Here is the initial approach doc, and questions just let me know:
First there are some important points:
1) Li-Ion cells are potentially explosive if heated too much (and this increases if you incorrectly fully charged them), so you do need some decent kit to do the soldering quickly. I include a good video below on soldering cells.
2) I was and are still OK with electronics as I did a 4 year apprenticeship back a long time ago when I left school and although you don't need to be an expert at all some of those techniques can be useful!
Finally I hope none of my comments are deemed to be patronising, they are meant to be helpful and I have never written anything like this before!
Shopping list and useful video at the end of this post:
So here goes:
Battery case prep:
1) You have to prize apart your beloved Speleo Technics battery case, this is needing some good effort as the batteries are partly potted in. So a) there is a danger of stabbing yourself with your screwdriver and b) you don't want charged batteries in case you stab through them (unlikely but...). My batteries were completely dead.
2) I found working with a strong but not too large screwdriver was best. I started in the middle of a long side of the case, stabbed in and worked on the overlapping part of the case top and bottom. Once I got all the way round the corners then its stage two, where you prize further down with your screwdriver so you are prising the bottom of the outside overlap against the edge of the inner one and hoping for the best. It will depend on how much potting has been added to how much fun this is. Amazingly it seems you can put some good sustained force into this and the case bends a bit but always bounces back (at least two of mine did).
3) Then eventually the lid gives and the case opens. Then you need to work more around the batteries that are sticking out to extract those and declare 'battery extraction success.
4) Desolder the cables to the batteries.
5) I then used a dremmel, very carefully, with a milling point to do two things - 1) Completely clean the case insides and the overlapping parts of the case of all potting compound ready for reseal and b) Carefully mill some of the potting that the wires are coming through (green in my case). This is needed because the batteries (3) only just fit. I halved the amount of depth of this green sealant, and I only did one side that was furthest away from where the battery wires came through as only one side of the case needs the extra clearance
6) Finally I trimmed the wires back a bit and added extra length of +ve and -ve cable by soldering and used heat shrink to cover these inline solder connections
Thats pretty much the case ready to go!
Battery prep:
I am using 18650 cells, they are called this because they are normally 65mm long and 18mm diameter! (see shopping list). They will come about 50-60% charged so I deemed those OK to solder, if not you can use an intelligent charging system to discharge them correctly if you prefer. I also bought flat top batteries, not button top batteries, I think though either will fit but flat top are the usual ones you solder
1) I measured both the depth (if looking at the battery box on your helmet this would be from front to back inside measurement) and height (top to bottom - again internal so you have to measure the top and bottom of the case as if they were resealed - and remember you only have the height to the green sealant that you have milled down a bit). You don't need the width because the batteries easily fit length ways across the width. So a finished battery with have the cells lying on their side unlike the ones you originally took out which were upright in the battery case
2) Using the depth and the height I drew that square box onto a piece of wood, so I know the batteries must fit into that square (when standing upright) if they are to go back in the case. This is important because once you solder them (if you use what I used in the shopping list), you cannot change the shape. You should find the batteries fit within that square by having the batteries not in a line but the middle battery offset from the others (so they are not in a straight line). You will also find they have to be right next to each other.
3) Because of that position they are fiddly to solder and get perfectly right, so what you want to do next is dab a small amount of silicone sealant along the length of them and on a flat surface with all the -ve terminals at the bottom of the batteries and fitting into your square you push the batteries together so the silicone holds them. Then leave till the silicone sets (come back later in the day or the next day), this hugely helps the soldering process and the shape should stay the same to fit in your case
4) Critical to the next stage is that you have a good quality soldering station (that goes to a high heat) and a good sized soldering tip. There is no point in carrying on with a cheap handheld soldering iron from Wilco's, it is unlikely to work. This is because when you solder the -ve ends this is trying to heat the whole battery case, and that takes some proper heating power. I have a temperature controlled soldering station. You also need soldering flux, soldering wire and tabbing wire (can be called other things). You could also do this by soldering a wire onto each battery end and joining each of the 3 ends together if you wish. I would recommend using tabbing wire though
5) So now it's time to solder the batteries - there is no point in me describing this as there is a very good video on this here:
I like this video as he gives you tips and explains the process and the challenges (not many buy...). The only difference here is as you are not doing the batteries in a straight line you cut 4 pieces of tabbing wire as you have to do each leg between the batteries separately. Note I always tinned with solder every component before I solder them together.
6) Once you have 3 batteries soldered, all +ve's one end soldered together and all -ve's the other and the joints are not dry (as in poor joints) you are rocking! I then tinned a part of my tabbing wire between each core soldered connection (if you like on the tabbing wire strip between the batteries) so that I added 2 wires to each end to connect to the original battery case wires. I did this for belts and braces in case a wire failed or a battery terminal failed (I don't expect that!) you would still get a connection and power
Putting it back together:
1) You then need to take your soldered onto your batteries wire (or if you did what I did two wires), and join these together with the original battery wire. You want enough wires to work with but not too long filling the case. the core thing is you need to route these int he gaps you have on either side of the batteries as they are offset you have spaces either side. Of course when you solder these you must ensure full heat shrink coverage.
2) You only have one orientation to fit the batteries back as you need to aim the outer battery on the side of the case where you shaved off some of the green sealant
3) I then made sure everything fitted, I checked the output with my lights, I check all soldering joints for a good connection, and then I used clear silicone sealant liberally added round the seals and used a small clamp to hold everything together until set. And that was that! 10,500mAh at 3.7v output, brilliant
Shopping list (besides the need for a quality soldering iron and solder):
1) Batteries - the 3500mAh are in short supply. I bought mine from 18650 and I looked at various reviews of them. These LG MJ1 batteries seem the best at the moment as unfortunately you cannot buy Panasonic versions as supply issues -
https://www.18650.uk/lg-mj1-18650-battery
2) Tabbing wire to make the core connections -
https://www.amazon.co.uk/dp/B095P118MP/ref=pe_27063361_485629781_TE_item
3) Soldering Flux
https://www.amazon.co.uk/gp/product/B07B511DDL/ref=ppx_yo_dt_b_asin_title_o03_s01?ie=UTF8&psc=1
4) Charger - as you need something to charge them (note I am sure you can buy cheaper!), I used the plug off my old speleo technics charger to connect to this -
https://www.amazon.co.uk/gp/product/B091638F81/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1
I think that's it, sorry this was way too long........
