JK BMS - Discharge Short Circuit Protection

[Lawrence_craig]

Hi all

I have a recent instal of a 300Ah LiFePO4 system with a JK BMS on my sailboat.

All has been working fine in the last week but today the BMS went out once on Short Circuit protection. I can't find anything on what causes or triggers this. Can anyone advise?

I was running my anchor windlass at times possibly during this time. Is it maybe that the windlass was pulling too much current? (1000-2000W)

 

[Q-Dog]

Yes, the BMS could see a heavy load as a short circuit.

 

[Lawrence_craig]

I have a 100amp fuse on the windlass so couldn't have been pulling any more than this. I guess the only parameter I can change is the SCP delay which is currently set at 1500us. Or is there anything else I can change? I get nervous over changing safety protections ?

 

[meetyg]

Most fuses are slow blow, so that doesn't mean much that it didn't blow. I suppose the motor surge current caused the short current protection to trip.

 

[Lawrence_craig]

Thanks, will keep a lookout the next time it happens.

 

[OffGridIdaho]

If the inverter is off power for more then a few minutes recharging the caps during inrush will blow the scp on the bms.

Here is how I diagnosed and fixed.

Shut down for 30 minutes to clean the inside of the inverter (dust)
Turned JK BMS back on and instantly get the short circuit protection fault.
Went into advanced password settings and turned off discharge slider in the bms.
SCP counts down from 60 seconds and doesnt blow another error.
Turned on discharge slider and BAM SCP blows again.
OK I think I have this figured out.

30 minutes down time was enough to drain the caps on the inverter (see where Im going?)
Turned slider for discharge off again. BMS reset to GTG at T MINUS 60 seconds and its happy.
Flipped main breaker to inverter OFF, turned slider inside jk bms to on or discharge again, no error.
Sooooo I took a small wire and ran it from the neg side of the battery (not other side of neg bms) to the neg terminal of inverter. FLASH-SPARK it starts charging caps slowly. Slow enough not to have a huge in-rush and trip SCP in the jk bms.
I wait 60 seconds and remove wire and re-touch, no spark. I quickly flip the breaker back on ito the inverter
AND HOLY FLEEPIN BATMAN it doesnt blow the SCP error.

It was the massive in-rush to the inverter caps blowing the SCP fuse inside the jk bms.

 

[Familymandan]

Hello, I have the exact same issue. I had (3) Daly BMS in parallel and switched to JK BMS and now I have this error. I thought I had an inverter issue so I ordered and installed a new power board. I'm new to this forum so I wish I'd have checked here first. I have (5) 100V-6800mF caps. I've read elsewhere that a resistor was used to charge the caps. You sound like you did not use a resistor, just touched a wire between neg side of the battery to the neg terminal of inverter. Do I need a resistor to do this? If yes, can someone please explain the exact procedure for using the resistor. Wattage, ohms, where to connect both ends, how long does it take, is there a way to electrically test to see if the caps are charged...Anything I'm forgetting?

 

[Zwy]

Hello, I have the exact same issue. I had (3) Daly BMS in parallel and switched to JK BMS and now I have this error. I thought I had an inverter issue so I ordered and installed a new power board. I'm new to this forum so I wish I'd have checked here first. I have (5) 100V-6800mF caps. I've read elsewhere that a resistor was used to charge the caps. You sound like you did not use a resistor, just touched a wire between neg side of the battery to the neg terminal of inverter. Do I need a resistor to do this? If yes, can someone please explain the exact procedure for using the resistor. Wattage, ohms, where to connect both ends, how long does it take, is there a way to electrically test to see if the caps are charged...Anything I'm forgetting?

Use a light bulb when charging caps, when the bulb gets very dim or quits glowing then caps are charged. Just use a bulb rated for the battery voltage. I'm switching out my resistors for bulbs.

You can even use a pencil if you don't have a light bulb/pigtail handy as the graphite will conduct electricity.

 

[kommando]

, just touched a wire between neg side of the battery to the neg terminal of inverter. Do I need a resistor to do this? If yes, can someone please explain the exact procedure for using the resistor. Wattage, ohms, where to connect both ends, how long does it take, is there a way to electrically test to see if the caps are charged...Anything I'm forgetting?

The small wire acted to limit the current.

