# Voltage sag



## cruisin (Jun 3, 2009)

Dougnutz said:


> So how much is too much?
> I recently updated some tired old floodies for Thundersky 160ah( 45 cells) batteries and the truck drives wonderfully. I don’t have a lot of miles on the new batteries but I’ve noticed now that the cold weather has rolled in that the battery voltage sag is significant. I’m finding myself having to stay below 1.5C in order to keep pack voltage above 130 (about 2.88v/cell). Resting voltage is still 143.5v. I only drive short trips 6-7Kwhr max, and this sag is detectable pretty much immediately. Is this just a function of the cold weather? It’s staying in the upper 30’s to lower 40s most of the time. I suppose I could add a heater and see what sort of affect it has. The documentation that came with the cells says that discharge to 2-2.5 volts will not damage the cells. I have no intention of discharging them that low but I fear that if I drive more than 20 miles or so the pack voltage may drop low enough that the voltage sag will make it nearly impossible to keep pack voltage above 2.8v per cell, which is my warm and fuzzy number.


You need to install a reliable BMS to protect the cells with LVC. Also, a SOC meter would tell you how much energy you have in the cells instead of using the voltage. There is only one way to do it, the right way.


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## Dougnutz (Aug 22, 2011)

cruisin said:


> You need to install a reliable BMS to protect the cells with LVC. Also, a SOC meter would tell you how much energy you have in the cells instead of using the voltage. There is only one way to do it, the right way.


I don't really want to get into the BMS debates there are plenty of those threads already. Suffice it to say I'm not using one but I do use a CA to monitor the pack SOC. I very rarely use more than 20Ah and the most I have used is 35Ah on average I use 14Ah. But I see this sag within one mile of my driveway "hot off the charger" so to speak.

I'm really just wondering what other people who are using similar batteries are seeing.


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## EVfun (Mar 14, 2010)

I don't know what is normal for TS cells because mine are little 60 amp hour units and they seem to have less sag that others report for the larger capacity cells. Still, they sag more when cold. I find a difference of about 0.10 to 0.13 volts per cell between normally warm (around 70F) and kinda cold (about 50F.) The thing is, I find that shift at 7C.

A BMS won't make them sag less.  You should make sure the pack is balanced if you choose to run without one. Top or bottom balance is really your choice, you just don't want any cell at a significantly lower SOC as that could lead to cell reversal which would look like serious sag.


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## Dougnutz (Aug 22, 2011)

EVfun said:


> I don't know what is normal for TS cells because mine are little 60 amp hour units and they seem to have less sag that others report for the larger capacity cells. Still, they sag more when cold. I find a difference of about 0.10 to 0.13 volts per cell between normally warm (around 70F) and kinda cold (about 50F.) The thing is, I find that shift at 7C.
> 
> A BMS won't make them sag less.  You should make sure the pack is balanced if you choose to run without one. Top or bottom balance is really your choice, you just don't want any cell at a significantly lower SOC as that could lead to cell reversal which would look like serious sag.


Ya I did do a top balance a couple weeks ago. One week after initial installation. I have probably put < 100 miles on the pack total. This truck is more or less a back and forth to the store type of vehicle. 

Anyway at 7c what sort of voltage sage are you seeing? That seems like a pretty steap discharge rate, I assume you don't hold that for long.


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## cruisin (Jun 3, 2009)

EVfun said:


> I don't know what is normal for TS cells because mine are little 60 amp hour units and they seem to have less sag that others report for the larger capacity cells. Still, they sag more when cold. I find a difference of about 0.10 to 0.13 volts per cell between normally warm (around 70F) and kinda cold (about 50F.) The thing is, I find that shift at 7C.
> 
> A BMS won't make them sag less.  You should make sure the pack is balanced if you choose to run without one. Top or bottom balance is really your choice, you just don't want any cell at a significantly lower SOC as that could lead to cell reversal which would look like serious sag.


