nickel strip sizing advice

[MTH07]

I am new to battery building, and trying to make sure that I am thinking through my first battery build to power my BBSHD powered ebike.

My question is on sizing of the nickel strip connections - I am looking at using samsung 25r cells in a 14s4p configuration. The controller on the BBSHD is 30 amps. I figure on using a 40A BMS so there is a little margin of error there above the 30 amps that the controller could demand.

I know that there are higher capacity cells that would give me a higher capacity in that configuration, but I am leaning towards the 25r cells for this first build due to the lower cost and they are likely to run nice and cool no matter how hard I flog the bike.

Since those cells are 20A cells, the battery would be theoretically capable of producing up to 80A, but it would be limited by the 40A bms. Do I need to size the series connections for the maximum theoretical possible (e.g. 0.15mmx8mm strips at 5A each for a total of 16 strips needed), or the BMS limitation (8 strips needed)? If I need to accomodate the full 80A, I probably need to look at other options like nickel/copper sandwiching etc. If I can get away with 2 strips per connection, I will probably just go with relatively thin pure nickel to keep things simple, but I don't want to create an unsafe condition obviously.

Thanks in advance for any insights on this!

 

[amberwolf]

What is the maximum current you will ever need to pull from the battery?

That is the target all of your interconnects need to meet.

Secondarily, the better the connections, the lower the resistance, the less voltage drop across them and the less wasted power as heat inside the battery, and the more power you will get under the max load at the controller/motor.

Also, if you look at specs for any cell, on the manufacturer spec sheet, you'll find the test conditions under which those max currents are supplied, and the voltage drop caused by various currents. (if the manufacturer spec sheet doesn't have this, then you can use places like lygte-info.dk for their battery tests and charts, if your version of your chosen cell was tested there.

 

[MTH07]

Thanks for the insights.

The most I forseeably plan to pull off the battery is 30A. My logic with running a 40A BMS is that the 30A current draw is only at 75% the capacity of the BMS so it should be quite comfortable running at that amperage. Also, if for some reason the battery got hooked to something that drew more than 30A, the BMS would limit the flow through the cells to 40A, which is why I was thinking that I could get away with 8 total 0.15mmx8mm nickel strips to make the connections.

But, given that the cells could theoretically produce much more amperage than those strips could handle (say, in the event of a short, or a failure of the BMS to limit the current), I wanted to make sure that I wasn't building something that goes against accepted safe practice.

 

[amberwolf]

A BMS doesn't actually do any limiting, not in the sense that a charger or controller does.

It just turns off the output (and/or input) whenever one or more of it's limits is exceeded. That could be over or under voltage (HVC / LVC) or overcurrent, or overtemperature. Some have other limits, such as an imbalance between cells greater than some limit (often 0.1v). Basically anything that could indicate a serious problem with the pack or anything external that could damage it based on the BMS chosen for the specific cells, interconnects, and usages.


Regarding the limits of each part--as long as everything in there is more than capable of the actual current to be used, worst case, ever, in any usage for it, then it doesn't matter if some parts are more capable than others--nothing forces current thru, but instead it is drawn from outside based on the load connected to it at any instant.