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I was wondering where the connection point should be on a "massive" parallel block of lithium batteries. Let's say there is a pack of 100 LiFePo4 18650 batteries all spot welded together with nickel tabs (1S100P). Nominal voltage is 3.2V and total capacity of 100AH (each cell is 1000mAh).

Where would the best place for the positive and negative wires attach to the pack? Is 4 gauge wire sufficient for the "short" run to the connector termination (assuming a 1C pull on the pack)?

My thought is if you were to connect the positive and negative leads to the single cell closest to the exit that it would receive the most abuse with the load becoming less and less until the furthest cell is reached, but this doesn't really make sense as they are all a single cell now that they are welded together.

Thanks!

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  • \$\begingroup\$ With a busbar it shouldn't matter where \$\endgroup\$ – Sunnyskyguy EE75 Feb 24 '17 at 23:54
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It depends on the discharge rate. I'm assuming that your paralleling them for a quick discharge. Regardless, your going to have to calculate the max discharge rate and current which is dependent on the load. The terminals can only handle so much current, otherwise there will be ohmic heating and destruction of the terminals.

You could attach a parallel cable at multiple points along the bank, the terminals all going to be near same potential anyway. Or since your so good at wielding spot wield another strip of metal along the top of all the terminals that will be sufficient for the 'current' requirements of the bank.

You have a nice bomb waiting to go off, handle with care.

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The spot welded tape connections are your weak point. If you connect 100 cells together in parallel then use only the connection to the last cell as the battery charge/discharge point you have a long line of very small resistors between your batteries.
While the resistors may be small values (fractions of milli Ohms) they will impact the performance of your aggregate cell at high current discharge rates.

I'd suggest you should connect the batteries in groups to reduce the number/length of connections and connect the center point of the group to a copper busbar. While this does not eliminate the small resistors within a pack, it minimizes and balances most of the problem (with lower per group peak currents through you nickel tape connections). I'd suggest groups of 10 cells would be the most I would consider.

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Nowhere.

You'd never design such a battery system.

Rechargeable batteries need to be balanceable, so there will never be a situation where you join all batteries on both sides. If you did that, the more-degraded batteries would always discharge and stress the less degraded ones.

Also, you'd never use the taps that come with the batteries for joining. These tabs are rated for the current one cell can provide, so you'd find something like a bus bar and attach your multiple batteries to it.

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  • \$\begingroup\$ "stressing the less degraded ones" is how they balance in parallel. The risk is 1 failure shunts all. \$\endgroup\$ – Sunnyskyguy EE75 Feb 24 '17 at 23:48
  • \$\begingroup\$ Not so @Marcus. Connecting cells in parallel avoids the need to balance the cells (in parallel). It's only when you place cells (or multiple parallel connected cells) in series you need to balance them. For those connected in parallel the voltage/charge characteristics would be essentially identical (assuming they are all they same model cell). \$\endgroup\$ – Jack Creasey Feb 24 '17 at 23:49
  • \$\begingroup\$ @JackCreasey, re: "characteristics would be essentially identical"... Is this why the battery industry desperately working on improving methods for "battery grouping" (aka "matching"), like in this article, mdpi.com/1996-1073/9/7/561 ? /sarcasm off :-) \$\endgroup\$ – Ale..chenski Feb 25 '17 at 4:13
  • \$\begingroup\$ @Ali Chen. Of course they are. This is exactly why Tesla do battery batch matching when building packs. \$\endgroup\$ – Jack Creasey Feb 25 '17 at 4:48

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