The BMS is to protect the cell from excessive sag. Without it you are taking a expensive chance of destroying a cell(s). Been there and done it.


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## drgrieve (Apr 14, 2011)

Obviously over discharge can damage a cell, but I've seen a few high power discharges take a cell to 2v to get as much power from a cell (for dragging etc) as possible.

Would like to see some tests that would show damage to a cell because of high discharge (short and long durations) that be would valuable to know.

OP: you'll find your question is fairly common for those that live in cold areas and easily solvable with low wattage heating pads while you charge and using insulated boxes.

Try the insulation first. You might find the cells keep themselves warm if you time the end of the charge not long before you drive etc.


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## EVfun (Mar 14, 2010)

Dougnutz said:


> Anyway at 7c what sort of voltage sage are you seeing? That seems like a pretty steap discharge rate, I assume you don't hold that for long.


The sag is roughly 2.68 vpc at 420 amps. It drops about another 0.11 volt when the pack is down to about 50F. I have my controller set to 840 motor amps and 420 battery amps so I can sustain 420 amps out of the pack for quite a few seconds at a time. Even with modest amounts of power I can only apply full power so long before the speed limit is a distant memory.


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## ElectriCar (Jun 15, 2008)

Doughnutz, here's a link to what I used to keep mine warm. Got the idea from DIYguy. They are in an insulated enclosed box as well. 

I too run a CA with no bms. I also have an overvoltage relay which is in my build thread. 

And if I'm not mistaken, Jack Rickard demonstrated that batteries don't fail because of sag as long as the SOC of the cell is above zero.


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## MN Driver (Sep 29, 2009)

The more voltage sag while under higher current, the quicker the batteries create internal heat. The difference between the resting voltage of the cell and the voltage under sag multiplied by your current is all going to be heat in the cell. So basically you are heating the cells by driving heavy current through them when cold. This heat will reduce the sag, just don't hit them too hard for too long as this can create hot spots in the cells if there are any areas that are more resistant to electrical flow. My personal minimum is 2.5v for 10 seconds at a time, let them rest to let heat spread a bit and hit them again. I don't have numbers on sag yet but there are some guys who made a video using what looks like Headway cells in sub-zero temperatures trying to start a car and each attempt is a stronger attempt than the last, they cranked for maybe 5 seconds, let it sit for 30 and hit it again and eventually the cells were hot enough to really crank the heat. It's all about getting the cells to heat themselves through their own inefficiency. If done right, it should be safe for the cells and more efficient during the whole trip if its done right. That video that floated around here with the college professor talking about LiFePO4 discussed using heavy current to warm the cells in just this fashion.


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## tomofreno (Mar 3, 2009)

Yes, the increased sag is due to cold. Heating them will eliminate it. I use 35W heater pads, one under each group of 4 or 5 cells. That low of power will not overheat a cell or cause "hot spots", but have kept mine at the heater set point on two successive nights of 5 F. I use a 65 F set point now and notice no difference in summer/winter performance. I have CALB 180Ah cells and recently recorded some cell voltages on a run at 30% SoC. At 521A discharge current, 2.89C, the lowest one sagged to about 2.86V at cell temperature of about 70 F.


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## dladd (Jun 1, 2011)

tomofreno said:


> I have CALB 180Ah cells and recently recorded some cell voltages on a run at 30% SoC. At 521A discharge current, 2.89C, the lowest one sagged to about 2.86V at cell temperature of about 70 F.


How are you measuring the individual cell voltages while under load?


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## Guest (Nov 16, 2011)

dladd said:


> How are you measuring the individual cell voltages while under load?


Why would you if your cells are balanced? What would you do with the information. Balance your cells and stay above the bottom and below the top. Matters not if its cold or hot. When its cold you're cells are sluggish but still work fine. Just don't hot rod your vehicle. As long as your not empty the cells will do just fine with a bit extra sag. It is when the pack is empty that you will have trouble if you continue but that is also the same cold or hot. You do need well balanced cells and not balanced cells from a forced BMS balance job. Pick your cells so the capacity is nearly the same and bottom balance. It will work just fine. Proven. No muss no fuss. 

