Testing C/D cells in Maha C9000

[Russel]

I made this adapter a while back for performing break-in, and testing capacity, on D cells with a MH-C9000.

(I apologise for the poor angle. The display is a little hard to read.)

I made the charger end of the adapter from teflon with brass ends.

That way I could feed wire through the center of the teflon body, solder then to the brass ends, and thread the ends into the body.

 

[MarioJP]

so I am guessing that the D cells are being charged in series??

 

[Mr Happy]

so I am guessing that the D cells are being charged in series??

What leads you to think that?

 

[Russel]

so I am guessing that the D cells are being charged in series??

No, Each cell is connected to it's own cell bay on the charger. They are independent of each other.

 

[MarioJP]

usually battery holders are in pairs. But I see what he did. Be no way to have one channel charge 2 D cells in series but thanks for the clarification.

 

[Russel]

I can see how it could be confusing. The cell holder that I used was made so that the batteries would be in series, but I cut the connections making each cell independent of each other.

 

[Mr Happy]

The usual challenge with that setup is that the steel springs used in cheap battery holders have much higher resistance than copper wire. You might consider bypassing the springs for better conductivity...?

 

[Russel]

Yep, the cheap cell holder is the weakest link in the adapter. I soldered the wires to all but one spring in an attempt to keep resistance at a minimum. The one that has a small socket head screw holding it had no where I could really solder to without melting the plastic. I only use the adapter for break in and discharge a low current levels. So, for now it works well enough.

I have been bouncing ideas around in my head thinking about how I would go about making a good cell holder. I would like to make something that didn't cover the main cell body to allow for good convection cooling. I am wondering if it would be better to have a spring loaded brass contact, or solid contacts at both ends of the batteries with some kind of screw between the cells to assure good contact pressure with the cell. If a screw between the cells was spring loaded, the electrical resistance of the spring would not be a factor. I like some of the 4AA to 1D cell adapters you can find here.

https://www.candlepowerforums.com/posts/2849142


I am just trying to figure out the best way of holding two D cells electrically independent of each other while still allowing good convection cooling.



Thinking out loud...

 

[Mr Happy]

I am just trying to figure out the best way of holding two D cells electrically independent of each other while still allowing good convection cooling.

See my other thread on analyzing batteries.

You can take thin strips of copper foil and attach strong magnets to one side of them using a dab of glue. Then solder your wires to the other end of the strips. The magnets will clamp the foil electrodes very firmly to the battery terminals giving an almost zero resistance connection. To make it even better you can silver plate the foil using some of the stuff used for circuit boards -- then you won't have to keep polishing the copper to remove oxidation.

If you get magnets with the shiny metal plating on the outside you can try soldering wires directly to the magnets like LuxLuthor does. But this takes a bit of skill to avoid destroying the magnetism with the heat of soldering.

 

[Russel]

I was considering something like the battery clamp I 'adapted' for my CBAII, but I would rather make something with the materials on hand rather than having to purchase more. Heck, I've got the machine tools, I need to use them to help justify the cost!

Here are a couple photos of my CBAII battery clamp.

I'll be sure to read through your other thread on analyzing batteries. Thank you for the suggestions.

 

[Apollo Cree]

I was considering something like the battery clamp I 'adapted' for my CBAII, but I would rather make something with the materials on hand rather than having to purchase more. Heck, I've got the machine tools, I need to use them to help justify the cost!

Here are a couple photos of my CBAII battery clamp.


I'll be sure to read through your other thread on analyzing batteries. Thank you for the suggestions.

The way those types of clamps work, it would be real easy to put too much squeezing force on the battery and damage it, perhaps causing leaks later.

Even if you don't overtighten the clamps, I wonder if thermal expansion during charging might cause mechanical stress.

Most battery holders have some degree of springiness in the design. I always thought this was in order to allow you to insert and remove batteries and still get a good contact, but I wonder if springiness is needed to prevent excess force on the cells due to mechanical and thermal expansion consideration.

 

[Mr Happy]

Even if you don't overtighten the clamps, I wonder if thermal expansion during charging might cause mechanical stress.

Most battery holders have some degree of springiness in the design. I always thought this was in order to allow you to insert and remove batteries and still get a good contact, but I wonder if springiness is needed to prevent excess force on the cells due to mechanical and thermal expansion consideration.

Cells do expand a bit when charging, especially using higher C rates. I recently noticed an AA Eneloop was domed a bit at the negative end when I took it off charge. I didn't think to measure its length, but it would have been a little bit longer than when it started. A fair bit of internal pressure can build up inside NiMH cells when on charge.

 

[Russel]

I agree that some contact pressure is needed, but that there should be some give to the holder. I am considering four strips of Teflon (I've got a sheet about 1 foot square, half inch thick) one pair of strips for each end of the batteries, with one of the strips in each pair cut to hold the batteries. Two batteries being side by side with spring loaded guide rods keeping the strips straight. That way there would be constant tension from the springs, but some give so that battery expansion would not be an issue.


At the very lease you should find my diagram humorous!

 

[Mr Happy]

At the very least you should find my diagram humorous!

 

[Apollo Cree]

A piece of rubber from a bungee cord between the copper strip and clamp jaw might also help.

It might be worthwhile to take a nail or punch from the backside and make some bumps on the battery side of the contacts for better contact with lower pressure.

The big thing would probably be for anyone who builds such a clamp to realize that you can easily damage the battery if you don't carefully clamp it gently. Be sure no one else like the kid or wife is going to decide to cinch it down for you.

It IS a neat idea.

 

[Mr Happy]

I'm gonna mention again that magnets are a seriously underrated technology when it comes to attaching wires to cells. Look up your local craft jeweler and see if you can obtain some thin silver foil. Silver wire would be easy, but silver foil may be possible. It would be perfect with a small neodymium magnet glued to the back.

 

[tsmccull]

Silver foil will eventually tarnish (that's why the wife haul's out the silver polish before a fancy dinner party). The right size & shape gold-plated electrical contact that would work would be a better long term choice.

 

[Mr Happy]

Silver foil will eventually tarnish...

I'm gonna mention again that magnets are a seriously underrated technology when it comes to attaching wires to cells. Look up your local craft jeweler and see if you can obtain some thin 24k gold foil. Gold wire would be easy, but gold foil may be possible. It would be perfect with a small neodymium magnet glued to the back.

 

[Russel]

Here is a slightly better diagram of the Teflon parts for the 2 D cell holder I posted earlier.

I figure that the cutouts to hold the D cells don't really need to be a half inch deep. That would allow me to place half inch diameter contacts with the wiring going in a groove cut into the back piece.

I'm not ruling out a method using magnets yet, I'm just exploring this idea to see where I can take it.

Note: I made this diagram with Google SketchUp. I'm just a SketchUp novice, but it surprised me how easy this diagram was to make. The best part is that it is free! Of course, there is a pro version that you can pay for if you want.

 

[Skinnysloth]

From the ideas posted in this thread I came up with this solution:

I'm using AAA to AA adapter to hold my end terminals in place. I took some 12 gauge speaker wires I had, crimped the end terminals and attached them to the batteries with magnets. The end terminals are sandwiched between two magnets and it held them pretty well.