Thanks for the sympathy! I have now learned about 'balanced charging' with LiPo Multi battery packs (like my4S). I was charging it as per a single 14.8V unit rather than 4 individual cells in series.
However, my so-called 'intelligent charger' (IMAX B6AC) reports a 'connection fault' when I try to used the 4-way balance charge option on the replacement battery pack. I checked the pins on the battery 4S plug and can measure 14.8v across it, and approx 3.7v pin to pin, so it seems to me the battery pack is fine, but the charger is not very intelligent. Can anyone recommend a decent, reliable and affordable balance charger for 14.8V 4S, 4500mAh packs? Thanks,