Working out safe voltage sag.

[MangDav]

So I been doing some research and can't find a concrete answer. I'm trying to find if there's a way to work out expected voltage sag of a cell/battery pack. Reason why I'm trying to find this is because I have a 20s14p Panasonic ncr18650ga battery which I use with a 150A Sabvoton controller, when I full throttle from a dead stop, I can see voltage drop as high as 8V-9V which I don't know if this is normal for 150A draw from a 72v pack, this is my first 72v build, I always messed with 48v systems and from experience, I know 3V-4V voltage sag is normal. The seller claimed that the battery pack's rated discharge is 150A with max continuous discharge current of 200A for 5 seconds so I am at the batteries discharge limit, atleast the BMS limit so could such a high draw cause such a drastic voltage sag? When I full throttle going 5MPH-10MPH I will only get 2V voltage drop which sounds normal to me.

 

[Hillhater]

Your measured 9 v sag at 200A would suggest a pack DCiR of 45 mOhm
You should calculate the theoretical DC iR for the pack, then use
Ohms law to find the THEORETICAL sag at 200 A
Then measure the actual DCiR to see if there is a significant difference

 

[harrisonpatm]

The seller claimed ...

Sorry to say, those are frequently famous last words.

I looked around for a datasheet on your cells, found this:
https://www.orbtronic.com/content/Datasheet-specs-Sanyo-Panasonic-NCR18650GA-3500mah.pdf
At 14p, a 150amp draw is just over 10 amps per cell. Says those cells are rated at 10 amps continuous discharge current. And the graph I linked shows something like a .5V sag at 10A draw, so multiply by 20s, then yeah, pulling the max recommended current from the pack should show around 8-10V drop.

 

[MangDav]

The seller claimed ...

Sorry to say, those are frequently famous last words.

I know you should not trust seller claims but the battery is from a reputable seller who provided proof that the cells were brand new not recycled and even provided pictures of the construction which in my opinion is over built with solid copper plates on both sides. I did open the pack myself when I bought it and everything checks out, the BMS specs, cells and the construction so if he didn't lie about those, I don't have a reason to be suspicious about his other claims.

 

[harrisonpatm]

That's totally fair, but also as I pointed out with the spec sheet, 10A per cell is their max rated discharge. So it does make sense for you to be seeing your reported voltage sag at such a discharge.

@Hillhater mentions that your reported sag suggests a DCIR of 45mOhm. I couldn't find IR on any spec sheet for this particular cell, but 45 is not unreasonable for an 18650. Not great, especially considering its for an EV, but not unheard of.

You said you are seeing said voltage sag, indicating that you have a live voltage display during riding. May I ask, are you also reading amp draw? If so, using what instruments? I'm wondering if you know what your actual amp usage is during acceleration, and whether it's possibly higher than even 150.

Last variable to mention is that the weather is getting colder. You'll see higher voltage drop in colder temperatures; its possible this is one contributing factor as well.

 

[MangDav]

That's totally fair, but also as I pointed out with the spec sheet, 10A per cell is their max rated discharge. So it does make sense for you to be seeing your reported voltage sag at such a discharge.

@Hillhater mentions that your reported sag suggests a DCIR of 45mOhm. I couldn't find IR on any spec sheet for this particular cell, but 45 is not unreasonable for an 18650. Not great, especially considering its for an EV, but not unheard of.

You said you are seeing said voltage sag, indicating that you have a live voltage display during riding. May I ask, are you also reading amp draw? If so, using what instruments? I'm wondering if you know what your actual amp usage is during acceleration, and whether it's possibly higher than even 150.

Last variable to mention is that the weather is getting colder. You'll see higher voltage drop in colder temperatures; its possible this is one contributing factor as well.

I'm using an SVMC72150 controller with a UKC1 display, that's how I'm able to get a voltage reading. The DC current setting inside the controller is set to 150A, it's a locked controller so it doesn't let me set it any higher, only lower. I'm not sure how accurate that setting is but the peak draw should be 150A max. The display does not have an AMP reading but it does have a watt reading, the highest peak I ever seen was 9200 watts which would indicate between 110A and 150A draw depending on the voltage which I didn't pay attention to at the time, that's if that watt indicator is accurate, I know some display can be off.

 

[harrisonpatm]

Sometimes off by a lot, I know mine was. I currently have two ways to measure amperage mid ride, I'm in the process of adding a third.

Anyway, it doesn't seem like you're doing/have done anything wrong, and as the datasheet and prior advice mention, you just might be at your cells' discharge limits, and there really isn't anything you can do except accelerate less and go more slowly overall if you're worried about triggering LVC.

I'm responding to your post in particular because I can sympathize. I thought I was overbuilding my battery as well, I thought I understood voltage sag and what a battery's "continuous discharge rate" meant, but ultimately I could have done better in either the number of cells in parallel, or selecting a different battery with lower IR.

 

[MangDav]

Sometimes off by a lot, I know mine was. I currently have two ways to measure amperage mid ride, I'm in the process of adding a third.

Anyway, it doesn't seem like you're doing/have done anything wrong, and as the datasheet and prior advice mention, you just might be at your cells' discharge limits, and there really isn't anything you can do except accelerate less and go more slowly overall if you're worried about triggering LVC.

I'm responding to your post in particular because I can sympathize. I thought I was overbuilding my battery as well, I thought I understood voltage sag and what a battery's "continuous discharge rate" meant, but ultimately I could have done better in either the number of cells in parallel, or selecting a different battery with lower IR.

I'm planning to add a temp. meter to my handlebars to be able to see motor temp. at all times as my motor already has a built in temp. sensor. While I'm adding a temp. meter, it might not be a bad idea for me to put an amp meter on my handlebars aswell. This is my first 72v build and trying to find a good quality 72v 150A pack for decent price appeared to be challenging. I been messing with 48v systems for few years now and finding a good quality 48v battery is easy not so much with 72v probably because they are less common, I had few sellers that flat out refused to tell me the cells that they were using which was a red flag for me, this battery that I have right now does get the job done, just didn't expect 10V voltage sag at 150A to be normal for it, I was expecting 3V or 4V. Wanted to make sure that there isn't something wrong, it's not a deal breaker, just means I have to be careful when doing full throttle pulls with lower voltage to not trigger LVC. I do plan in the future to upgrade to an unlocked controller as right now I'm limited to 50A flux weakening, while I'm switching controllers, I might as well upgrade to a 200A controller while I'm at it. I will have to find a new battery and I already know finding a 72v 250A rated battery with quality cells and construction is going to be challenging. I have been considering building a battery myself but I already know that every time I'm out riding, I would be paranoid the whole time that I did something wrong and the battery will suddenly combust in flames. I prefer to leave the battery building to experienced people plus it's nice to have some sort of warranty.