Help with battery pack

[sclakediver]

Hi guys, I decided to build a can light as a winter project. There are great sources to be found in the DIY forums. Thanks alot. I have built the cannister and light head, but am having trouble with the battery pack.

I am using a dozen NiMH D-cell batteries(three stacks of four) The problem I am having is soldering them together. I have tinned the terminals of all the batteries, I just can't solder them together. I have a 80W iron and made my own hammerhead tip. I made the tip out of a steel bolt and brass nut.

I need more heat. Is my iron too small? Did I make the tip out of the wrong materials? I'm at a loss. Any hints, tips, or suggestions would be greatly appreciated. My project will be complete as soon as I have a battery pack. Thank you for your time.

 

[cool_hardware52]

Hi guys, I decided to build a can light as a winter project. There are great sources to be found in the DIY forums. Thanks alot. I have built the cannister and light head, but am having trouble with the battery pack.

I am using a dozen NiMH D-cell batteries(three stacks of four) The problem I am having is soldering them together. I have tinned the terminals of all the batteries, I just can't solder them together. I have a 80W iron and made my own hammerhead tip. I made the tip out of a steel bolt and brass nut.

I need more heat. Is my iron too small? Did I make the tip out of the wrong materials? I'm at a loss. Any hints, tips, or suggestions would be greatly appreciated. My project will be complete as soon as I have a battery pack. Thank you for your time.

Making good solder joints is not easy. You need the right solder, flux, heat source and technique. Without seeing the problem it's very hard to provide specific advice.

As to the heat source, you need the right wattage, and temperature.

The first place I'd look is the tip on your iron. Is it clean? Is it tinned? Is there a commercial tip available?

You should never attempt to solder directly to the end of a cell. It sounds like your cells have solder tabs. A solder tab is a strip piece of nickel that is spot welded to the end of the cell and extends out a bit.

Why no soldering to the cell? Soldering requires that the base metal be hot enough to melt the solder. Directly heating the end of a cell with a soldering iron to the ~200C required will damage the cell.

It's counterintuitive, but a brief high temp spot weld inputs much less energy to the cell.

If you wan to limit the heat introduced in the cell you also want to be careful how long you heat the solder tabs. Pre tinning them is a good idea.

Here's a link with some good tips.

Electronics Tips: Soldering Techniques

Good luck,

Tobin

 

[sclakediver]

Thanks for the reply Tobin. As always very informative.

However, I disagree with never attempting to solder directly to the end of a cell. True it is very easy to destroy a battery by doing this, but it can be done successfully. RC airplane enthusiasts do this all the time. You need a hot iron, and have to work quickly. I have made a couple of small battery packs this way, but never with a battery larger than a AA. Maybee a D-cell is too big of a heat sink.

I'm thinking I should have ordered batteries with tabs.

 

[cool_hardware52]

Thanks for the reply Tobin. As always very informative.

However, I disagree with never attempting to solder directly to the end of a cell. True it is very easy to destroy a battery by doing this, but it can be done successfully. RC airplane enthusiasts do this all the time. You need a hot iron, and have to work quickly. I have made a couple of small battery packs this way, but never with a battery larger than a AA. Maybee a D-cell is too big of a heat sink.

I'm thinking I should have ordered batteries with tabs.

Ni-Cads and Nimh cells have built in Over Pressure Vents, to prevent cells from bursting if abusively over charged or in certain high current discharges.

These OPV's use a polymer disk under the positive end cap. Overheating can damage this OPV. You may not see immediate failure, but over time you are likely to see the cell express the KOH electrolyte.

Hobbyists get away with all sorts of "bad practices" because they are their own "customer"

Tobin

 

[sclakediver]

Wow, I did not know that.

So would you suggest NiMH cells with tabs?

 

[cool_hardware52]

Wow, I did not know that.

So would you suggest NiMH cells with tabs?

Yes, with tabs, unless you have a spot welder.

Tobin

 

[sclakediver]

We have a spot welder at work, but I wouldn't be able to weld them end to end.

 

[cool_hardware52]

We have a spot welder at work, but I wouldn't be able to weld them end to end.

I was joking about the spot welder. The welders used for batteries are specialized device, not what is used in general sheet metal work.

Look here Miyachi Unitek Corporation - 100ADP / 300ADP Capacitive Discharge Welders

Assuming you had one there are two common techniques for end to end cell connections.

The entire case of the battery is the negative terminal, and the insulated "button" on the end is the Positive.

There are nickel pieces made that are shaped like a shallow "cup" The id is just large enough to slip over the negative end of the cell.

There is a ring shaped insulator placed over the positive end of the first cell, and then this cup is place open end up over the ring insulator. The center of the cup contacts the positive terminal and is spot welded in place. Then the neg end of the next cell is placed into the cup and is welded around the perimeter. Only after welding is the cell stack covered in heat shrink

The 2nd common means to connect cells is to use the same ring insulator, the the two cells are placed side by side, one neg end up and one positive end up. A Strip of nickel is welded across the ends of both cells, and then the strip is folded as you bring one cell in line with the other.

This can be approximated using solder tabs by soldering the tabs from two adjacent cells together. Don't leave out the ring insulator on the positive terminal. Make sure the two solder tabs are flat, and don't leave globs of solder on the joint as it will not let the bent tabs lay flat. Works pretty well. It helps to have a little extra nickel strip if the tabs are too short or oriented inconveniently.

If you use the "fold method" be aware that if the strips are too long the "fold" may project beyond the diameter of the cells. This can become a short circuit point if it penetrates the final heat shrink.

Tobin

 

[sclakediver]

I know you can't use a conventional spot welder to put the tabs on batteries. We have an avionics shop, but probably not the equipment needed.

I think I'll see if batteries plus can build a pack for me. So I guess it's not a DIY project afterall.

Thanks for the help Tobin. I'll seek your advice on a wing later.

 

[coldsmoke]

I got the guys at our local batterys plus to spot weld my batteries together for me. They charged me $10 and they even put some bomber heat shrink around the pack. It was worth it to me. Good luck.

Hunter