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Old 01-10-2022, 07:16 PM   #1
reiter284
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Default DIY Lifepo4 Daly BMS cell voltage drop issue under load

I finally got around to building the Sergio lithium set up but used lifepo4 batteries and a Daly BMS.

Everything works as advertised on my 2009 RXV with the stock controller, freedom mode, 23" tires.

However, under even moderate loads, there is a significant voltage drop across the cells. They are 105ah 1C continuous, 3C peak rated.

If I stand on the throttle, 3-4 cells will show 2.4 volts and the BMS will cut the accelerator via relay as its supposed to. However, they all return back to 3.3 volts immediately. I've driven it several miles and the cells appear to all have relatively good capacity.

Has anyone experienced this? I've checked all the connections, and its multiple cells across the pack. Could it be the bus bars?
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Old 01-10-2022, 08:31 PM   #2
Volt_Ampere
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Default Re: DIY Lifepo4 Daly BMS cell voltage drop issue under load

It is the internal resistance of the cells. 3C rating is only 300 amps peak. You may be pulling that much or more under heavy load with larger tires. Make sure that your bus bars are adequate for the high current - as some of the drop could be across them. The BMS can't tell the difference. Focus on the cells that are dropping the most. Where did you get the cells? Some Chinese sellers lie about the quality of their cells and even lie about whether they are new or not.
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Old 01-10-2022, 09:15 PM   #3
reiter284
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Default Re: DIY Lifepo4 Daly BMS cell voltage drop issue under load

I bought cheap Chinese cells. LiitoKala was the brand, they were like $40/each.

They seem to have pretty good capacity and they stay really well balanced. But even under medium acceleration, the voltage drop is .3 or .4 which is significant.

I checked and tightened all the connections.

Are BUS bars that sensitive typically? Some of the BUS bars are doubled up because I ran out of washers and the bolts are a little long. But that doesn't seem to correlate around the cells that are dropping more than others.

I've not done much with these types of batteries, but they seem like a pretty significant conductor compared to wire or other terminal connections.
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Old 01-10-2022, 09:20 PM   #4
Volt_Ampere
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Default Re: DIY Lifepo4 Daly BMS cell voltage drop issue under load

My guess is that they are not really able to put out 3C due to high internal resistance. You don't get good cells for that price in that capacity range. I also guess that they are not new cells or A grade cells either.
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Old 01-10-2022, 10:16 PM   #5
reiter284
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Default Re: DIY Lifepo4 Daly BMS cell voltage drop issue under load

******. I knew I wasn't buying quality cells and was prepared to take the risk of ruining them or losing a little capacity, but I didn't take into consideration the voltage drop making them unmanageable with the BMS.
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Old 01-10-2022, 10:26 PM   #6
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Default Re: DIY Lifepo4 Daly BMS cell voltage drop issue under load

Where are you at? If it is below 40 degrees, they will have excessive voltage drop. Does it do this only going uphill or on flat ground?
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Old 01-10-2022, 11:02 PM   #7
Volt_Ampere
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Default Re: DIY Lifepo4 Daly BMS cell voltage drop issue under load

Problem is the BMS thinks that some cells are discharged so it's shutting things down. As WalterM6 says, cold weather increases internal resistance a lot. (it just about doubles for every drop of 10degC in temp. Do you know what current your cart is pulling under load? I doubt it's more than 300 amps which would be about 3C for your batteries.
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Old 01-11-2022, 08:17 AM   #8
reiter284
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Default Re: DIY Lifepo4 Daly BMS cell voltage drop issue under load

I'm in Houston, Batteries were at 18C or 65F. I don't know the amps my cart pulls. I love everything about the setup. I think my options moving forward are:

Lower the voltage cutoff for individual cells to 2.2V and the overall pack to 41.6V (2.6V per cell) and monitor the balance of the cells regularly. This will help with the instant dips of a few cells.

Buy a new set of batteries maybe 200AH?

Will a navitas controller be the only way to limit the peak current?

Or is there a BMS that will can manage voltage over time? So 2.75V for 4 seconds is a cutoff vs instant 2.75V?
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Old 01-11-2022, 08:27 AM   #9
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Default Re: DIY Lifepo4 Daly BMS cell voltage drop issue under load

It may very well be the cell(s) drop if the draw is big enough. I did the "hook up the laptop" to get the readout when I first installed my lithium pack.

When I would drive up the steepest hill I encounter (about 30-35 degree) my cart would draw 392amps. It did not drop any cell below the LV cutoff and I changed my controller so the max draw would be 300amps. I re-tested the cart going up the hill again and encountered no perceptible decrease in performance.

You may want to look at your controller and change the settings (dunno if you can do this with your stock controller or not). Or you may want to consider upgrading your stock controller to an Alltrax where such adjustments can be done.

FWIW
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Old 01-11-2022, 08:45 AM   #10
WalterM6
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Default Re: DIY Lifepo4 Daly BMS cell voltage drop issue under load

What is the Max Amp rating of your BMS?
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