Me formulated the question :
I have JS5IN1 (jump starter) which hasn't seen the light of day for some
time. The battery is flat, the worklight come son quite brightly but
otherwise nothing. I have attempted to charge it using it's own charger plug
with little result, the compressor will run for a short time but that's it.

I have a "smart" battery charger the instructions of which suggest it might
be able to revitalise a flat battery. Can I connect this directly to the
JS5IN1 across it's jump cables without doing any damage? Taking the battery
out to charge it would probably be a better option but none of my
screwdrivers will fit the screws I assume hold the case together.

There is a lot of varied expertise in this group so any thoughts?

Thanks.

I think once it gets to that stage, it's knackered. As others have
said, if it won't charge from its own charger....

Always found those sort too big anyway and no use for the compressor,
so I went for one of these instead https://tinyurl.com/yasal9sp.
Absolutely brilliant - started a mate's 4.3 litre Lexus LS430 no
problem at all, and the car had stood unused for 6 months.