Originally Posted by billdelong

I have personally experienced this first hand many years before SMC published their article... I had no idea what caused one of my packs to prematurely swell... I had no IR meter, nor was I rotating the packs, but I definitely experienced premature LVC issues and all kinds of reliability issues when running packs in series for 10 min mains... no issues with 5 min quals, but 2x2S-5Ah in series was a disaster and 1x4S-5Ah gave me no issues.

I believe the extra wiring/connectors for series wiring is what causes the additional IR on the circuit, totally different from having a single pack wired internally, plus a single pack has matched cells where you can't possibly guarantee matched cells from 2 separate packs. Simply put, running packs in series is just a bad idea all the way around