Speedlights and battery issues

[Simpo Two]

Needed to take some record photos of a house yesterday - nothing special but used an SB800 on the hotshoe. NiMH batteries had been fully charged about 3 weeks ago, then left.

I switched the flash on and although it whistled happily the ready light didn't come on. Anyway, it flashed with the test button so I started work.

Normally you'd expect a flashgun with low batteries simply to give dark photos. But in this case the exposure was all over the place. Sometimes it was right, sometimes 2-3 stops under, sometimes totally black (although the flash fired so it must have been a synch issue).

I took the batteries out of the camera, turned the flash off and on, tried manual exposure and still the exposure was all over the place - one of my wedding nightmares! (and no, it wasn't set to bracket mode).

Eventually I fitted a fresher set of batteries, the ready light came on and then everything worked fine.

I just wondered if anyone else had experienced the same issue. Normally when flash batteries die after a long day's work they just start giving underexposure.

[Simpo Two]

Thinking back, the battery in the camera was low too and had to be changed after maybe 20 shots. But even so, normally all that happens is that the camera simply refuses to fire - not throw in random exposures and synch failures.