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Do I really need another battery to feed Victron MPII?

corn18

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Sep 9, 2021
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I got my MultiPlus II 12/3000/120x2 installed today. I am feeding it with 2x206 Ah 12V SOK LiFePO4. According to SOK, these are rated for 100A discharge each. Well, I was pulling 230A for several minutes today. Got scared and turned off the loads. I read on the SOK Facebook page that several users have tested these batteries and found that they will discharge @ 150A and charge @ 100A.

My question: If I can pull 300 amps from these things, isn't that enough to feed my MPII? To be honest, I will never need more than 2000W for short periods, so I may never add a third SOK battery just because of that.
 
The spec sheet says 100A max continuous. 200A surge for 3 seconds. BMS disconnect at about 330A.

So somewhere between 100A and 200A it goes from full-time down to 3 seconds. I have no idea what the curve is between the two. Who knows what you do to the battery life if you try to use, say 125A, for an hour once a day or whatever. It may be fine, it may cause issues to the cells, or it may shorten the life of the BMS.

How were you pulling 230A? At 13.6V that's 3128W. That's way over what the 3000VA inverter is rated for (not counting the brief surge load it can handle).

With your two batteries in parallel at 13.6V and pulling the rated 200A from the batteries you can safely use over 2700W. Or maybe 2300W if you throw in an 85% conversion factor.
 
The spec sheet says 100A max continuous. 200A surge for 3 seconds. BMS disconnect at about 330A.

So somewhere between 100A and 200A it goes from full-time down to 3 seconds. I have no idea what the curve is between the two. Who knows what you do to the battery life if you try to use, say 125A, for an hour once a day or whatever. It may be fine, it may cause issues to the cells, or it may shorten the life of the BMS.

How were you pulling 230A? At 13.6V that's 3128W. That's way over what the 3000VA inverter is rated for (not counting the brief surge load it can handle).

With your two batteries in parallel at 13.6V and pulling the rated 200A from the batteries you can safely use over 2700W. Or maybe 2300W if you throw in an 85% conversion factor.
That's a good question. I do know the battery voltage was 12.2V not 13.6V under load. And I was also getting some amps from solar. I will have to redo the testing tomorrow to see if I can figure out where the BMS cuts off. My breaker is only 250A, so I know that's a limiting factor. I tripped that main breaker today when I turned on the electric fireplace on high. Saw 3100W on the GX display before it tripped. The BMS did not trip, though.
 
You'd have to pull 4kW to make the BMS shutdown. That could happen with a big enough surge.

3100W at 12.2V is 254A so the breaker did its job well.
 
You'd have to pull 4kW to make the BMS shutdown. That could happen with a big enough surge.

3100W at 12.2V is 254A so the breaker did its job well.
I really like my Blue Sea Systems breaker. Do I need to be worried about it? I was going to replace t with a class T fuse, but then I have to add a switch to disconnect the batteries.
 
It seems like the breaker works under "normal" loads.

From what I understand, the battery fuse with LiFePO4 batteries needs a really high interrupt capacity to properly deal with the massive current flow during a short. Blue Sea Class T fuses have an interrupt capacity of 20000A. The ANL fuses are 6000A. The breaker you have is only 5000A.

So while your breaker may handle a simple overuse situation like you saw recently, it may not safely handle a much larger surge caused by some sort of fault. Maybe someone with more knowledge of such issues will chime in.
 
Thanks @rmaddy. I have a 400A class T fuse in a holder sitting on my workbench. It's a beast. If I install that, I won't need the breaker.
 
If you have a 250A breaker then you should probably use a 250A fuse. 400A is only safe on 4/0 cable. And if you do add the fuse, remove the breaker, especially if you use the 400A fuse.
 
If you have a 250A breaker then you should probably use a 250A fuse. 400A is only safe on 4/0 cable. And if you do add the fuse, remove the breaker, especially if you use the 400A fuse.
That's what I'm thinking. Remove the breaker and replace with a 400A class T fuse and an HD Victron disconnect.

DC wiring 290RL t.jpg
 
EFE3DBFD-D19A-4965-9534-09426AE25C85.jpeg

I turned on as much as I could. Pulling 221A for 10 minutes. Need to hook up my heat gun and see where the inverting limit is. Also, looks like .8 conversion from DC to AC. Is that normal?
 
You are actually getting 85%. 2711W - 275W = 2436W. 2069W / 2436W = 85%. Though that still seems a little low.
 
7790BAA1-C948-4983-B7F2-4536E0BF82DB.jpegHeat gun wasn’t a very adjustable load. It was either on or off just varied how long based on the heat setting. What else is a good variable AC load?

I did determine my breaker cuts off right at 254A. Need to get that out of the way to see what these SOK batteries are capable of.
 
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