This article is about the third alternative. If you are not fairly skilled in working on electronics, this is not necessarily a good alternative. But, it is possible. Personally, I was inspired to do this the first time because the small (3-cell) battery for my ASUS laptop was very ill and I couldn't find a replacement.
Financially, it also looked like a good choice. The out of stock new battery cost $129. I found the cells for a bit over $5 each. So, I ordered the cells and, expecting to have a success, ordered six more for a sick battery for one of my T23 ThinkPads.
The first trick is to open the old battery without destroying the plastic or anything important inside. What you need to do is find where the two parts are glued together and work on the glue line with something sharp until you can open the battery.
In the case of ThinkPad batteries, there are labels over parts of the glue line. So, first cut through the labels with a sharp knife. I then chose to use a wood chisel to first find parts of the plastic that will flex because there is open space behind them and then start opening in these areas. You may have better luck with a utility knife.
Once you get the opening process started, a utility knife works best in some parts, the wood chisel in others. Just be careful not to cut too deep or you could damage circuitry or wiring inside. Also, remember that the goal is to be able to glue the battery case back together and have it fit in the laptop so cutting out a little plastic (possibly with a fine saw) may be better than an attempt to just open it up with the result of warping or breaking the plastic.
Once you have the battery pack open, it is analysis time. The most common battery packs contain three, six or nine Li-ion cells of size 18650. Typically they are in strings of three in series (that gives you about 11V) and then each string is paralleled. But, that may not be the case. Do a bit of analysis before you start cutting wires.
Note that the cell size is not something that indicates that there are at least 18,649 other choices. It is actually the diameter of the cell in millimeters (18) followed by the length in millimeters (650). The big advantage with this system is that if the cell isn't marked with a size, a metric ruler can help you out.
As Li-ion batteries need special "care and feeding", you will find a circuit board within the battery pack, Wires will go from the circuit board to each cell junction so that the voltages can be monitored. Also, there will be a thermistor and a thermal circuit breaker within the pack.
There are lots of places that sell replacement Li-ion cells. One is Battery Junction. On their site you will find all the common battery sizes. They also have both PC boards to protect sets of cells and a special 18650 cell with a built-in protection circuit. Assuming you are replacing cells in an existing pack, you just need to buy regular cells. One thing you do need to watch for, however, is tabs. Typically you can ask to have solder tabs spot welded to the cells. This is the right way to go rather than trying to solder directly to the cell which could easily damage the seals.
Once you have all the pieces, it is time to actually build the new battery pack. I cannot empathize enough that an error here can be dangerous. First, you are dealing with batteries that if abused, can explode or burn. Additionally, the monitor circuit is always active so you are working on operating electronics. Consider yourself warned.
I have found the best way to build the new pack is first take all the cells, the circuit board and the connector out of the plastic case. Figure out how things are wired. That is, which cells are in parallel, which are in series and, most important, which is the positive and which is the negative end of each cell. There is also usually a thermistor (it looks like a little lump on the end of two wires) and a thermal breaker attached to the batteries. Figure this out and, better still, take a picture of it.
In the T2x series ThinkPad batteries, things are quite well organized. The black wire goes to the negative end of the cell string. The red and orange together (one is the thermal breaker) go to the most positive. White goes to the 3.7V tap and yellow to the 7.4V tap.
You are most likely going to have to solder the battery tabs together or connect them with short pieces of wire. Try to make the connections as compact as possible. While everything may appear fine, when you try to put the top on the battery pack you may discover you need to re-solder connections and/or file down any lumps.
I start building the new pack within one half of the battery pack case. Double-stick tape will hold the cells in place as you work on the battery. Hot glue can be used to stick cells together and hold down the thermal breaker and thermistor.
When you get all the cells in place, start soldering the wires from the circuit board. Once again, remember that the circuit is live so touching a wire to the wrong battery terminal can mean so long circuit.
Once all the connections are made, it is a good idea to check the voltages to see if they make sense. Starting at the negative end, each cell or set of parallel cells should add about 3.7 volts to the total. The exact voltage will depend, of course, on the state of charge of the cells but a fairly fresh cell should read 3.5 volts or more with no load.
If everything checks out, put the top on the battery case. If it doesn't fit, file, adjust, re-solder and such until it does. It is then a good idea to test the battery in the laptop. A bit of tape can be used to hold the battery together for testing.
If all goes well, you need to glue the two battery halves back together. While model cement can be used, I prefer a hot glue gun. Work slowly, preparing one side or part of a side, holding it together and let the glue set. Then move along until the entire battery is sealed. Then, with a knife, cut off any glue that extends beyond the seams.
That's it. You just proved you could save $100 by spending half a day playing with hand tools and glue.
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