BMS not updating

JohnJayMack

New Member
Joined
Dec 22, 2020
Messages
25
I built a 2P4S prismatic battery with an estimated total capacity of 560 Ah. This is not my first build. I am using the android app to monitor the BMS, an iCharger X8 for charge controlling, and a Fluke 87V meter to check the other two voltage readings. Of the Five batteries that I have built and tested, this is the first BMS to report these unusual values.

Using an iCharger X8, I charged the pack looking for a top balance value of 3.65 volts per cell. My initial charge current was 20 amps, and I lowered the charge current to 1 amp during the final part of the charge.

After a full charge, I did a load test using a 1500 watt inverter to get a discharge current of @106 amps until the voltage dropped to low to support this current. I reduced the discharge to @30 amps until The BMS shut the discharge off at 2.5 volts per cell.

I then charged the battery back up to a top balance value.

On the dashboard view, the app shows 100% charge but Remaining -115.99AH. That is a Negative number.
The Battery State page shows Nominal Capacity -104.97AH, Remaining Capacity -115.99AH, Percentage Capacity 98%.
The Parameter View shows Nominal Capacity 550.39 AH, Cycle Capacity 448.0AH. Both Positive numbers.
The Parameter Setting View shows Nominal Capacity of 550.39AH, Cycle Capacity 448.0AH.

Originally, I put in a pack value of 560 AH based on the spec sheet.

Finally, I noticed that the reported cell voltages were very different between the reported cell voltages from the BMS, the cell voltages measured by the iCharger X8, and measured voltages taken using a Fluke 87V meter. Here is a snapshot of readings taking during the second charge cycle.

BMS iCharger Fluke 87
cell 1 3.426 3.514 3.506
cell 2 3.534 3.509 3.502
cell 3 3.572 3.507 3.508
cell 4 3.345 3.552 3.545

The iCharger and the Fluke readings have the closest correspondence and the BMS has the greatest variance.

I am open to comments.
 
Last edited:

sunshine_eggo

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Oct 26, 2021
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Errors in BMS voltage readings are not uncommon. Would triple check you don't have a sensing harness issue. Loose connector or bad crimp.

Most BMS don't behave well SoC-wise until a battery has been fully charged while passing the current through the BMS.
 

JohnJayMack

New Member
Joined
Dec 22, 2020
Messages
25
All good suggestions.

First check I made was to verify sensing harness connections end for end. I did both resistance checks with the wires lifted, and voltage checks with the wires attached. My first assumption when errors occur is that I made the error.

The charge, discharge and charge were made through the BMS.
 
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