Lifepo4 Charging confusion

[webbbn]

I guess the BMS is closing down the charge path. When this happens the battery volts seen at the terminals is around 0.6 volts less than the Internal volts due to BMS volt drop.

I assume that's just for lower-end BMS that likely just use a diode to prevent reverse current when charging is disabled? I don't see that on my JK BMS.

 

[mikefitz]

assume that's just for lower-end BMS that likely just use a diode to prevent reverse current

Sort of, its the diode built into the off MOS, should be similar for all fet bases BMS.

 

[webbbn]

Sort of, its the diode built into the off MOS, should be similar for all fet bases BMS.

I finally had a chance to (I think) fully test this, and with my JK BMS (200A, 20S), I could not detect any voltage drop under any condition. I did find a post that describes the reason for the voltage drop, and it makes perfect sense, but I don't think I'm seeing that effect.

I've wondered how they block one direction and not the other without risking dropping a huge amount of power over some sort of diode, which they work around by only turning off a single MOSFET when the current is low, but I don't see any difference in the output voltage when I turn on/off charging with no (only the draw from the meter) current, or with 1-2A of discharge current.

I don't know if it's relevant to this discussion, but it seems like JK has an improved design that doesn't have this voltage drop.

I think you could use an ideal diode to make a bidirectional MOSFET switch, and, I didn't look into this in detail, but it looks like TI is making an ideal bi-directional power switch, although that is off-topic, but here's the paper:

https://www.ti.com/lit/an/slva948/slva948.pdf

 

[datsolardude]

Solution found, it was a faulty circuit breaker I had from Renogy Positive output to battery. All fixed