Voltage Dives with Any Load

[Pokerking1993]

Hello,
I built my 4s LiFePo bank a couple of years ago and never used or tested it until now. I ran small fans and a heat gun but never pulled more than 10A on it until I hooked up an electric burner (since this is supposed to be backup power) and pulled a peak of 52A. Ever since that test, ANY load I put on the bank the voltage dives to basically 0. I just want to confirm that my cells indeed bad before shelling out for another set.

Example: I have a 4A load on it right now and the voltage dives from 13.9v to 10v

 

[Q-Dog]

How big is this battery? BMS? What size wiring?

 

[Hogheavenfarm]

Are you considering amps at 12v or 120v? Sounds like you are using the 120v amp data and not allowing for 12v conversion, so your 9 amps (120VAC) would be 90 amps (12VDC). (basing this off the heat gun comment, and your 13.9v reference).

 

[Pokerking1993]

How big is this battery? BMS? What size wiring?

The listed cell capacity is 320Ah per cell for a total of 1280Ah. BMS is a RadioB Tech Smart BMS 4S 12V 150A. I have I think 4 gauge wire coming off of the pack but my load tester is using some small 18 gauge wire. The pack voltage is 12v. I had it hooked to a 1500w inverter when I was testing with the heat gun and burner. As of now, the pack is unhooked from the inverter and I am using this load tester to put a load on the cells. https://www.amazon.com/dp/B07HB3XK7L?psc=1&ref=ppx_yo2ov_dt_b_product_details

 

[timselectric]

The listed cell capacity is 320Ah per cell for a total of 1280Ah.

It's only 320ah total.
In series, you add the voltages and the amperage stays the same.

 

[Zapper77]

The listed cell capacity is 320Ah per cell for a total of 1280Ah. BMS is a RadioB Tech Smart BMS 4S 12V 150A. I have I think 4 gauge wire coming off of the pack but my load tester is using some small 18 gauge wire. The pack voltage is 12v. I had it hooked to a 1500w inverter when I was testing with the heat gun and burner. As of now, the pack is unhooked from the inverter and I am using this load tester to put a load on the cells. https://www.amazon.com/dp/B07HB3XK7L?psc=1&ref=ppx_yo2ov_dt_b_product_details

Your package voltage (12v) is way to low.

What are you using to charge your pack?

 

[Xhumeka]

I would start by testing the voltage of each cell, then try charging each cell individually. If the cells take a normal charge and are acting fine and the problem still exists, it must be your BMS misbehaving.

 

[Zapper77]

Check for poor connections.

 

[Pokerking1993]

Your package voltage (12v) is way to low.

What are you using to charge your pack?

Bench power supply at 14.6v.

 

[Zapper77]

Bench power supply at 14.6v.

What's the current resting voltage of your pack and each cell!

 

[Pokerking1993]

What's the current resting voltage of your pack and each cell!

I'll have to charge it again and get back to you on this.

 

[Pokerking1993]

What's the current resting voltage of your pack and each cell!

Alright, charged it about fully a couple of hours ago. Pack voltage holding steady at 14.2v. Cell 1: 3.557v Cell 2: 3.558v Cell 3: 3.563v Cell 4: 3.569v

 

[Zapper77]

Alright, charged it about fully a couple of hours ago. Pack voltage holding steady at 14.2v. Cell 1: 3.557v Cell 2: 3.558v Cell 3: 3.563v Cell 4: 3.569v

Those all look great.

Apply a load and check for heat buildup at the connections.

 

[Pokerking1993]

Those all look great.

Apply a load and check for heat buildup at the connections.

With or without the BMS influencing?

 

[Zapper77]

With or without the BMS influencing?

Both. Try to check the voltage of each cell when you have a 10 amp DC load.

 

[Pokerking1993]

Both. Try to check the voltage of each cell when you have a 10 amp DC load.

Hmm. So I think it was just a poor connection. I have a quick disconnect and I think it was just that... I hooked jumper cables from the pack to the inverter and pulled 30A no problem. Gonna figure out another quick disconnect solution. Sorry for wasting time on this.