A cautionary tale about soldering to batteries

[Mr Happy]

After a few years of use the battery wore out on my cordless phone. I bought an aftermarket replacement battery, but it was crap and within a few weeks it was performing as poorly as the original.

I thought therefore I would try to make a replacement pack out of some quality retail AAA cells.

It is often mentioned on these forums that it is a bad idea to solder connections to batteries, but since I lack a battery welder I thought I might give it a try and see how it goes.

Knowing that speed is of the essence, I had roughened the battery terminals with fine grained sandpaper and used extra flux. However, on one of the cells I had only touched the iron to the positive terminal for a second before an internal short circuit developed and the cell got really hot and went into meltdown.

So there's the thing. The internals of a rechargeable battery are very delicate and can easily be damaged by the heat of a soldering iron. You can try to solder them and you might get away with it, but you might not. You really can damage batteries this way.

If you like your batteries and don't want to damage them, don't try soldering them.

 

[VidPro]

that shouldnt have happened.
usually there is a tiny bit of space between the top tab and the electrolyte roll.
course with an enloop it has the seal right there under the nipple.

i soldered 4x4 16 bats for my phone replacments, and they still work some 10+ years later.

were you using a Weller 140W gun or something?

Ohhh AAAs, that is what i get for not reading well enough, they will heat up fast.

 

[Mr Happy]

Yeah, the cell was an eneloop (well Hybriloop actually, but same thing).

 

[VidPro]

so that is just a minor setback? or are you going to use duct tape now :laughing:

 

[HarryN]

I wonder if you could get away with some silver filled epoxy (120 F cure) or some high indium content solder.

 

[xenonk]

I've soldered to batteries before, but you really do need a hot iron and to be very quick. I don't rough up the cells though as the nickel plating tends to be rather thin and nickel is a useful surface for soldering.

The way I do it is to pre-tin the connection tabs and apply a tiny bit of flux, then press the battery to the tab and apply heat to the backside of the tab so that the solder bead melts and whets to the battery terminal. I first practiced on some cheap old "heavy duty" batteries to get a feel for soldering them before I started building packs, because the margin for error isn't very large.

 

[45/70]

xenonk has the solution, hot and fast. :devil:

I too have made packs for cordless phones and other low drain devices. I use flatops though and only AA, never tried any AAA's.

Dave

 

[Mr Happy]

Well I tried again and this time I was successful. I made sure to be really really quick with the iron.

Now I am waiting with interest to see how well it works. I used Hybriloops with a genuine 800 mAh capacity and I put them through a conditioning cycle before assembling the pack. The cells all tested within 1% of 820 mAh so the pack should be nicely balanced.

 

[Remon]

Try to find cells with soldering lips that are actually made for creating battery packs. Finish the home made pack with a heat shrinking hose.

 

[Mr Happy]

Try to find cells with soldering lips that are actually made for creating battery packs. Finish the home made pack with a heat shrinking hose.

Yes, you are quite correct, if I were doing the job right that is what I should do. However, I would have to go on a special search for tabbed cells -- and I was also curious to find out how eneloops would perform in my phone since I know those cells are robust and long lasting.

Does anyone know a source for flat topped, pre-tabbed eneloops?

 

[45/70]

Does anyone know a source for flat topped, pre-tabbed eneloops?

I don't remember seeing a tabbed version on their site. I'd just continue on as you are, Mr H. You seem to have things under control, as usual.

Let us know if they work out well. I've never thought of using eneloop's in such an application, as I figure their LSD properties wouldn't be much of an advantage in something that gets used all the time. They do seem, to be superior cells though, and may actually work out to be a better choice.

Dave

 

[kramer5150]

You shouldn't have needed to rough up the surface. Just pre-tin with low melting point solder and paste. I use an 80 watt weller pencil, and have never had a problem. I have soldered ~30-40 (give/take) 6-cell packs for my RC hobbies over the past ~10 years and never had a problem. Then again I have never done AAA cells.

You need not only a HOT iron, but you need one that can maintain temperature (IE stay hot) as the heat is soaked up by the cell. A good station is superior in this regard... IMHO.

 

[Mr Happy]

You're right. The solder took OK on the last cell where I didn't rough up the surface. I didn't know that at first though and on the first cells I was trying to be extra careful about providing optimum conditions.

 

[BoarHunter]

If you remove the nickel plating, you end up soldering on steel and it prefers an other kind of soldering flux than the one commonly used for copper wire !

 

[VidPro]

i always buff them up first, never had a problem, i thought i had to get all the finger oils, and road film and slime and oxidation off first.
a quick wisk with a green scouring pad or 400grit, doesnt take off a whole layer.

 

[45/70]

You guys may have a valid point about scrubbing through the nickel plate when roughening up the surface. I'll tell you from experience though, when I used to just solder to the shiny surface, I had problems with the connections popping off. Cordless phones in particular get dropped, fall off the couch and what not a lot around here. Years ago I started roughening up the surface a bit before soldering, and I haven't had a problem since.

I think VidPro has the solution, just don't go too deep. I use something like 600 grit paper that has already been used for lapping parts. I suppose crocus cloth would work as well. I rub the surface just enough that the "mirror" quality is gone. Seems to work well for me anyway.

Dave

 

[zipplet]

The couple of times I've soldered to cells (both times CR2032) I would *gently* rough the surface by first of all using a fibreglass pen to clean the contact, then gently scraping in a hash pattern with the tip of a screwdriver. Worked well for me

 

[xenonk]

I haven't had any issues with the solder letting go after the devices were dropped or even tumbled down the stairs. The solder should bond rather strongly with the nickel. Maybe it's just luck, but I attribute a good bond to the use of flux.

Now that I think about it, the acidic effects of the flux when heat activated probably contribute some microscopic roughing of the surface in addition to removing oxidation.

 

[lctorana]

Does anyone know a source for flat topped, pre-tabbed eneloops?

Nothing OEM, just a few aftermarket suggestions.

 

[WildChild]

Last year I soldered 3 Duracell 2650 mAh to make a replacement battery pack for a 900 mHz cordless phone. Still keeping its charge for 3 weeks after over one year of use and all cells in the pack are still balanced.