I have the same issue with a 4s using a venom pro charger but I believe the issue lies with the pack and not the charger.

I have 2 4s 5400's and after running the first pack in my rc8be it hit lipo cutoff on my LRP, slowed down and then the speedo flickered. I took the battery and put it on my venom charger. The charger recognized 3s so I turned off the charger and back on thinking that it needed to be reset and the charger still only saw 3s. I took my other pack, never used at this point, and dropped it on the charger and it saw 4s. This makes me think the balancer connector on the pack is faulty and not the charger.

I did notice that the charger only dc's at 1 amp, so far this is the only downside. And besides, I can dc from the car faster if I *need* that done quickly

Hope this helps someone else having the same issue.