18, 2010)

Durr said:


> Say you bought a lifepo4 battery pack. Upon using the pack, you notice that it makes the cells sag down to 1.8v per cell and you can see this on a cell monitor. Ok, but you've only used a few AH from the pack. It's nowhere near completely used yet, would that still kill the cells?
> 
> Is it voltage sag that kills cells or is it amount of depletion?


I want to ask a question here. WHERE are you taking this measurement from, and how?

It rather matters. 

The short answer is that the sag voltage will not kill the cell. It's a symptom, not a cause. But you have to be careful with these measurements. If you measure the voltage at any distance from the cell terminals, on the same cables that carry the current, you are measuring something else. If you have a high impedance voltage measurement device connected directly to the cell terminals, and are measuring there with a load on the cells, you may have another problem.

But I'm betting you are measuring a voltage drop across cables and terminals.

Jack Rickard
http://EVTV.me


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## Durr (Apr 18, 2010)

It was just a theoretical question. So it's really the heat that kills them then. I do have a lifepo4 cells but the sag isn't as bad as in the question. So if after I use my battery, I come home and feel the temperature and it doesn't appear to be warm then maybe it will be okay. I'd just like them to last at least 3 years. I've never felt the cells to be very warm after usage. Definitely not hot. Maybe slightly above room temperature after usage.


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## Voltswagen (Nov 13, 2008)

Jack
If what you are saying is true then those of us who measure voltage off a shunt located a few feet from the battery pack are getting mis-information? Or am I confused?

Another answer to his question and possibly the culprit in deep sag on LiFePo is that the battery pack is undersized for the application.
If you're trying to propel a 3000 lb vehicle with say 96v of LiFePo it's likely that you will experience deep sag on acceleration.
Spend the extra money and correctly size the pack.

Roy


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## Guest (Apr 19, 2010)

Voltswagen said:


> Jack
> If what you are saying is true then those of us who measure voltage off a shunt located a few feet from the battery pack are getting mis-information? Or am I confused?
> 
> Another answer to his question and possibly the culprit in deep sag on LiFePo is that the battery pack is undersized for the application.
> ...


No. You're not confused. But why would you be measuring voltage off a shunt? 

You would of course use a shunt to measure current. But if you are measuring your pack voltage, you definitely want to be connected right at the most positive and most negative terminals.

Some of the "sag" talk I see I suspect is caused by this. If you run 500 amps through 0.01 ohms, you're off by 5 volts. So any resistance from terminals and cables can be a problem. 

I actually have had cable clamps on terminals and measured at the handle of the clamp instead of on the terminal and introduced significant errors. 

Any voltage measurement should be from a ring terminal bolted onto the cell with the cell terminal bolt. If you take it a few feet away on a shunt, you are not measuring cell voltages.


Now you can run sense wires 50 feet and get an accurate reading. This is because your measurement device does not load the cells and there is really very tiny current in the wire. But if you are taking your measurement even a few feet from the terminals along cable that carries significant current, there will be a voltage drop when you have current flow.

Jack Rickard

Jack Rickard


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## Voltswagen (Nov 13, 2008)

Jack
So if I understand this correctly....because I'm taking the voltage reading off one side of the 500amp shunt...........when the reading is static (no current flow) it might be correct but when I'm crusing (current flow) the voltage will register lower than it actually is. (ie sag or more sag than is actual)

Thanks - Roy


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## atzi (Jun 26, 2008)

jrickard said:


> ....... you definitely want to be connected right at the most positive and most negative terminals...... So any resistance from terminals and cables can be a problem. ......... there will be a voltage drop when you have current flow.
> Jack Rickard
> Jack Rickard


One way to test if a location is inaccurate is voltage drop.
Place the positive lead of your voltmeter at the most positive battery terminal and your negative lead at the location you wanted to put positive lead. Run a heavy load and if voltage does not drop below a couple volts there is very little resistance between the points . Do the same on the negative side.


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## Guest (Apr 19, 2010)

Voltswagen said:


> Jack
> So if I understand this correctly....because I'm taking the voltage reading off one side of the 500amp shunt...........when the reading is static (no current flow) it might be correct but when I'm crusing (current flow) the voltage will register lower than it actually is. (ie sag or more sag than is actual)
> 
> Thanks - Roy


That is correct.

Jack Rickard


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