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intra-battery pack connections

Hi everyone

I have 8x 400AH Winston cells that I'd like to use to create a 24v pack in my boat. I have space longways but not width, so the configuration needs to be different from normal. This means that one connector will be significantly longer than the others. I have planned to get a bunch of cables made up, all the same maximum length that you can see in the diagram (220mm) so that this does not affect the performance of the pack. I expect no more than 300A out of this pack, but will design for 400A maximum. This would require at least 70mm2 cables rated at 400A between the batteries. The cables would need to loop around to cover the 65mm connection gap. It's a bit messy. Would this work? Or do I just go with copper connection bars and ignore the difference in length? Any feedback most appreciated. Also, will the Tao shunt deal with a 400A load, do I need a bigger shunt?

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Hi Phil,

Cable length is important when connecting cells or batteries in parallel so that all the parallel strings of cells / batteries have exactly the same resistance.  Like that they will draw the same current (otherwise you will rapidly have an unbalance between parallel strings) .

Here all your cells are in series, so the same current will go through each cell = no risk of unbalance due to the length of cables / links.  

I would use the standard short links except for the 220 mm length where I would use a conductor with a larger section (so that longer link has a resistance close to the resistance of the short links - although it is not critical). 

When the BMS measures cell voltage and computes cell resistance it includes the cell + the link to (or from) the next cell (depending where the cell taps are attached).  If the links resistance are not all the same you will see that difference in the voltage measure and the cell resistance.  But it will be minimum if the connections are properly done.  

Note that the BMS calculates the cells' open circuit voltage (excluding the voltage drop due to the cell / link resistance) and use that data to decide if balancing is required.

The standard TAO shunt will not work for 400 A .  You will need a shunt for example 500A/50mV which is common.  With those high current it is better not to have multiple shunts in your system, so if you already have a shunt (for example the Victron shunt), you can mount the TAO shunt PCB (electronics board) on that existing shunt.(just need to drill and tap 2 holes on the side of the shunt).

Philippe

Skokie has reacted to this post.
Skokie

In case of using the Victron shunt 500A/50mV and add the TAO Shunt PCB to this, are there any adjustments necessary in the BMS?
Needs to be reprogrammed? Or just fit & use?

Jens

Hi Jens,

You will just need to change the value of the shunt resistance in the BMS parameters (see attached screen shot).

The standard shunt resistance is 200 µΩ - the Victron shunt  resistance is 100 µΩ.

Philippe

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