I can confirm this behavior of the DP811. I just tested it with a second PSU instead of a Li chemistry battery. Actually it seems to be even more weird because this shorting out / latch-up of the input appears to occur only at certain voltage ranges.
The sequence to induce the latch-up condition is as follows:
1. With the mains power to the DP811 switched off, apply a voltage (current limited supply between 1 and 4V or above 8V) of correct polarity to the DP811's output. At that time, there won't be any considerable current observable.
2. Power on the DP811 -- as soon as the switch is flipped, even before the DP811's display shows anything, the externally applied voltage gets shorted or pulled down to a residual voltage of 0.95V @ 3.5A.
3. The DP811 apparently boots normally and doesn't show anything abnormal about its output condition on its display.
4. If now the DP811's output is enabled, the instrument directly goes into CC mode and the full, selected current is read on the display. The voltage displayed matches what can be measured across the output terminals. This voltage is now slightly higher than when only the external PSU is fed into the DP811, in my case it was approx. 1.25V @ 3.5A externally supplied and the DP811 set to 10A CL and enabled. The load stays in this condition also if the external PSU is switched off as long as the DP811's output stays enabled.
5. If now mains power to the DP811 is switched off again, the current supplied by the external PSU keeps on flowing and the DP811 only resets itself if the external supply is momentarily disabled.
So I guess this exactly matches MegaVolt's findings.
I assume there's some kind of crowbar SCR device installed across the DP811's output that for some reason gets triggered when the power supply is switched on. Apparently this is only related to the output since all the other circuitry appears to be working normally. Considering that there doesn't seem to be any current limiting device present between the output and the supposed SCR, it would be a very bad idea to try to charge a powerful battery with the DP811. If there's mains power cycle for whatever reason, the whole setup may end up in flames.
Maybe I'll tear down my DP811 to check if this shortcoming could be revised.
Cheers,
Tom