How are server rack batteries paralleled within the system?

[SilverFox333]

In your video on building your own 24v LiFePO4 battery pack out of (8) 3.3 volt cells, you state that one should not parallel more than 2 such battery packs. But in all the server rack battery systems I see, each individual 48v rack module is paralleled with the other ones in the rack. Some are paralleled with daisy-chain cables and some are paralleled with big vertical bus bars that each 48v module attaches to. So a 600Ah rack might have (6) 100Ah battery modules in it, all of them paralleled together. So - how do they do that? Over time, can one individual module have a lower internal resistance, and hence 'take over' the charging/discharging? Is module-to-module balancing sometimes necessary? If so, how best to do that?

I very much appreciate your clear explanations of so many things battery related that you have given us in your videos. Can you help me understand this one?

 

[Shimmy]

Not sure exactly which video you are referencing, but generally you want to make sure that you aren't using a BMS with paralleled cells, although two cells in parallel isn't necessarily the end of the world. It really comes down to the quality and consistency of the cells, or at least common binning to match cells.

The server rack batteries can be paralleled up to the limitations of the individual BMSs.

 

[Adam De Lay]

I believe the paralleling limitation you’re referring to has to do with daisy chaining the batteries together. Since the current has to flow through all the terminals daisy chained together, there has to be a maximum amount of current recommended.

When you connect to the vertical bus bars in the cabinet, those bus bars are designed for more current that what a daisy chained stack of batteries are designed for.

I’m not familiar with the official numbers for the EG4 batteries, but a lot of the server rack style batteries I’ve seen can support a maximum of 4 in parallel (daisy chained together).

 

[timselectric]

I think that you are confusing parallel with series.

 

[SilverFox333]

Sorry about the vague YouTube reference. The video I'm referring to is:

Go to 24:43 to hear Will say "If you have more than 3 strings in parallel, you're asking for problems." Will goes on to explain why.

Will - can you chime in here please? How do rack servers with (5) 48v modules running in parallel do it?

Is all you have to do to run 3 or more strings in parallel is have big fat paralleling cables or big bus bars? I would think if it was that simple Will would have mentioned it in the video. Maybe the communication link in the modules in a rack server can talk to the BMS's in each module?

When I see big $ for complete rack servers buy much less $ for 280Ah rack-sized kit, I'd like to build my own rack server with multiple modules.

Will - what is the correct way to parallel multiple battery modules?

Thanks again for all your great insight and help with our battery systems.

 

[Supervstech]

To get Wills attention @willdoes the trick

 

[Shimmy]

@Will Prowse Does this advice still hold in your mind, or what has changed?

@SilverFox333 I'd tend to say that advice is obsolete, at least for properly matched cells. You do need to make sure your cable arrangement allows for balanced discharge though. Current sharing among paralleled batteries is not perfect, but it usually is good enough.

 

[Supervstech]

Sorry about the vague YouTube reference. The video I'm referring to is:

Go to 24:43 to hear Will say "If you have more than 3 strings in parallel, you're asking for problems." Will goes on to explain why.

Will - can you chime in here please? How do rack servers with (5) 48v modules running in parallel do it?

Is all you have to do to run 3 or more strings in parallel is have big fat paralleling cables or big bus bars? I would think if it was that simple Will would have mentioned it in the video. Maybe the communication link in the modules in a rack server can talk to the BMS's in each module?

When I see big $ for complete rack servers buy much less $ for 280Ah rack-sized kit, I'd like to build my own rack server with multiple modules.

Will - what is the correct way to parallel multiple battery modules?

Thanks again for all your great insight and help with our battery systems.

Most server rack setups have data collection and monitoring, also a well designed bussbar configuration.

This handles the banks safely.

 

[timselectric]

Quality cells
Good bus bars
Good wiring
No problem

 

[sunsurfer]

I have 8 SOK LFP 12v 206ah batts all in parallel (20kwh). I have massive aluminum buss bars. They are charged with 7 Victron 100/30 SCCs. I currently have 2 inverters pulling off the batts.They have been in service for about 8 months this way with no problems.

 

[Shimmy]

Quality cells
Good bus bars
Good wiring
No problem

I would add Good BMS.