@RCinFLA i may be wrong about your statement but on my benchtop observation when discharge is turned off even if i increase current over 5A and voltage drop across is 0.6 V it will not start charging ( considering cells are not fully saturated at given voltage and benchtop power supply is set same as voltage measured before JK BMS. Initial drop across BMS will result in no current flowing because voltage is less than true battery pack voltage. Raising pack voltage on benchtop power supply above 2 V than pack voltage measured it will trigger automatically switch for discharge ON and voltage difference will drop to 1-2 mV , where now if this is not something user knows it will charge pack 2 Volts higher ( I'm talking about pack Voltage here not cells voltage). Please correct me if I'm missing anything in my observations.
JK BMS issue with charging
- Thread starter Becaris
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I've written about this in the past here:
diysolarforum.com
Andy from Off Grid Garage also showed this phenomenon - watch the "Discharge" status on the screen recording of the app:
BMS MOSFETs Explained
I'm writing this post since it seems there is a lot of confusion (especially in the comment section of a certain youtuber in Australia) on why both MOSFETs in a typical single port BMS are controlled individually and why/how this works when they're in the same series circuit. It will hopefully...
diysolarforum.com
Andy from Off Grid Garage also showed this phenomenon - watch the "Discharge" status on the screen recording of the app:
RCinFLA
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In the first video it shows confused Andy. Start video at 9:00 timestamp.
At 13.31 timestamp is the critical action. He enables charging on BMS (discharge still disabled) and raises power supply voltage so charging current goes to 5 amp, and voltage drop across BMS goes from 360 mV to 3.5 mV.
To be specific, and because he did not know what was happening, he did not specifically look for what BMS current sense overrode the discharge disabled setting. At the 5 amps of charging current, just where he happened to set power supply, it was overridden.
At some point between 0 and 5 amps of charge current the BMS override of discharge disabled occurred. If you gradually raise the charge current and watch for when voltage across BMS drops to low level it will tell you the disable override current. So, my statement of 5 amps may not be accurate. It might take less current through BMS required to override the disable setting. The BMS needs some margin on getting too close to zero current since its current measurement at low current is not great and there is delay in its reaction time to turn it back ON.
Realize there are 20 large diodes in parallel during disabled setting mode. Because of the diode V-I exponential curve there will not be as much diode voltage drop at low currents. BMS net heating will be the diode drop times the BMS through current. Above about 10 amps it would begin to have significant heating so they need to turn the disabled MOSFET back ON to avoid the diode voltage drop additional heating.
At 13.31 timestamp is the critical action. He enables charging on BMS (discharge still disabled) and raises power supply voltage so charging current goes to 5 amp, and voltage drop across BMS goes from 360 mV to 3.5 mV.
To be specific, and because he did not know what was happening, he did not specifically look for what BMS current sense overrode the discharge disabled setting. At the 5 amps of charging current, just where he happened to set power supply, it was overridden.
At some point between 0 and 5 amps of charge current the BMS override of discharge disabled occurred. If you gradually raise the charge current and watch for when voltage across BMS drops to low level it will tell you the disable override current. So, my statement of 5 amps may not be accurate. It might take less current through BMS required to override the disable setting. The BMS needs some margin on getting too close to zero current since its current measurement at low current is not great and there is delay in its reaction time to turn it back ON.
Realize there are 20 large diodes in parallel during disabled setting mode. Because of the diode V-I exponential curve there will not be as much diode voltage drop at low currents. BMS net heating will be the diode drop times the BMS through current. Above about 10 amps it would begin to have significant heating so they need to turn the disabled MOSFET back ON to avoid the diode voltage drop additional heating.
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RCinFLA
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Maybe this diagram will help explain the charge/discharge disable function. The diagram at bottom shows the inverter capacitor pre-charge function which only occurs when BMS turns itself on. (it won't help limit current if you use your series circuit breaker when the BMS is already ON)
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I have watched Andy video but my test is giving me different results. Will have to watch one more time when I get the chance to see where Andy and me are going different in observation. But for everyone else this is not something you should be worried about JK BMS it just needs attention to this .
RCinFLA
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Andy still doesn't fully understand what is happening.
Instead of adjusting the power supply voltage he should have set power supply to desired absorb voltage, as if it was a normal charger, and use the variable current limit to adjust the charging bulk current allowed to go through the BMS, as if it is from a PV controller with variable PV sunlight power output.
If he did this, he would just see the voltage drop across the BMS suddenly reduce when the current reached the point where BMS overrides the disabled discharge setting. If he then adjusted current limit back down, he would see when BMS re-allowed the discharge disable and voltage drop across BMS would increase again.. This of course assumes battery state of charge is low enough not to limit the bulk constant charging current by getting too close to absorb charge voltage limit set on power supply.
The greatest ramification is if charger relies on current taper off charge termination from absorb cycle. As soon as taper current drops low enough the BMS voltage drop will snap increase by diode drop and the charger may immediately exit absorb cycle prematurely.
At a high enough current demand through the BMS, the BMS must override the disabled discharge or the BMS will overheat. All BMS's that allow independent charge/discharge disable must do this or risk overheating BMS.
This all applies the same way to discharge current if only charging is disabled. If inverter draw 50A to 100A with the diode drop in BMS still engaged the BMS will roast in a few minutes. More likely, the BMS over temp sensor will shut down the BMS and the inverter.
Instead of adjusting the power supply voltage he should have set power supply to desired absorb voltage, as if it was a normal charger, and use the variable current limit to adjust the charging bulk current allowed to go through the BMS, as if it is from a PV controller with variable PV sunlight power output.
If he did this, he would just see the voltage drop across the BMS suddenly reduce when the current reached the point where BMS overrides the disabled discharge setting. If he then adjusted current limit back down, he would see when BMS re-allowed the discharge disable and voltage drop across BMS would increase again.. This of course assumes battery state of charge is low enough not to limit the bulk constant charging current by getting too close to absorb charge voltage limit set on power supply.
The greatest ramification is if charger relies on current taper off charge termination from absorb cycle. As soon as taper current drops low enough the BMS voltage drop will snap increase by diode drop and the charger may immediately exit absorb cycle prematurely.
At a high enough current demand through the BMS, the BMS must override the disabled discharge or the BMS will overheat. All BMS's that allow independent charge/discharge disable must do this or risk overheating BMS.
This all applies the same way to discharge current if only charging is disabled. If inverter draw 50A to 100A with the diode drop in BMS still engaged the BMS will roast in a few minutes. More likely, the BMS over temp sensor will shut down the BMS and the inverter.
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Which is what he did in the video I posted, at the time stamp it's at.
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