Soldering wires to batteries safely?

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MrNaz

MrNaz

Enlightened
Joined
Jul 20, 2006
Messages
244
Location
Melbourne, Australia
I am trying to make a 3P2S array of 18650 batteries that needs to fit in a 40mm tube. Working out the geometry of the batteries alone gives me a required tube diameter of 38.8mm, so there's not enough space to use a holder or any sort, so I would like to make a battery pack with heat shrink over a set of 6 batteries like you can buy for R/C cars or the like, only bigger.

My problem is soldering wires to the batteries to ensure good connection once I put the shrink over them. I know that soldering to batteries is a no-no, but surely there are ways to ensure that wires can be permanently attached to batteries without damaging them or putting yourself in danger.

I guess I could buy 18650s with soldering tabs, but they are far more expensive. I can get non-tabbed 18650s from DX for under $4 each. Buying tabbed batteries from batterystation.com or something costs close to $20 per battery.

So I need a way to attach my own tabs or wires to the battery. What is the best way to do this?
 
I tried soldering some NiCADs end to end and they even had tabs but they got too hot and I destroyed them. For a different project I found that the local battery store makes packs. I brought them some cells and for a fairly reasonable price they made a pack for me and they even shrinked it. See if you have a local store with a battery welder.
 
It's hard. I made a 2p3s 18650 pack. I soldered wires from battery to battery. Use lots of flux and make sure your iron is is plenty hot. First tint the wire with plenty of solder, then I think i used my copper hammer head bit to touch the wire to the battery for less then a second. Heat is batteries worst enemy. I think i bought my hammerhead bit from cheapbatterypacks.
 
MrNaz said:
I am trying to make a 3P2S array of 18650 batteries that needs to fit in a 40mm tube. Working out the geometry of the batteries alone gives me a required tube diameter of 38.8mm, so there's not enough space to use a holder or any sort, so I would like to make a battery pack with heat shrink over a set of 6 batteries like you can buy for R/C cars or the like, only bigger.

My problem is soldering wires to the batteries to ensure good connection once I put the shrink over them. I know that soldering to batteries is a no-no, but surely there are ways to ensure that wires can be permanently attached to batteries without damaging them or putting yourself in danger.

I guess I could buy 18650s with soldering tabs, but they are far more expensive. I can get non-tabbed 18650s from DX for under $4 each. Buying tabbed batteries from batterystation.com or something costs close to $20 per battery.

So I need a way to attach my own tabs or wires to the battery. What is the best way to do this?
Click to expand...

Hi MrNaz,

First I would like to say that I have only done this with NIMH cells, but it can be done.

A couple of things you may already know but I feel the need to mention. In order to charge this pack properly you will need to have a charger that will balance the voltage of the individual cells during the cycle or install something called balance tabs or taps ( dont recall) to the ends of the cells.

That being said. In order to do this you need to go very hot very fast. What I mean is if your iron is not hot enough to melt the solder in about 1 second or less, you will begin to heat up the cell internaly which is very bad.

I use flux for each connection, tin each end of the cell, reapply flux if needed and then solder them together. I dont use wire in between the cells, accept at the ends, you could use that or a thin strip of copper.

Again the most important thing ( other than charging safety for the pack) is you must have a hot iron so only the surface of the cell gets the heat.

Get some flux and use it. Apply it to the cell and to the wire, tin them both and then add more flux. Make the connections and then take a cue tip with some alcohol on it and clean the connections. Flux is an acid that can weaken the joint later if not removed.

Good luck with your build, and please be careful.
 
I've done this successfully with Li-Ion cells before using this procedure:
a)Solder extremely quickly
b)Immediately upon removing the soldering iron, have a second person cool the cell with a damp cloth by dabbing the newly soldered area

Be careful not to short the cell by getting it too wet. In fact, if you have some distilled water that would be highly recommended. And this is definitely a two person job - someone quick with the soldering iron to solder and someone right there ready to cool the cell off. If you fail to make the solder joint properly in a second or two, cool the cell and wait before trying again.

I've done this successfully literally hundreds of times as a two man team in order to build very large li-ion packs (not flashlight related). However I take no responsibility if it doesn't work for you!

Good luck!
 
Heres a partial re-post from an earlier thread...

Soldering bus bars onto cells is not the most straightforward thing. You are going to need a good soldering iron (a station would be ideal), one that can maintain its temperature when it contacts the cell. Cells are heatsinks, and will "freeze" your iron tip when the two come in contact, unless your iron can sustain its own temperature enough to allow the solder to flow. I use at least an 80 watt weller pencil iron, and it has worked fine for my RC hobbies. The hotter, the better... but then again its paramount that the iron has th ability to maintain and sustain its temperature. The generic "pencil" tips that are OEM for most irons and stations will not work very well. I have always found the more blunt tips with flats on them to work best... pic here.
dscn6156.jpg


Cell surface preparation is paramount. Iso-propyl alcohol (99% will suffice) , some firm scrubbing, flux application to the area. You can use a dremel ball grind attachment to scuff the surface, but I have not found this to be necessary all the time. A seasoned RC heli enthusiast once told me that solder flows where flux goes. So a little flux goes a long way to ensure good solder contact.

Deans battery bars and 12AWG wet noodle are pictured. I have seen guys make their own battery bars using tinned copper stock, cut to shape.
dscn6155.jpg
 
So I grabbed some dead AA alkies, set my Hakko station to 900F and I had decent results. As mentioned earlier in this thread you have to (in this order) flux, tin, flux, then solder. You must do this to both your battery and wire. Using this method I was able to solder batteries end to end as well. Good luck, get a decent station, get some flux, and maybe practice on some dead cells first.
 
