Can I use magnets like this:
to make a button-top battery 5mm longer? Ideally I want to put 5 of these on the cathode of the battery. Not sure if this would be stable.
It’s unsafe and not recommended, even if you use just one.
I think it depends on the light. Never use a magnet in a light that uses a battery config that spins the button tops against a ring type contact. The BLF LT1, Sofirn SP36, & Q8 all come to mind as examples of light that should never use anything other than a real button top or a solder blob top.
A light that the battery is in line with the contact and the battery is not moved when compressed is likely OK for a magnet spacer.
When using the spacer, one that is too large in diameter will slip to the side when being used. Test this before using a given magnet.
If the light only needs a longer cell, (as opposed to a button top), putting the magnet on the base is a safer way to go. But remember tha spring may be spinning against that magnet and possibly making it move.
That being said, I have several lights designed for extra long 18650 cells. The type with built in charging.
The only way to run one of these with a normal battery is with some fairly long magnets.
Here is an example of a headlamp that needs a buttontop to work. Adding a small magnet lets a flat top work just fine.
After the top is on, the magnet is not going anywhere.
Just always be aware that a short can lead bad things and make your decision accordingly.
All the Best,
Jeff
I’ve done it on the ones in my GT because they were so loose, but they are superglued in place (tiny dots of around the edge). Never had an issue, just have to be a bit careful when they are not in the carrier, but that’s kinda obvious.
If you go down the soldering route, why even use magnets - just use a bit of copper sheet, or just a plain blob of solder.
Only reason I didn’t do it was I wasn’t crazy about soldering a cell, plus I had 8 batteries gratis out of an old malfunctioning drill battery pack ( 1 cell was knackered) the others were fine.
For single battery flaslights I use faucet joint 8/18mm in/out 1mm less tall than the magnet I use.
I’m just looking for a simple way to extend the length of the battery by about 5mm. It’s a button-top.
Just put a 12x5 mm magnet on negative side
Maybe this light was designed to work with 21700?
I have been soldering small piece copper to cells in order to convert them into button top. You can do the same with a thicker copper but take into consideration that soldering a cell need to be done with great caution
I’ve just ordered a protected (i.e. longer) 18650 instead, simpler 
I’ve had one overheat using just one neo mag.
Depends on the light and the cell; just be careful.
Experiments can and do go awry.
I keep a bucket of water nearby when experimenting.
A pair of pliers is also handy.
I have a professional person do a solder blob professionally when needed.
I like to have a pair of heavy duty zircon encrusted tweezers in hand.