Hotwire AAs???

[lightoftheworld]

I have a 2C flashlight with a body that is a little too big. The C batteries are loose when I put them in. I checked, and I would be able to jam 3AAs in there side by side if I pushed. I would like to solder them together in a series and electrical tape them up and push them in to leave them permanently in there. I have already figured the charging part out.

Could anyone please give me some info on how to solder together NiMH AA batteries in a series? 3 x AA on top of 3 x AA for a total of 7.2V

I've never soldered onto batteries before.

Thanks, Shane

 

[Doug Owen]

Shane,

Don't do it. Get AA cells (they're really not a battery until there's more than one of them....) with tabs already *spot welded* on for just this purpose.

The issue is the work (solder joint) has to be up to 'working temperature' (uniformly a bit past the melting point of the solder used) before cooling. This temperature will destroy the seals in the cell. This means you must be very good with heat control. Any beginner should count on wrecking a large part of such attempts. Even for highly skilled, it's questionable at best. You'll never find hand soldered connections in legitimate products, they all use the (standard) tabs.

And I guess I don't follow what the tape is for? You shouldn't need it between cells, and between the upper and lower set a plastic shield (say cut from a plastic bottle?) might be a better call. Watch for sharp edges in the finished package.

Anyway, buy the proper cells, they're only a few cents more.

Doug Owen

 

[WildRice]

If you already have the cells and really want to solder too them. 2 bits, 1, like Doug said, DO NOT OVERHEAT, use just enough heat to get the solder to flow. 2, clean the contacts. I use an emery board to really sand them down. They are coated (probably with zinc) so they dont corrode, and solder DOSEN'T like to stick to this material. There is useually copper underneth.
Jeff

 

[AilSnail]

I done it lots, ginseng too, and his packs looks much better than mine. To my knowledge I have not destroyed a cell yet.

I used an 80w iron,

clean it,
make sure the solder flows well on it,
if not then sand it down and tin it.
I used rosin core solder, if you have flux then smear that on the electrodes first.

OK: first, you apply FRESH solder (straight from the spool) to all the electrodes. Don't heat too long. Let it cool before you continue work on the same cell. You can cool with spittle on your fingers too. You want a powerful iron to heat the electrode fast so the tin will flow onto it. flux helps with this.

Then solder two and two cells together, heating both cells at the same time, then just smack them together before the solder hardens. A small dab of fresh solder on the iron tip helps get the conduction of heat to the tin on the electrodes get started.
It helps to put them in a groove for alignment.
Keep your iron clean, put fresh solder on it and wipe off with a cloth or paper towel or something.
If you stuff up and don't manage to slap them all the way together or they get misaligned you can always reflow the solder or break them apart and try again.

Now make sure you have no exposed metal that could get in contact with the other stacks.

Then you put the three stacks together with some braided copper wire or copper flashing. If you have solder wick that can be used, I suggest saturating the whole length you need first with tin, then solder it to one electrode, let cool, and solder it to the other electrode, then cut it.

Key is, at all times when you start soldering to a cell, all the parts should be cool, and the iron max hot. Thats what I think, anyway. I have been much sloppier than what I described and the packs still turned out useful.

 

[lightoftheworld]

Thanks. I connected all of the batteries in a series via Aluminum foil and electrical tape, and the pack worked good when I tried it, before stuffing it in the flashlight. When I went to put it in, it didn't quite fit, so I pushed it in with a little force, (hehe), and the batteries got real hot. When I pulled them out, I realized I had scraped off the outer plastic coating on some of the AAs to expose metal behind the plastic coating. I checked, and the whole metal body of the battery is the negative. The body of my flashlight is all metal and that is how the first bottom batteries negative gets up to the top of the flashlight. I think some of the other batteries negatives were touching the flashlight body and connecting. Is that why it got hot?

Thanks again, Shane

 

[Ginseng]

Oh yes. That would be why. Sounds like quite a dangerous situation. Batteries don't like to be dead shorted.

