Output of overkill bms when "off"

[james1]

Hi all.
I just connected 16 new Eve Lf280k cells in 16s to a new 100 a Overkill Bms. All cells at 3.291 except for a couple at 3.292 volts.

Pack voltage 52.67. So everything seems ok.
Did not start charging yet.

But when I click the button to disable discharging "lock the discharge FETs in the off state" the pack voltage is 15.8 volts.

Is that the designed output? I thought it would be zero.

Thanks

 

[MisterSandals]

Is that the designed output? I thought it would be zero.

I think we all went thru this at one time. Its best described as leakage and if any amps are pulled, it quickly drops to near zero.
So normal behavior, whether as designed or not, i dunno.

 

[james1]

I think we all went thru this at one time. Its best described as leakage and if any amps are pulled, it quickly drops to near zero.
So normal behavior, whether as designed or not, i dunno.

Thanks for such a quick reply.
Good to know.

I put this question into Microsoft's new Copilot app:
"is it normal for an Overkill Solar Bms to show 15.8 volts at the output when the discharge FETs are locked in the off state?"

This was it's response:
When the **discharge FETs** of a **48-volt battery pack** equipped with an **Overkill Solar BMS** are locked in the **off state**, it is indeed **normal** for the output voltage to display approximately **15.8 volts**. Let's explore why this occurs: 1. **Voltage Drop Across FETs**: - The discharge FETs are responsible for allowing current flow from the battery to the load. - When they are turned off, the charge FETs (which are typically not fully off) take over. - These charge FETs have intrinsic body diodes that introduce a voltage drop (around **0.7 volts**). - As a result, the output voltage remains slightly lower than the battery voltage. 2. **BMS Behavior**: - The Overkill Solar BMS prioritizes safety and cell health. - By maintaining partial activation of the charge FETs, even when the discharge FETs are off, the BMS prevents overdischarge. - This approach ensures that the battery doesn't reach dangerously low voltages. 3. **Power Dissipation**: - Keep in mind that the BMS must dissipate the power lost due to the voltage drop across the charge FETs. - At higher currents, this power loss can be significant. - Therefore, disabling charging entirely is not recommended unless there's an unexpected overcharge situation. In summary, observing a slightly lower voltage (such as 15.8 volts) when the discharge FETs are off is expected behavior due to the diode drop across the charge FETs. The BMS's design aims to balance safety and performance, even during idle periods.

 

[james1]

I think we all went thru this at one time. Its best described as leakage and if any amps are pulled, it quickly drops to near zero.
So normal behavior, whether as designed or not, i dunno.

And also a Big Thank You for all of your responses all over the forum. I always read your responses. Very informative.