The Battery Cell Box Idea

[safe]

The Battery Cell Box Idea

Here's an idea... (that probably has gray hairs :wink: )

You build a box out of something like aluminum. Inside the box you stack an entire set of standard cells of whatever flavor your desire. (NiMh, NiCad, LiFePO4) Then you drill holes in your box and install suspension fittings that will carry the electrical current. The fittings design MIGHT include something like a rubber washer that allows the tip to move a little so that when the battery cell heats and expands it has room to move. The entire thing is then sealed up by closing the box and using long bolts to apply pressure. You could even add little spacers between the cells to allow for air circulation. The wires that extend outside the box would add some resistance, so it would be advised to use a very heavy gauge wire.

This is a variation of the "tubes" concept and allows you to do easy maintenance of your battery pack. You could likely extend the life of your pack if you rotated your cells after starting with a few spares. You would just look for problem cells and swap them out when you find them.

 

[safe]

Resistance

Correct me if I'm wrong, but solder is something like 10 times less conductive than copper. So it's possible that the wires actually are a small percentage of the resistance in a typical pack. (depending on how well the soldering job might have been done) The negatives about soldering might be enough to warrant the wires.

Hmmmm.... :?

 

[safe]

Suspension Fitting Design

The "ideal" fitting would somehow be a seamless copper part that blends into the copper wire. By focusing on getting this right and also adding a little silver paste at the tip where it meets the battery cell it's possible that the overall resistance would be low.

Pick an issue and run with it...

 

[TylerDurden]

Correct me if I'm wrong, but solder is something like 10 times less conductive than copper. So it's possible that the wires actually are a small percentage of the resistance in a typical pack. (depending on how well the soldering job might have been done) The negatives about soldering might be enough to warrant the wires.

Solder may be more resistive, but the current doesn't have to go very far through it, compared to the wires.

 

[safe]

Solder may be more resistive, but the current doesn't have to go very far through it, compared to the wires.

Here's the situation...

By eliminating the solder and tabs and switching to all copper it "should" work pretty well... you don't need to have long wires, not any longer than the tabs anyway.

The really "extreme" idea would be to connect the cells in a kind of "circuit board" that was built into the aluminum...

 

[Toorbough ULL-Zeveigh]

Correct me if I'm wrong, but solder is something like 10 times less conductive than copper. So it's possible that the wires actually are a small percentage of the resistance in a typical pack. (depending on how well the soldering job might have been done) The negatives about soldering might be enough to warrant the wires.

Solder may be more resistive, but the current doesn't have to go very far through it, compared to the wires.

According to the textbook on proper wiring technique, it's the wires that provide the primary conductive path for the current & should be in good secure physical contact with each other b4 any solder is applied, precisely for the reason that solder is not as conductive as copper.
The solder is only there to keep the wires from separating & can only serve to lower the overall contact resistance by providing some additional parallel path.

 

[Beagle123]

You could take the box idea one step further, and make a double box where the bottom of the box is one big piece of aluminum, and the batteries on the left side of the box point down, and the ones on the right point up. Then you don't need to solder anything on the bottoom part of the box. You could make two tops for the left and right. The voltage of the two tops would be double the cell voltage. Then you would have cut your problem in half.

 

[safe]

You could take the box idea one step further, and make a double box where the bottom of the box is one big piece of aluminum, and the batteries on the left side of the box point down, and the ones on the right point up. Then you don't need to solder anything on the bottom part of the box. You could make two tops for the left and right. The voltage of the two tops would be double the cell voltage. Then you would have cut your problem in half.

There are a lot of ways to do this...

One idea would be to build a box where the top and bottom are built like:

1. Aluminum layer for rigidity on the outside.
2. Thin later of rubber for insulation and flexibility. (or buttons)
3. Thin layer of copper sheet to link all positive or negatives together.

This would create a "Parallel Box" that would have all the cells tied together in parallel. The copper sheet links it all together. The only thing here is that maybe the thin rubber layer would be better if it were a bunch of little rubber buttons that would act like suspension and PRESS the copper sheet into the tips of the cells to provide a strong connection that is also flexible. (since the rubber flexes)

The only wires would be the series connections. (all the parallel connections would be through the copper sheet... the entire unit would then need a single charger to charge it)

 

[fechter]

Yes, it does have grey hairs. I can't find the post now, but there was a good picture of it.

