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JBD Voltage drop when charge disabled?

meetyg

Solar Enthusiast
Joined
Jun 4, 2021
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Hi.
I recently noticed some strange behavior on one of my DIY Lifepo4 batteries, using a 4S JBD:
When charge and discharge are enabled, I get the correct voltage at the terminals (using a DMM) as the BMS app states. But when charge is disabled, there is less voltage on the terminals (around half a volt less).

I went back to check two other 12v Lifepo4 batteries of mine (also with JBD BMSs, but different models) and the same behavior happens.

I wonder why this happens ?
Do other "smart" BMSs behave this way?

It may not be that critical, but it could confuse some SCCs or other peripherals connected to battery, that measure its voltage.
Having charge disabled could be intended, or may happen as the BMS decides it should (like overvoltage or overcurrent).

Is there a setting to change this behavior?
Has anyone else noticed this?
Thanks
 
If you're simply toggling the charge function and getting 0.5V difference, that's odd.

If you're comparing "disabled" to the voltage when actually charging, there should be a difference.
 
That's normal and inherent to how the BMS does the charge/discharge switching. Basically if both aren't enabled there's a diode in series, hence the 0.5 V drop. In theory the BMS should close the MOSFETs again passed a certain current to not fry said diodes (which are actually the body diodes of the MOSFETs themselves), try charging at like 10 or 20 A and you should see the drop disappear ;)
 
If you're simply toggling the charge function and getting 0.5V difference, that's odd.

If you're comparing "disabled" to the voltage when actually charging, there should be a difference.
No, not while charging, just measuring voltage.
 
That's normal and inherent to how the BMS does the charge/discharge switching. Basically if both aren't enabled there's a diode in series, hence the 0.5 V drop. In theory the BMS should close the MOSFETs again passed a certain current to not fry said diodes (which are actually the body diodes of the MOSFETs themselves), try charging at like 10 or 20 A and you should see the drop disappear ;)
OK, as long as its normal behavior.
Thanks.
 
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