Pete


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## EVfun (Mar 14, 2010)

gottdi said:


> [snip] not balanced cells from a forced BMS balance job. [snip]


What is a forced BMS balance job?


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## Dougnutz (Aug 22, 2011)

ElectriCar said:


> Doughnutz, here's a link to what I used to keep mine warm. Got the idea from DIYguy. They are in an insulated enclosed box as well.
> 
> I too run a CA with no bms. I also have an overvoltage relay which is in my build thread.
> 
> And if I'm not mistaken, Jack Rickard demonstrated that batteries don't fail because of sag as long as the SOC of the cell is above zero.


Thanks for the link, I'm confident now that I need to insulate the batteries and add some sort of heater. And I also wondered about the SOC being the real indicator not the actual voltage. The info from Jack Rickard seems to agree with what the product documentation indicates. Presumably driving the voltage too low, just like SOC too low, can hurt the cell with hot spots.



tomofreno said:


> Yes, the increased sag is due to cold. Heating them will eliminate it. I use 35W heater pads, one under each group of 4 or 5 cells. That low of power will not overheat a cell or cause "hot spots", but have kept mine at the heater set point on two successive nights of 5 F. I use a 65 F set point now and notice no difference in summer/winter performance. I have CALB 180Ah cells and recently recorded some cell voltages on a run at 30% SoC. At 521A discharge current, 2.89C, the lowest one sagged to about 2.86V at cell temperature of about 70 F.


Do you have a link to the pads you used? I obviously need to do something.  

Is there any danger in letting the batteries get too cold? My truck is parked outside and we are headed into the 20's this weekend. I'm wondering if I need to be concerned.


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## Guest (Nov 16, 2011)

EVfun said:


> What is a forced BMS balance job?




Well I figured you get the idea. No BMS required.


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## dladd (Jun 1, 2011)

gottdi said:


> Why would you if your cells are balanced? What would you do with the information.
> Pete


i would determine which of my cells is the lagger that is causing a low voltage alarm (2.70v) when my pack voltage is sagged to 2.77vpc under a load.


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## steven4601 (Nov 11, 2010)

I saw it being mentioned that low temperature increase the impedance of the battery and thus also increase the losses thus also heating up the battery more quickly. 

From my (limited) knowledge, this is not exactly true. 
The current collectors (copper & aluminium) actualy reduce in electrical resistance when colder. It is the chemical process (for instance the graphite has a negative t.c.) that makes the battery slows down which translates to voltage sag (some sort of apparent resistance). Because the chemical reaction slows down and no specific large amounts of additional heat is produced when sagging due to cold.


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## Guest (Nov 16, 2011)

dladd said:


> i would determine which of my cells is the lagger that is causing a low voltage alarm (2.70v) when my pack voltage is sagged to 2.77vpc under a load.


If you have a cell that is lets say in the 2.7 volt region and the rest are in the 3.2 region then your cell is out of balance with the pack and it would tell me your top balanced maybe instead of the safer bottom balanced. If you bottom balance you will see at the top of the charge where your low capacity cell resides. You want your cells to safely all reach into the same voltage zone when you are near the end of your charge and at least for me when I sag hard during hard take offs. If your pack is full it really does not matter. If your alarm is on at 2.7 volts you will always get an alarm during real hard amp pulls. It may just happen sooner during cold weather. But Id for sure bottom balance your pack. It is at the bottom and hard amp pulls that is the issue with bottom vs top balance. No matter you will still always only use the most that your lowest capacity cell will provide. That is why if you don't go with a BMS you should bottom balance over top. If you KNOW for sure you will never get into the bottom (not likely) you can top balance if you like. It is your pack not mine. 

You choose. 