You could use a piece of heavy copper clad PC board material and drill holes in it for each cell. Bring the cell tabs through the holes and solder them to the copper. If you think you want extra cooling, drill more holes that line up with the spaces between cells. This would be nice and vibration resistant. PC board material can be very strong (the fiberglass kind). Then you just need a layer of insulating/cushioning foam stuff outside that.

If you're Mr. safety, you can also put a separate fuse for each cell on one side.

 

[safe]

Bring the cell tabs through the holes and solder them to the copper.

Then we would be going backwards into the soldering concept...

The main idea is to spring load the box so that all the cells have direct and pressurized contact with the copper. By eliminating the non-copper elements it should lower the resistance value.

With something like this you could simply throw all the cells into a box and you can also open the box and do individual cell maintenance later on. Out of 100 cells you are always going to have 3-4 that are weaker and if you don't have a mechanism to remove the flawed cells you end up bringing the entire pack down to the level of the weakest cell. This is the primary reason for doing the box.

Think "mass production"... if you were given the task of producing 1,000,000 battery packs you could invent a "battery box" and just throw the cells into it very quickly OR you could hire a few hundred illegal immigrants to solder them at minimum wage. (or outsourcing it to China) Aside from the moral question of promoting low wage jobs it also comes down to cost effectiveness. If you had a circuit board like box with all the necessary components included the boxes could be built in about 5 minutes or less... the soldering approach might take 10 times that long.

Of course by that point the battery manufacturers themselves could be asked to produce the batteries in 12 volt blocks about the size of automotive batteries which is where it should all eventually end up anyway...

 

[fechter]

The resistance of a good soldered connection will be very low, and stay low forever. For a compression connection, all the batteries would need to be precisely the same length, and stay the same length when they heat up (which they won't). A compression connection will be prone to corrosion and loss of continuity. In order to maintain a good compression connection, the forces on the connection need to be very high, like you get with a bolted connection. Most batteries will crush with this kind of force. Look at the circuit board in a computer. All the components in there were not individually soldered by child labor in China. It is done by a wave soldering machine, which is very fast. Same thing could be done with batteries. Changing individual cells would be a pain if they were in the middle of the pack somewhere. Just figuring out which cell is bad would be tough. In a mass production / maintenance scenerio, the entire sub pack would be replaced, not individual cells. Same as a DeWalt battery.

 

[vanilla ice]

I like to visualize a fuse when I think about resistance vs. length. The little wire in a fuse probably sucks at conducting relative to everything else in line with it, but it doesn't matter because its so short.

 

[fechter]

Right about the fuse.

In a typical soldered connection, the electrical path is only a few thousandths of an inch long, and has a relatively large diameter. Nano-ohms of resistance.

Another possible approach might be to have welded tabs on the batteries, but individually bolt them to the plate. Don Harmon's LifeBatt batteries come with threaded studs on the cells, so that would work well if you wanted to parallel a bunch of them.

 

[safe]

Flate Plate Battery Box

The idea here would be to use two flat plates of aluminum to serve as the frame for the batteries in a kind of sandwich. The sides would be closed with appropriate clamps/fittings/etc to make a good connection. You end up with units of 12 volts which you can combine in series or parallel in whatever way you need in order to run the bike or charge it.

A very flexible design and you can take the batteries out and test them whenever you wanted...

 

[safe]

The Fitting...

 

[marty]

Safe,
Look at these battery holders.

Go here for my thoughts from a few months ago.
http://endless-sphere.com/forums/viewtopic.php?t=993
Call Rabbit Tool people http://www.rabbittool.com They will explain all about battery holders. Can make all different sizes.

 

[safe]

Well it definitely confirms that the idea is a winner... 8)

 

[quadcopterman]

that's my Gary Fisher bike with the rabbittool 36 v pack it worked very well for a D cell battery's

 

[Hillhater]

Wow ! 8 years sleeping and the thread is revived ?.!
Well done on finding your bike buried so long ago . :wink: