SCP Delay shutting down JK-BMS when trying to run air conditioner

[Diddsy04]

I have a 500Ah LFP battery in the canopy of my 4wd. I am using a JK-B2A8s20p BMS. I have a Victron 12/3000/120 Multiplus. I connect the 230v ac to my caravan. I have tried to run my air conditioner in my caravan and the Short Circuit Protection kicks in and shuts down the BMS. I changed the delay from 1500us to 5000us and the air conditioner will run. ( I have changed it back to 1500us). I have seen the startup I as high as 424 amps for a very short time. The maximum discharge I for the BMS is 350A. My air conditioner is an Air Command Heron 2.2 split system.
Will I blow up the BMS if I permanently change the setting to 5000us?
Is there any way of solving this problem without putting another 500Ah battery in parallel , e.g. super capacitors?

 

[niktak11]

You may be shortening the lifespan of the BMS. I recommend splitting it up into at least two packs anyways for redundancy.

 

[HRTKD]

I have two 120 amp 4s JBD BMS, each on it's own 4s 280 Ah set of cells. I have no problem starting my rooftop air conditioner through my Victron 12/3000/120. I specifically went with a two BMS/battery setup so I didn't have to worry about drawing too much current through a single BMS.

 

[Diddsy04]

Thanks for your replies. My battery consists of 4 x 500ah cells so I would have to get another four cells which are heavy, expensive and take up a lot of room which I don't have. Before I changed to LFP I had 390ah of agm batteries and they ran the air conditioner without any problems although it dropped the voltage very quickly so I swapped them out for a lithium battery expecting it to easily handle the load. The specs for my air conditioner stated the maximum locked rotor current was 19 amps which is about 380 amps at 12v. I must admit I didn't check that before installing the LFP battery or I would have got 2 x 300ah batteries instead. I will have to decide how much I really want to run the air conditioner.

 

[HRTKD]

Have you considered a soft start device for the air conditioner?

 

[MisterSandals]

The specs for my air conditioner stated the maximum locked rotor current was 19 amps which is about 380 amps at 12v.

19A x 230V = 4370W
4370W / .85 inverter efficiency / 12V = 428A

 

[mikefitz]

Although not common practice, some have installed two BMS in parallel to the same set of cells to increase the output current.

The suggestion of using a soft start in the AC feed to the air conditioner has a high chance of solving your problem. My company was one of the first, ( many years ago), to offer such units for single and three phase systems. In most applications they reduce the start current significantly.

Mike

 

[Diddsy04]

Have you considered a soft start device for the air conditioner?

I have considered one but I can't find an air conditioning technician who can confirm that my air con can have one fitted. Nobody wants to touch caravan air cons. I can understand why because my compressor is tucked into a tiny space under my fridge.

 

[Diddsy04]

19A x 230V = 4370W
4370W / .85 inverter efficiency / 12V = 428A

Thanks for that calculation. It means my air conditioner hasn't gone faulty and that my current reading is almost spot on.

 

[Diddsy04]

Although not common practice, some have installed two BMS in parallel to the same set of cells to increase the output current.

The suggestion of using a soft start in the AC feed to the air conditioner has a high chance of solving your problem. My company was one of the first, ( many years ago), to offer such units for single and three phase systems. In most applications they reduce the start current significantly.

Mike

I have never considered two BMS in parallel. Not a bad idea. I would like to install a soft start but I want to pay someone to provide the soft start and to install it. So far I haven't been able to find anyone and I have made a lot of phone calls.

 

[DIYrich]

Maybe: if you have access to the thermostat, and there is a Run line for the compressor, you could make a timer/relay switch that would close for one second when the Run line is activated. The circuit would short the negative line around the BMS so most of the current flows around the BMS for that one second (or however long the timer is set).

If you go the super cap route, my guess is 1,000 f per second for a 0.1 voltage drop. Don't know how it would be wired in (deliver 100 amps of the 425 required, limit charging draw, etc).

 

[Diddsy04]

Maybe: if you have access to the thermostat, and there is a Run line for the compressor, you could make a timer/relay switch that would close for one second when the Run line is activated. The circuit would short the negative line around the BMS so most of the current flows around the BMS for that one second (or however long the timer is set).

If you go the super cap route, my guess is 1,000 f per second for a 0.1 voltage drop. Don't know how it would be wired in (deliver 100 amps of the 425 required, limit charging draw, etc).

Thanks DIYrich. I'm pretty sure they installed the compressor and then built a cupboard around it. I can't get the compressor out without dismantling the cupboard. I think the easiest but most expensive way is to install another battery in parallel.