I have built quite a few soldered packs using A to D size cells. There are several methods guides as to how to solder cells but here are some tips I've picked up.

1. Surface preparation is key. I sand, then wipe with orange solvent to remove oils. Then I wipe with 90% alcohol to remove everything else. Then I immediately swab on a high activity liquid rosin flux. Now it's ready for the next step.

2. I use a hammerhead flat tip on a 40W iron. Keep all tips clean and tinned. Tin the cell contact pads with lead-based rosin core solder. I use thin guage solder. For some reason, the lead-free rosin core solders just don't flow or wet like the lead-free. Each tinning should last no more than 2-3 seconds. I then touch the solder pad with a heavy steel implement to draw off heat.

3. When joining cells, a jig is imperative. This allows you to precisely position the cells, apply even heat to both cells and bring them together in precise alignment quickly. I re-flux all surfaces and apply the hammerhead for no longer than 3 seconds. Blow on the cells or hold in hand to draw off heat. Usually, you cannot even feel that they have warmed.

Of the packs that I have built and used, they are all holding charge and have demonstrated no loss of capacity even after repeated, punishing 4-9 ampere discharges. This indicates that the seals have not been grossly compromised. It is not something to be done casually or by the novice, however, if you want to do it, I'd say buy at least a half dozen cells to practice because you will make mistakes your first time out.

I would love to do resistance soldering like the big pack builders do. For one, the joints are much more tougher/flexible (if you use tabs or cups) and for moderate current draws (sub 3A or thereabouts) solder tabs are fine. However, my work requires the absolute lowest resistance and this can only be achieved by direct soldering. All in all, building your own packs is a very satisfying experience. It provides a degree of flexibility and economy that custom production packs are hard pressed to match.

Wilkey

 

[AilSnail]

Yea, forgot to mention: don't short your cells /ubbthreads/images/graemlins/grin.gif

 

[K-T]

Good that this subject was brought up since I have a question that was troubleing me for some time now. This is especially for the "hotwire" folks like Ginseng. If you build battery packs for high-current-applications do you do something different or are connections made the way described above enough to withstand 5-10A? I have done it like you guys described for 3-5A but was wondering if this is adequate for higher demands.

Klaus.

 

[Ginseng]

Klaus,

The short answer is, it's fine. However, it depends somewhat on the cells you use. In a recent test of "regular" high-capacity type premium AAs, I found that they dropped voltage much faster and heated up much more quickly and more intensely than roughly equivalent high-current cells. This is due almost certainly and entirely to the internal resistance of the cells. In one case, 30 milliohms and in the other 6 mO. Also, the smaller the cell, the worse the heating...up to a point, and depending.

The upshot is this: if you use the appropriate cells, this technique will serve you well up into the 30A+ discharge rate. If you are melting your solder joints, then something seriously wrong or abusive is going on. I mean solder will not melt until at least 374F (alloys for temperature sensitve electronic operations) so if your cells are seeing that, you can be damn sure your seals are destroyed and the cells will become inert in no time. I have accidentally dead shorted a pack for several seconds and the wiring became very warm as did the pack but all solder joints were still intact and the cells came back up to capacity. I estimate this was a discharge of at least 45-50A.

The RC guys have been using this technique reliably for up to 65A+ pulsed demands and 30A+ continuous and the history is good. Confidence is high as long as your technique is good.

Our pack designs are somewhat different geometrically than the flyer guys so that implies some other nuances in terms of pack building but the general principles are the same.

Wilkey

 

[K-T]

Thanks, Ginseng.
I have never experienced solder joints to break other than due to mechanical "stress", nevertheless I was wondering. The cells (some RC-Sanyo High-current cells) I am using right now are supposed to have some internal resistance of 6-8mO, my "build-your-own-light" path came from a different direction, nevertheless the stuff you guys did inspired me a lot. /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/buttrock.gif