This is OK for joining batteries that are not end to end, but if one was to heat shrink batteries in series, how would you go about ensuring that the end to end connection was maintained? Is there a way to join these easily? Is there a conductive paste that you can use to ensure that a good connection is maintained?
- Naz.
 
Let me start to say I'm no expert on batteries.
That said, I did solder a tip on top of an 18650 battery which holds really well.
When soldering electronics like computer chips you have to use an soldering iron of low wattage (and thin solder) so not to ruin the electronics by too much heat. I use an 11 Watt soldering iron for that and that's the one I used to solder the tip. When the iron is heated up, you heat the battery (+ of -) top. Ideally the solder would never touch the soldering iron but would go fluid when it touches the surface you're heating. With a battery you won't get that done with 11 Watt and it probably wouldn't be wise. So I heated the top for 10 seconds or so and then applied the solder at the point where the soldering iron heated the top. The battery was less warm then it is when being charged and the soldered tip is quite solid.
I think it's safe to do it this way and haven't noticed any problems, but anyone feel free to correct.
 
When soldering batteries you'll absolutely want at least a 50W iron, preferably more towards 65-85W. The key is to get the job done FAST. So get the tip heavily tinned and use plenty of solder to transfer heat to the surface you want to solder, have everything ready and tinned.

Personally I tin the wire with a bit more than I normally use for connections, heat the battery surface adding excess solder to improve thermal conductivity, once everything is wetted solder the wire onto the surface and let cool. This should not even take 10 seconds from heating to done.
 
MrNaz said:
This is OK for joining batteries that are not end to end, but if one was to heat shrink batteries in series, how would you go about ensuring that the end to end connection was maintained? Is there a way to join these easily? Is there a conductive paste that you can use to ensure that a good connection is maintained?
- Naz.
Click to expand...

If you are wanting to join the cells in series here is a great vid on doing that.

Also post #38 in this thread will help.
 
e1sbaer said:
Let me start to say I'm no expert on batteries.
That said, I did solder a tip on top of an 18650 battery which holds really well.
When soldering electronics like computer chips you have to use an soldering iron of low wattage (and thin solder) so not to ruin the electronics by too much heat. I use an 11 Watt soldering iron for that and that's the one I used to solder the tip. When the iron is heated up, you heat the battery (+ of -) top. Ideally the solder would never touch the soldering iron but would go fluid when it touches the surface you're heating. With a battery you won't get that done with 11 Watt and it probably wouldn't be wise. So I heated the top for 10 seconds or so and then applied the solder at the point where the soldering iron heated the top. The battery was less warm then it is when being charged and the soldered tip is quite solid.
I think it's safe to do it this way and haven't noticed any problems, but anyone feel free to correct.
Click to expand...

I appreciate your help, but I just have to say that this advice is wrong. :grin2:

You have to use a very high wattage iron to ensure minimum thermal energy is transferred in by keeping contact as short as possible. Furthermore, if you heat the battery terminal enough such that solder melts onto it, you're heating up the internals enough to degrate the electrolyte and will greatly reduce battery capacity.
 
OK I think I've found the best way to do this: Electrically conductive epoxy.
http://www.epoxies.com/elec.htm
The 40-3900 product looks like exactly what I'm after. It may even be suitable for use instead of welding the battery bus strips.

I'm sure there are many projects here that could benefit from this stuff, so spread the word!
 
MrNaz said:
I appreciate your help, but I just have to say that this advice is wrong. :grin2:

You have to use a very high wattage iron to ensure minimum thermal energy is transferred in by keeping contact as short as possible. Furthermore, if you heat the battery terminal enough such that solder melts onto it, you're heating up the internals enough to degrate the electrolyte and will greatly reduce battery capacity.
Click to expand...

Yes, that's why I said that in case of a battery it's probably not wise to let the solder melt (on the battery terminal). About the wattage, I'll use a higher wattage for batteries next time. That's why I like this forum :)
 
MrNaz said:
I appreciate your help, but I just have to say that this advice is wrong. :grin2:

You have to use a very high wattage iron to ensure minimum thermal energy is transferred in by keeping contact as short as possible. Furthermore, if you heat the battery terminal enough such that solder melts onto it, you're heating up the internals enough to degrate the electrolyte and will greatly reduce battery capacity.
Click to expand...

this advice is correct... you want the iron to be HOT (80-120W) in order to limit exposing the cell chemistry to heat. The longer the cell is exposed to heat, the more at risk it is of damage. Using an 80-120 watt blunt tip weller pencil, the longest I need to hold the iron onto the cell is ~3-4 seconds. You shouldn't need to pre-heat the cell, and you definitely do not want to hold the iron onto the cell any longer than what it takes to flow the solder.

The other thing I do is use a damp sponge to cool the solder joint down and prevent the further spread of heat into the cell. The solder will solidify but retain a lot of heat after the iron is removed. You want to eliminate this heat source quickly before it enters further into the cell.

Its like searing a steak on the BBQ grill. You want to sear/brown and alter the chemistry of the outside without adversely affecting the chemistry inside.
 
Last edited:
This is how I've done it:

1. Use a dremel, or similar to roughen the contacts.
2. Secure the cells, you only have two hands.
3. Use a good Soldering iron, add some solder to the ends, it should flow by itself.
4. Add whatever you prefer to connect the cells to each other.

As others said, keep the thermal stress as low as possible, practice on some old NiCD's, they won't explode.

As also mentioned before, you'll need to balance while charging, therefore, you need a charger which has a balancer, or an external balancer.

You need extra cables and an appropriate connector for the balancer. Pin 1 goes to ground, Pin 2 goes to + from Cell 1, Pn 3 goes to + from Cell 2, and so on.

I'll do a quick guide on this, as soon as I find the time, but don't wait for me.
 
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