The problem is, hooking up a load AND this charger in the same device can cause it to "just barely" drop enough to engage in fast charging again. The dv/dt drop may not show up, and it could blindly charge the batteries thinking they're low, not full, until the temp sensor realizes the batts are overheated and shuts down (hopefully). But it'll murder the batts to do that, because, starting from a cold state and using a fast-charge current, they'll take a significant overcharge before they get so hot as to trip the temp detection. It has to heat up the entire cell to trip that.