I have a selection of different diameter & thickness neodymium magnets that I use to change flat tops into button tops or for use on short cells that need to be lengthened.
Unless the light is an out & out hot rod I don”t think any extra resistance from the magnet is noticeable in light output.
If you are concerned about the magnet moving (I have never had this happen) you can use some superglue on the magnet edge or a small bead of silicone around the magnet.
You can also buy purpose built cell connectors such as sold by Jaxman which can be used either to allow flat tops to make contact in series or to extend a cells length https://www.aliexpress.com/item/Connector-for-flat-battery-series-instead-of-little-magnet-Flashlights-parts/32667378904.html?spm=2114.01010208.3.247.bqXKNv&ws_ab_test=searchweb201556_0,searchweb201602_5_10057_10056_10065_10068_10055_10054_10069_10059_10058_10073_10017_10071_10070_10060_10061_10052_10062_10053_10050_10051,searchweb201603_3&btsid=eb76dc2f-c73e-4bd7-a271-239417b36d88
Cheers for the input … a good idea!!
I normally have a selection of them lying around from when I used to fly RC Planes but when you need them can I find them … like F…. can I … always the way 
I’ll jump online and order some I was thinking 5mm x either 0.5 or 1mm … if I go with the 0.5 and buy a few if ever I need thicker just use more then one.
I would go with the 1mm. The .5mm magnets seem to shatter just by looking at them. Even the 1mm shatter easily. For this reason I use 5mm x 1.5mm and they hold up much better than the thinner ones. Others may have different experience?
This is the set I got early this year: 100pcs N50 Dia.8mmx1.5mm Strong Round Rare Earth Neodymium Magnet :THUMBS-UP: . Perfect power, size & price balance. :-)
Cheers ^:)
Barkuti, could we Please see a few pictures of this?
Muchas gracias,
George
PICSTIME! :-) 
Enjoy! 
Cheers ^:)
Original post date: Sun, 09/11/2016 - 06:50; swapped a couple of slightly defocused pics. :-)
@Barkuti, Muchísimas Gracias for the great pics. 
Best Regards,
George
Yep. Neodymium magnets .5, 1, 2, or even 3mm thickness and various diameters because it all depends. Also the stronger N50, N52 magnets are best if not using something additionally to bind the magnet. They won’t move too easily but it is still possible and sometimes can be more likely than others depending on how they will seat in the flashlight. I think Amazon names like CMS Magnets and Applied Magnets are sellers with reliable claims. I know that all of the magnets I’ve bought on Banggood have fallen well short of claimed N52, N50 and were more like N28 or N32. Still strong but chimp strong, not gorilla strong.
Also had success fitting several batteries with solder blobs. Touch the contact with a dremel or sandpaper then touch on the contact a little with the hot iron and get that blob down quick. I don’t recommend doing this unless you are aware of what the dangers and your tools/skills are up to the task.
@Barkuti, I like it a lot. Never would have occurred to me to do this. It is clean looking too. Thanks.
Solder blobs are a 2 second affair . There’s no need to have any discussion at all and the methods have been discussed here at length . Rough up the end add flux load up the iron with a massive amount of solder and just make a blob ….bang …
no heating the battery and it’s not coming off .
NOT doing solder blobs is one of the dumber things i did for years . Feel sort of stupid for ever even thinking it could be anything but a great ideas .
magnets have dented/deformed ,moved a bit and were a dumb idea … the answer is FLUX
Already screwed up some Ni-MH cells by soldering on them in the past. Wherever the solder has to flow into needs to reach fused solder temperature for the thing to work (proper wetting of the surface). This means you'll need a lot of heat because batteries are like big heatsinks and they don't like high temperatures, which is one of the reasons spot welders are for. 
With regards to the pics, I used 10 layers of aluminium foil; conductivity should be equivalent to 0'1mm copper sheet. The conductive metal wrapping is also a convenient dampening protection for the magnet 
. Less hot glue can also be used, a careful dab on each corner should do nicely and be easier to remove if needed.
Cheers ^:)
Original post date: Sun, 09/11/2016 - 10:18; grammar fix edition.
Nice idea ‘Barkuti’, thanks for sharing it. :+1:
Hello!
Being a perfectionist, I already thought I had to replace pics 4 & 5 (which weren't meeting my standards 
). There you are. 
Cheers ^:)
P.S.: papaslightsaber, I don't know what you'd call a strong magnet but, as you can see in the last pic, my ∅8×1'5mm (≈1'4mm actually) magnets are well above 100g of pull force.
what about copper washers with a conductive adhesive?
@Burkuti , Yes, it’s all good. I just know that magnets coming from certain retailers are quite noticeably harder to pull off of metal given the same sized magnet. I’m not saying that any functionally decent neodymium magnet won’t do the job as much as I’m saying there are stronger ones easily available for roughly the same cost and they can ship out in one day too. The magnets that I’ve personally received from Banggood have been weaker than the ones from CMS Magnets or other big magnet distributers on Amazon (not applicable to all locations). When used for our purposes discussed, the extra reassurance is, frankly, reassuring. Of course given your method, which is quite excellent, this point would be much less of an issue. However, when using the magnets naked on the positive contact, I personally am much more comfortable ordering known N52. I don’t have a gauss meter to show the difference of the Banggood claimed N52 and the CMS or other reputable distributer’s actual N52 but I most certainly feel them as having a stronger pull force. Only cost a few cents more too. Sorry if this comes across as defensive and neurotic, I’m really just trying to clarify my point.
Trying to load images from Flickr is not working. Anyone help? Was going to just show my neodymium magnets for charging 10 or so cells on a hobby charger but can’t load an image. Thanks
Don’t you just copy and paste a link … I’m not very familiar with Flickr … tend to use drop box myself.
Re: Magnets.
While it shouldn’t be an issue, do check the space between the + and - points on the driver. It could be potentially easy to cause a direct short here if a magnet is too big, or moves. But it will depend on the driver and fitment in question.
I often put magnets on the - end of the battery instead. No risk of shorting and if it’s just the battery is a little short, it’ll solve the problem.
Agree with Boaz, just solder a blob on top of the battery and be done. Never even used extra flux, just rosin core solder wire does the job: load the solder iron with a bit of solder, pre-heat the battery top with that for a few seconds, feed extra solder wire until the blob is to your liking, done.
I did go the magnet route but mainly because it’s the easiest to undo if ever I wanted …… they arrived today and an initial test worked a treat … I haven’t tried wrapping them in foil / hot gluing the corners yet but will certainly give it a try!!!
The ones I bought were 4 x 1.5mm they possibly could be a bit bigger diameter and as they’re so cheap I may order a couple of other sizes any way as useful to have to hand.