Soldering on Battery Terminals

[engr_joni_ee]

Hi,

I am wondering if it is OK to solder directly the battery cell terminals for electric cycle. I am going to replace new cells in the battery. I have watched a video in which they do Spot Welding with Nickle Strips. Any suggestion which method is more reliable.

 

[betwixt]

Spot welding uses far less overall heating power. It is intense but only for a very short period.
Soldering requires the surface to heat up to solder melting temperature and on a cell that might mean heating a substantial amount of metal. The risk of boiling the cell electrolyte is high and it could cause an explosion by gassing.

I spot welding isn't feasible and the battery and wires are held firmly, you can use conductive resin glue to make the connection. It generates negligible heat and makes a good connection but is quite expensive. Look for "Silver loaded epoxy" for more information.

Brian.

 

[engr_joni_ee]

Hi, You mean that conductive resin glue method is better then spot welding ?

 

[betwixt]

No, but it is an alternative that doesn't risk the danger of soldering directly to the battery.

Spot welding is probably the best solution if you have the equipment to do it. Intense heat is generated in the weld but only for a fraction of a second. Soldering uses a lower but still significant heat source but has to heat the surface of the battery to solder melting point before it will flow, this is what creates the danger.

Conductive resin is very expensive but almost no heat is generated.

Brian.

 

[c_mitra]

The battery terminals are also tough to solder; somehow the solder does not like to stick. It works for Zinc surface with some difficulty.

Welding with thin Ni strips is not very tricky - I saw sever you tube videos but I do not know how well they work.

If you want to try the epoxy route, you can make some using graphite powder and regular epoxy- but you need to hold how the connecting wire. Mix some epoxy using regular glue and add an equal amount of graphite powder to get some conducting paste.

 

[kripacharya]

I would think that epoxy/ graphite method would add several ohms to the current path. Obviously not desirable for a voltage power source, but may be the easier hence better route to take for some applications.

 

[betwixt]

Conductive epoxy is loaded with silver powder, not graphite. It can provide very low resistance connections but as it contains real metallic silver it is obviously rather expensive to buy. As pointed out, the resistance of graphite is far too high for it to be used in current carrying applications.

Warning - I know someone who tried spot welding by connecting a car battery to one battery terminal and touching a connecting strip through a thick wire to the other car battery terminal. Their intention was to generate intense heat at the point of contact and form a weld. It worked, they welded the strip to the battery and then couldn't remove the car battery connections because they welded too! It resulted in the insulation around the car battery wires catching fire and the wires burning out. They were lucky the car battery didn't explode.

Brian.

 

[BradtheRad]

Is the question about small AA or C or D cells? These are available with solder tabs (a few dollars each).

I confess I solder wires directly to rechargeable C and AA cells. It keeps my electric shavers operating (draw is 1/2 to 1 Amp). It's not the safest method but it uses up my aging stock of rechargeables. I solder as quickly as possible, for fear the heat might explode the cell.

I use my 100W soldering gun, letting it heat up first. The aim is to work quickly as possible on the smallest area of metal. I apply a dab of flux paste. The gun usually has a bit of solder on the tip. I tap it to the battery a few times, watching that heat is intense enough to transfer a bit of solder so it tins the terminal (an area about 1/8 inch diameter).

While it cools I tin the wire (stranded, 18 or 20 gauge). With the gun I mash the tinned wire to the tinned spot. Usually heat transfer is rapid so the solder melts together. I hold the wire in place for half a second so the solder hardens. If it takes longer than that then I suppose I risk shortening the battery's useful life.

 

[c_mitra]

Conductive epoxy is loaded with silver powder, not graphite.

I know but the paste will be still effective with graphite (graphite powders are easily available from science labs: that is where I got them!)

Most of the epoxy must have about 30% (v/v) of silver otherwise the particles will not touch each other. But 30% v/v comes to about 70-80% w/w because of the density differences.

Most manufacturers use less than 30% (v/v) and use a particle size distribution so that good conductance is obtained. But for special applications 29-30 (v/v) applications is needed. For military applications, gold is used (30% v/v becomes very high when you consider w/w proportions).

Because a blob is used to connect, the lower conductivity is not the issue; the physical strength of the joint is more important. Polymers as a rule do not stick well to metals (porous surface is ideal).

Graphite may be 100 times lower conductivity compared to copper or silver (or gold) but does that matter for this application? If silver contact will give you a uohm resistance, graphite paste will give a 0.1 mohm resistance. It is still negligible if you compare with the regular load impedance.

 

[kripacharya]

I would think - after a quick crosscheck on the web - that resistivity of graphite is of the order of x10, 000 or more as compared to silver. Maybe x100,000. So where a silver-epoxy joint might have 10 uOhm, same graphite-epoxy would be around 1 ohm!

So definitely the usage has to be carefully selected.

 

[engr_joni_ee]

Hi,

I am trying to get conclusion for soldering the batteries for electric cycle. Do I really need to do spot weld with nickel ?

 

[c_mitra]

Do I really need to do spot weld with nickel ?

Simple questions do not have simple answers.

Most common cells (except the very old C and D type ZnCl2 cells) have contacts that are Ni plated. They are easier to solder and still easier to weld.

Soldering is easy if you have a relatively high power soldering iron - but do it quickly. Use lots of flux. For this, copper is the preferred metal because solder wets copper easily (tin forms an alloy with copper).

Tin also dissolves in Nickel (forms an alloy) and if the contacts are Ni plated you can solder them without much problem. But you should do it quickly.

For spot welding, you apply far less energy for a shorter time in a very small area. The metal melts and fuses but only at the point of contact.

For spot welding, Ni foils is the most suitable because Cu can corrode easily (Cu is also not easy to spot weld). So if you have the setup, go for spot welding.

The spot welding setup need not be complicated: a bank of capacitors and a high current SCR may be all that is needed.

If you are planning to do this on a regular basis, I suggest that you go for a spot welding setup.