60 secs would be plenty time

The resistor I use is 12 ohm 25W

You need 2 wires, a positive and negative, I put a resistor in the positive and a switch in the negative.

With the switch in the negative wire off you by pass the breaker with the negative and positive wires.

Turn the switch on and the reduced current will slowly feed the inverter caps and the inverter turn on with no complaints from the BMS, 60 secs will do.

Now turn the breaker on and no inrush as the caps are full

remove the 2 wires including switch and resistor

 

[Familymandan]

You need 2 wires, a positive and negative, I put a resistor in the positive and a switch in the negative.
Connect a small wire from the battery + thru the resistor/bulb to the inverter +?
Connect a small wire from the battery - to the inverter - thru a switch?

Turn the switch on and the reduced current will slowly feed the inverter caps and the inverter turn on with no complaints from the BMS, 60 secs will do.
Does the inverter have to be in the ON or OFF state?

 

[offgrid-curious]

You can also check out FilterGuy's doc on designing a pre-charge circuit switch: https://diysolarforum.com/resources/inverter-disconnect-switch-with-precharge.60/

 

[kommando]

Does the inverter have to be in the ON or OFF state?

On if you want the caps to be filled.

 

[meetyg]

On if you want the caps to be filled.

No, OFF!

The capacitors are usually directly connected to the battery terminals.

No high current flows in the On/Off switch.
It's just a switch for the controller boards in the unit.
Of you leave inverter on, you will charge the capacitors slower, or maybe the inrush will be even bigger.

Just pre-charge for a few seconds with inverter off, connect battery terminals quickly, then you can turn on.

 

[kommando]

No, OFF!

The capacitors are usually directly connected to the battery terminals.

No high current flows in the On/Off switch.
It's just a switch for the controller boards in the unit.
Of you leave inverter on, you will charge the capacitors slower, or maybe the inrush will be even bigger.

Just pre-charge for a few seconds with inverter off, connect battery terminals quickly, then you can turn on.

SMA has to be on, without turning on its own breaker the caps are isolated, so the answer is depends on the inverter.

 

[meetyg]

SMA has to be on, without turning on its own breaker the caps are isolated, so the answer is depends on the inverter.

OK, so I was referring to the MPP/Voltronic style inverters with a small ON/OFF switch, not a breaker.

Thanks for the clarification.

 

[OffGridIdaho]

You need 2 wires, a positive and negative, I put a resistor in the positive and a switch in the negative.
Connect a small wire from the battery + thru the resistor/bulb to the inverter +?
Connect a small wire from the battery - to the inverter - thru a switch?

Turn the switch on and the reduced current will slowly feed the inverter caps and the inverter turn on with no complaints from the BMS, 60 secs will do.
Does the inverter have to be in the ON or OFF state?

This is WRONG info as the JKBMS only controls the Negative power flow I bypassed the JKBMS by running a wire from the neg battery to neg input on inverter. YOU DO NOT NEED 2 wires. The positive runs straight from the pos battery to the inverter. JKBMS only breaks NEG current flow.

 

[Familymandan]

This is WRONG info as the JKBMS only controls the Negative power flow I bypassed the JKBMS by running a wire from the neg battery to neg input on inverter. YOU DO NOT NEED 2 wires. The positive runs straight from the pos battery to the inverter. JKBMS only breaks NEG current flow.

I understand your point. But the + and - between the batteries and inverter run thru a disconnect. If you turn the disconnect off, then neither the + or - is connected to the inverter. In order to complete the circuit, I ran a wire from 1 of my batteries + thru a light bulb to inverter +. I ran another wire from the same battery - thru a 3A CB to inverter -. This bypassed the BMS and disconnect. I turned on the breaker, the bulb lit for a few seconds then off. I removed the added wiring, turned on my disconnect, enabled the JKBMS discharge and it was happy, No more Short Circuit Fault.

 

[kommando]

Why are you bypassing the BMS when you are also employing a resister or bulb to limit the amps, if the amps are limited then the BMS will not trigger an alarm in circuit. I use the wires and resistor to by pass the breaker I have between the BMS and the inverter and the BMS is on and does not trigger an alarm while the caps are being filled and the breaker is off. Once the caps are filled I can turn the breaker on and full amps can flow but as the caps are full the BMS is happy and the wires can be removed until next time.