I’ve just got a cheap ebay light with a 8.4V 6400mAh battery.
In the shed I also have some old vistalite nightstick batteries, which are 6V 2200mAh.
Can I use these old batteries as spares for the new light? Clearly it will not be as bright and not last very long, but is there anything else to worry about?
Cheers

doubtful, and probably more hassle than they’re worth IMO.
how many LEDs in your light pack? typically you need a bit of headroom for current regulation.
spare packs are very cheap if you really need the capacity, and your old nightsticks are probably down on their capacity now, through age, and if you’ve kept them in the shed and they’re nimhs they don’t particularly like the cold either.

Depends on the light – Some have got “buck” type driver circuits in that can boost the input voltage: We’ve got a big, round single LED lamp with a decent driver circuit: roughly constant power (~8W) from ~4.5V to 8.5V in.

We’ve also got two ‘Solar Storm’ X2 lights, one has a buck driver (a pretty poor one), the other has a simpler PWM regulator. Externally virtally identical.

It might be worth a try if you’ve already got the batteries and are only looking for some ‘insurance’.