JK BMS not allowing charging when recovered from low voltage.

[hOOPSNAKE]

Hello. I have 2p, 4s lifepo4 batteries @12v. Each has its own JK bms. It’s winter here and we only get ~4h of direct panel sunlight a day. A couple of times the batteries have drained so much that the bms disconnects discharge (2.65) and remains at, or around that voltage. When the suns up and my CC (EPEver Tracer 40a) is free to make power, the BMS doesn’t allow any charging. I have to manually use emergency switch to allow the CC to start charging. I have the UVP at 2.65v and the recovery at 2.75v.

Any help would be great.

 

[sunshine_eggo]

Hello. I have 2p, 4s lifepo4 batteries @12v. Each has its own JK bms. It’s winter here and we only get ~4h of direct panel sunlight a day. A couple of times the batteries have drained so much that the bms disconnects discharge (2.65) and remains at, or around that voltage. When the suns up and my CC (EPEver Tracer 40a) is free to make power, the BMS doesn’t allow any charging. I have to manually use emergency switch to allow the CC to start charging. I have the UVP at 2.65v and the recovery at 2.75v.

Any help would be great.

This is a common issue.

When you hit low cut-off, the battery stops delivering voltage/current. The MPPT needs power from the battery to operate. It's not that the BMS is not allowing charge it's that the MPPT is not getting power from the battery. Nothing is providing voltage to the battery, so the BMS stays in protection.

A Victron controller does not have this limitation. They can be powered exclusively by PV and will output voltage to jump start a dormant BMS.

 

[hOOPSNAKE]

Thanks for your reply.

Actually, I’m pretty sure the CC can be powered from solar. It’s Bluetooth module is external of the battery cabinet and it’s always lit up. Could it be a voltage sensing issue?

 

[sunshine_eggo]

Thanks for your reply.

Actually, I’m pretty sure the CC can be powered from solar. It’s Bluetooth module is external of the battery cabinet and it’s always lit up. Could it be a voltage sensing issue?

If it doesn't sense a ~12V battery at the terminals, it doesn't output anything.

 

[hOOPSNAKE]

If it doesn't sense a ~12V battery at the terminals, it doesn't output anything.

Ok. So, if the JK is doing this by design and not allowing the CC to detect the voltage, how can this be “bypassed”? Could a small voltage sense wire be directly connected to the CC to allow morning startup? I realise this would drain the battery even more but think the CC rest voltage is pretty low( I’ll have to check the spec sheet). If this worked, and the CC started to charge, even though the discharge is disconnected, would the thin charge sense wire burn out or would the energy default to the 8g wire? I hope I’ve explained that well enough.

 

[sunshine_eggo]

Ok. So, if the JK is doing this by design and not allowing the CC to detect the voltage,

Sorta. It's just not providing voltage, or it's providing an arbitrary low voltage to indicate it's in protection mode.

how can this be “bypassed”? Could a small voltage sense wire be directly connected to the CC to allow morning startup?

You would have to wire directly to the cells and bypass the BMS. This would effectively disable the BMS and provide an alternate path for charging/discharge effectively defeating the purpose of the BMS... You could still charge and discharge outside of limits.

I realise this would drain the battery even more but think the CC rest voltage is pretty low( I’ll have to check the spec sheet). If this worked, and the CC started to charge, even though the discharge is disconnected, would the thin charge sense wire burn out or would the energy default to the 8g wire? I hope I’ve explained that well enough.

The sense wires would carry current according to the ratio of their resistances. In the event of BMS protection, the sense wires would carry all the current and likely burn out.

You need to insure that all your loads disconnect BEFORE the BMS triggers protection.