Had the same problem. It was the protection board. The battery charges up ok, (and turns green), terminal voltage measures ok at >8 volts, but as soon as you put it under load, one side of the protection circuit switches a pair of the batteries out and the voltage dives to 4 volts. Depends how comfortable you are, but I’ve removed the circuit and just run direct. The board is there to limit current draw, over voltage and under voltage. The first two tend to happen in failure conditions (so you have bigger problems), the latter when the battery is run down. If you run the batteries below a critical voltage, there is a risk of damaging them. With the DX light, the head unit has voltage monitoring, just switch it off when the light on the back turns red and you should be fine.
Just one minor thing, I wouldn’t trust either the DX batteries (regardless of protection circuit) or the cheap charger. Strongly suggest you don’t leave for long periods unattended. I charge mine in the garage with a timer switch to shut it off after 3 hours, and I put the batteries in a fire proof box (well, a Quality Strret tin), while they are charging.