How is this voltage sag on my 22S2P LFP battery

[Zambam]

I recently completed a 22S2P battery pack by soldering 44 Gotion 32135 LFP cells together using 0.3 mm copper strips. Cells are rated 15Ah. Today, I did a screen recording of the BMS app while doing full throttle uphill so I can observe the V sag. The pack already had 28 miles on it. With no load, pack voltage was 71.5V. Full throttle, it sagged down to 68.3V, putting out 2128W (31.1A).

That works out to 4.5%. Is that a good number for V sag?

What V sags are you fellas seeing?

Here's a screenshot of the screen recording.

 

[Zambam]

Note the Cells delta of 0.059V during the sag. I had theBMS cell balance turned off for charge and discharge to see how well the cells stay in balance by themselves. A short while after the sag test (about an hour), the cells delta is back to normal.

 

[bengxe]

The math is pretty simple. You're a little over the spec for the cells at ~9mOhms, they claim <8. I couldn't tell you if this is a problem. Edit: I missed that you had already ridden it 28 miles, I would re-do this test with a fully charged battery.

Gotion 3.2V 15Ah 32135 cylindrical lifepo4 battery cell

Gotion 3.2V 15Ah 32135 cylindrical lifepo4 battery cell

www.evlithium.com

V/A=R=100mOhms, divided by 22 in series and multiplied by the 2 parallel groups.

 

[BatteryMooch]

You're a little over the spec for the cells at ~9mOhms, they claim <8.

You also have to add in the resistances for the spot welds, cell interconnect material/wiring, solder joint to the main leads, main leads, and connectors….can sometimes be pretty substantial.

 

[bengxe]

Can be but shouldn't be. In this case I suspect if it were fully charged the total resistance would match the manufacturers spec for just the cells. Since these voltage readings come from the BMS there shouldn't be much wiring to compensate for.

 

[BatteryMooch]

Can be but shouldn't be.

Agreed!

In this case I suspect if it were fully charged the total resistance would match the manufacturers spec for just the cells. Since these voltage readings come from the BMS there shouldn't be much wiring to compensate for.

Good point. All depends on whether the pack voltage is the sum of the cell voltages (where most of other resistances don‘t matter) or it was measured at the main terminals (where most of other resistances do add on).

For accuracy though it should be at the main terminals IMO as that’s the voltage the user actually works with.

 

[Zambam]

Looks like my 22S2P LFP voltage sag is not too bad per these 2 threads.

What is the voltage sag on your ebike?

What is the voltage sag on your ebike when you have turned the full throttle from 0 mph / kmh? I'm wondering the voltage sag on the other thread and I'm interested to know what kind of voltage sag you get on your ebike. What is the normal and what is not. Thanks!

endless-sphere.com

Working out safe voltage sag.

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...

endless-sphere.com

 

[neptronix]

I'd say this is within the acceptable range. The only problem is, as the pack ages, that sag will go up. In freezing condtions, the sag will be 4x worse.

This is why i like to overspec the hell out of the amp rating on any battery. But you didn't do too bad, you have less voltage sag % than the average factory bike