Pete


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## pm_dawn (Sep 14, 2009)

My pack sags from about 129v-130v under no load to 116v at 160A (1C load on my 160ah cells.) That way today at about +3degC and about 50-60% SOC

Pack is 40 Thundersky 160ah before the yttrium came. has been rolling for over two years now heading for its third winter.

Drove the car first winter in -28degC, batteries really sluggish, had open boxes under the car and no heating.

Drove the car last winter in -30degC, now with closed insulated boxes but NO heating, batteries did much better.

But bevare that really cold batteries does not have same capacity as warm batteries. Atleast according to spec.

Regards
/Per


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## dladd (Jun 1, 2011)

I have a shunting BMS, so I don't think bottom balancing works for my setup. Regardless of balance, I've got enough safety measures in place to protect the cells. Charger shuts off at 3.5vpc, BMS shuts off if any cell hits 3.6v, my controller is programmed to keep my pack above 2.77vpc, and my BMS will alarm if any cell hits 2.70v. I feel safe.

Here's my recent data. On a fully charged pack, I can sag it to 133v pretty easily (2.77vpc) but no single cell triggers the 2.70v BMS alarm even if I hold it there for an extended time (10 seconds or more). Same pack with about a 50% SOC when sagged to the identical 133v (less current though) will sound the alarm within seconds. I'm just wondering which cell is doing this.

Personally I don't understand the fear of data. I get that it has been deemed safe to run without a BMS, but I don't understand the resistance to gathering info on what's happening at the cell level.

sorry for the thread derailment... though I am still wanting to know how Tom is measuring cell voltages


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## ElectriCar (Jun 15, 2008)

Look at a split pack voltage monitor, don't over charge or discharge and your pack should last longer and you don't waste energy with your bms bleeding off power. You can buy quite a few extra cells for what a BMS costs you. 

The split pack monitor will instantly show you if you lose a cell or for some reason if half your pack voltage doesn't compare to the other half. At that point you know you have a problem. When you get home you just have to find the bad cell which will only take a minute with a meter. Same thing if you have a bad connection, the meter will indicate a bad connection between cells which only shows up under load. Will a BMS will show you that? 

If you have a bad connection, current during discharge will artificially make one battery appear over charged. Will the BMS then while you're driving start shunting to reduce the apparent over voltage on that cell? If that happens then it's going to be undercharged compared to the rest of the pack and then your distance is cut down by the amount that cell lost. If it loses 10% charge, your total capacity is cut by 10% due to your BMS misdiagnosing a problem and trying to correct it and when you let off the throttle if that cell SOC is low enough, it's going to show a LV condition and shut you down.

We who run BMSless are charging to just about the knee of the charging curve as the voltage just starts to spike. You may be adding maybe a half mile of charge to take your pack to full charge and at the same time you may be shortening the cell life, may be, I don't know. I never leave my "tank" near empty so half a mile or even a mile of extra charge is immaterial to me.


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## EVfun (Mar 14, 2010)

Since you are seeing the sag immediately Doug the whole issue of top balance, bottom balance, BMS, no BMS isn't even relevant. You mentioned you recently balanced the pack. I would recommend you recheck that after a drive, just to make sure sure you didn't make a cell adjustment mistake during the process or have a cell that is prematurely failing. If you have a cell or few cells significantly more charged than the rest you could have charging terminate before most of the cells are charged or be cooking a cell on charge which can increase its internal resistance. If you have a cell or few cells significantly behind the rest it could be just those cell(s) that are crashing under load.

So long as the pack balance is good you are seeing the effects of cold on LiFePO4 cells. The later TS and Winston cells that are LiFeYPO4 (with Yttrium) are rumored to be more resistant to cold sag. Mine are these newer TS cells and I still see some extra sag in the cold. Much below 50F the little convertible doesn't leave the garage so I won't get data about more serious cold. 

You should look to the manufactures spec sheets to make sure you don't charge them when they are too cold. Lithium cells can be discharged at lower temperatures than they can be charged. According to reliable sources, charging below the minimum temperature will damage the cells and can create a fire risk due to Lithium plating.


