DIY Lifepo4 Daly BMS cell voltage drop issue under load

[Volt_Ampere]

One side of the bus bar straps to the other. That way the voltage drop of the strap will change where it shows up in your BMS display. Think of each strap as a resistor even though the circuit shows it as a common point to both battery posts. The voltage drop of those straps shows up as a change in the cell voltages depending on where the BMS sense wires are connected.

[DaveTM]

To address the bus bars, I purchased (maybe a little overkill....but they sure work!) a "Multipurpose 110 Copper Bar, 1/8" Thick, 3/4" Wide, 2 Feet Long" from https://www.mcmaster.com/catalog/127/4090 The product number is 8964K5 and you can just go to the web site and punch in that product number.

I then drilled the holes in the right place, and also drilled and tapped the holes for my BMS to hook up to.

Just an option that may work for you.

And just for kicks, I totaled up your individual cell voltages on your post. At rest the pack voltage is 53.17 and under load it's 47.477. That is a pretty steep drop of 5.693V.

Just a thought, but are your existing busbars getting warm when you do quick, multiple accelerations? Heat indicates "wasted current".

[Pat911]

Jumping in on this a little late but like others here, I would suspect the bus bars. Also, ensure that the connections are torqued down correctly. You'd be surprised at the voltage drop across a seemingly tight, but inadequately torqued connection.

[reiter284]

Originally Posted by DaveTM

To address the bus bars, I purchased (maybe a little overkill....but they sure work!) a "Multipurpose 110 Copper Bar, 1/8" Thick, 3/4" Wide, 2 Feet Long" from https://www.mcmaster.com/catalog/127/4090 The product number is 8964K5 and you can just go to the web site and punch in that product number.

I then drilled the holes in the right place, and also drilled and tapped the holes for my BMS to hook up to.

Just an option that may work for you.

And just for kicks, I totaled up your individual cell voltages on your post. At rest the pack voltage is 53.17 and under load it's 47.477. That is a pretty steep drop of 5.693V.

Just a thought, but are your existing busbars getting warm when you do quick, multiple accelerations? Heat indicates "wasted current".

They don't get hot, but the severe voltage drop is only for maybe for 2-3 seconds. Just cruising flat ground at top speed the voltages hover above 3.1.

I should have ordered copper, but the aluminum flat bar is on the way and hopefully its enough. 1/4" x 1" x 2.25"

If that fixes it, I will do a write up on my build. I think I'm all in for around $1000 with batteries, BMS, charger, electrical components.

[Volt_Ampere]

If you have access to a good DVM, you can measure the drop across the bus bars. I once worked on electric cars that were pulling very high amps and we had to work on every connection to the old fashioned FLA batteries. (We're talking 1973)

[WalterM6]

One more thing if you don't mind. Could you post screen shots of the BMS settings. Also did you top balance your cells?

[Pat911]

One other thing I noticed from your screen shots is that cell one is consistently the highest cell. That cell would be measured with the balance leads connected directly to it without any bus bars. Pack negative would be on its negative terminal and cell 1 lead would be on it's positive terminal.

[reiter284]

Originally Posted by Pat911

One other thing I noticed from your screen shots is that cell one is consistently the highest cell. That cell would be measured with the balance leads connected directly to it without any bus bars. Pack negative would be on its negative terminal and cell 1 lead would be on it's positive terminal.

Yes, and 16-15-14-13 are doubled up busbars because I ran out of washers. 12 is always the lowest cell under load. Hopefully, the bigger busbars fix it.

[reiter284]

Originally Posted by WalterM6

One more thing if you don't mind. Could you post screen shots of the BMS settings. Also did you top balance your cells?

I lowered some of the low voltage thresholds just so it wouldn't cut off. But here are my current settings. Yes I top balanced. At rest they are really close.

[WalterM6]

Did these batteries ever work correctly, or did they start messing up on day one?