Ive gotta device that uses 4x aaa 1.2 nimh cells when the voltage runs down to around 4.2v-4.3v the device shuts off to protect from over discharge the cells.The problem is under load one of the cells has dropped to 0.850v whilst the collective cells still are within the low voltage cutoff threshold ie
> 4.2v- 4.3v any ideas ?

 

The one cell is weak. It may have become damaged by repeatedly being discharged too far, overcharging, charging in series, defective, or just plain beyond its' service life.

It's best to replace all of the batteries at once. If one of them has failed, the others may not be far behind.

 

If you are going to replace the cells, try to use the Eneloop type.
I think other manufacturers have these types also, but I'm not familiar with their commercial name.
I replaced all of my NimH, which I had in my digital camera's, with Eneloops.
A normal Nimh will self discharge quite a bit in a month's time if not in use.
The Eneloop type will hold 90% of their charge after a year of non use.

 

As Sgt. Wookie has suggested replacing all 4 NiMH batteries is the best option. The three that seem to be O.K. could still be used in devices that require 1 or 2 cells. As for the bad cell, well, recycle it according to local regulations. The main reason for replacing all the batteries is that if you replace just the bad cell you are going to run into the same scenario again, over discharging some cells and stressing the others.

 

The cells have only been though about 10 cycles over a couple of weeks of and have never been allowed to drop below 1v or exceed 1.45v when charging.

Is it worth giving the weak cell a couple of cycles on its own to see if it will recover ?

Is the low voltage threshold set about right for 4 x aaa 1.2v nimh cells at 4.2v -4.3v ?

 

The cells have only been though about 10 cycles over a couple of weeks of and have never been allowed to drop below 1v or exceed 1.45v when charging.

Apparently one of them has dropped below 1V as you stated above.

Is it worth giving the weak cell a couple of cycles on its own to see if it will recover ?

Some chargers will not even accept the battery for charging if it is below a certain level....don't know what that magical number is or if there is one with your charger. If they are only 2 weeks old I suggest returning them to the store or call the manufacturer.

Is the low voltage threshold set about right for 4 x aaa 1.2v nimh cells at 4.2v -4.3v ?

It sounds about right for 4 NiMH's. Some specs may allow as low as 0.9V/cell but I myself would play it on the safe side as your charger seems to do.