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## Dougnutz (Aug 22, 2011)

pm_dawn said:


> My pack sags from about 129v-130v under no load to 116v at 160A (1C load on my 160ah cells.) That way today at about +3degC and about 50-60% SOC
> 
> Pack is 40 Thundersky 160ah before the yttrium came. has been rolling for over two years now heading for its third winter.
> 
> ...


Do you keep your car parked in a warm area? One thought I have is that the batteries may not warm each other up with just insulated boxes. Since most of my trips are so short. 

Oh, do you do any venting in the summer months or leave the box closed and insulated?


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## Dougnutz (Aug 22, 2011)

EVfun said:


> Since you are seeing the sag immediately Doug the whole issue of top balance, bottom balance, BMS, no BMS isn't even relevant. You mentioned you recently balanced the pack. I would recommend you recheck that after a drive, just to make sure sure you didn't make a cell adjustment mistake during the process or have a cell that is prematurely failing. If you have a cell or few cells significantly more charged than the rest you could have charging terminate before most of the cells are charged or be cooking a cell on charge which can increase its internal resistance. If you have a cell or few cells significantly behind the rest it could be just those cell(s) that are crashing under load.
> 
> So long as the pack balance is good you are seeing the effects of cold on LiFePO4 cells. The later TS and Winston cells that are LiFeYPO4 (with Yttrium) are rumored to be more resistant to cold sag. Mine are these newer TS cells and I still see some extra sag in the cold. Much below 50F the little convertible doesn't leave the garage so I won't get data about more serious cold.
> 
> You should look to the manufactures spec sheets to make sure you don't charge them when they are too cold. Lithium cells can be discharged at lower temperatures than they can be charged. According to reliable sources, charging below the minimum temperature will damage the cells and can create a fire risk due to Lithium plating.


Thanks for the advice about charging when cold. I'll recheck the datasheet that came with the cells. 


I did recheck the balance last night after the drive and one group of cells in the pack were off a little more. Nearly the entire pack was 3.27 but three cells in the middle of the string were 3.24. This doesn't seem like much but I thought that it was odd that they were all next to each other. It might be meaningless since they were also all on one corner of the pack's physical arangement and may have been subjected to cooler air flow somehow. I'll recheck the balance tonight after I charge everything up.


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## tomofreno (Mar 3, 2009)

> How are you measuring the individual cell voltages while under load?


 A Cell log8S.


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## tomofreno (Mar 3, 2009)

> Do you have a link to the pads you used? I obviously need to do something.


 I used Farnum pads from www.kta-ev.com, but there are other options. Do a search here, and check out what others did. Here is a photo of a Farnum pad in the front box with 1/2" insulation, ready for a group of 4 cells to be lowered onto it:


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## tomofreno (Mar 3, 2009)

> I have a shunting BMS, so I don't think bottom balancing works for my setup. Regardless of balance, I've got enough safety measures in place to protect the cells. Charger shuts off at 3.5vpc, BMS shuts off if any cell hits 3.6v, my controller is programmed to keep my pack above 2.77vpc, and my BMS will alarm if any cell hits 2.70v. I feel safe.


 I agree, you should be fine.



> ...though I am still wanting to know how Tom is measuring cell voltages


 Cell log 8S from Hobby King. Do a search here and you will find some threads discussing them. Make sure you get the datalogging model if you get one (about $28) not the one that just monitors (about $14). I like data too. Interesting to see what is going on, especially if it only cost $28.


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## pm_dawn (Sep 14, 2009)

Dougnutz said:


> Do you keep your car parked in a warm area? One thought I have is that the batteries may not warm each other up with just insulated boxes. Since most of my trips are so short.
> 
> Oh, do you do any venting in the summer months or leave the box closed and insulated?


No I park outside in the cold !
Insulated boxes makes a real difference compared to open mounting.

I do not have any venting during summer. It rarely gets above 25degC here were I live during summer.

Regards
/Per


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