This instructable will document the process of removing the dead Li-Po batteries from a LEGO NXT battery pack and replacing them with Li-Ion batteries. This does two things. 1) It may save a bundle of money on replacing a battery pack. 2) It doubles the mAh rating of the battery.
In addition, the mod is not clearly visible, so it may get past inspection at robotics competitions. No promises here.
I apoligize for not having photos of the before or of the insides of the intact LEGO NXT battery. At the time I did not think to make this an instructable as I was just exploring and did not intend to rebuild the battery. Later, when I discovered that Li-Ion batteries would fit this became an idea for an instrucable.
Step 1: Dismantling the NXT Battery Pack
Using a small drill, carefully drill out all five "rivets" that hold the battery together. These are marked with red arrows in the photo. (note: the rivets are not shown as this is the after photo.) You only need to get just the flat head of the rivet off, taking care to not damage the plastic or drill far into the rivet. This makes removing the remaining part of the rivets easier later.
Carefully pry the case apart. This is harder than it sounds as the Li-Po batteries have sticky tape all over, holding everything together. There are NOT plastic tabs holding the pack together. The sticky tape is all that is holding it together. Possibly using a hair dryer to warm the pack up would help.
Once you get the case apart you can see the innards.
Step 2: Remove the Innards of the NXT Battery Pack
Most everything will be removed and not used. Pictured are the parts that are removed. The parts to keep are the metal tabs that make up the battery contacts.
Getting it all out is a bit difficult due to the sticky tape that holds it all together. Use heat if it helps.
WARNING: The Li-Po batteries are easy to puncture or cut. Doing so with a metal instrument will cause shorting, smoke, sparks, and a bad smell that is possibly hazardous to breath. It may also be possible to cause a fire.
I got sparks, smoke, and smell.
You should be left with the two halves of the case and the contacts still in the bottom half. The postive contact will probably come out, so pay attention to how it goes back in. It's not hard, just annoying.
Step 3: Acquire Li-Ion Cells Cheaply.
More and more there seems to be an abundance of single Li-Ion cell charging devices. These are identified by a slender shape and USB ports on one end. I got two of these for $3 at Frys on sale. Normal prices range from $10-$30. LEGO sells new batteries for $80 (plus you have to buy a $30 charger for the newer white batteries they sell now). The total cost of this project for me was a bit less than $20. Results will vary.
The second photo above shows the electronics from the donor device and two cells (already in the LEGO case from later steps). The USB charger board can be tossed. It would be great to be able to charge this pack from USB, but alas, it only works with a single cell, not two.
I recommend removing the USB charger board from the wires. Leave the wires on the batteries, as they come in handy later.
Step 4: Prepare the Case Halves.
It turns out that 2 Li-Ion cells fit, but they are slightly too fat. The case doesn't quite go together completely, but the pack will fit on the NXT since plastic bends. I don't like this and I'm not sure it would not cause long term damage to the NXT brick. So I made more room in the case.
As shown in the photos, I removed material from the contact radius along the length of both halves. I used a dremel sanding drum as shown in the photo. Be careful not to go too deeply, the plastic is thick enough to remove some, but it is possible to go through. I did not experience this, but I could feel the plastic getting quite thin.
I didn't remove much, but it was enough to get the case to fit together beautifully.
Also, this is the step where you should remove what's left of the rivets. They can be pulled out using pliers. Hopefully there is enough metal left to grip and none of the holes were damaged by the drilling process. I ended up with 2 usable holes since I was not being careful.
Step 5: Acquire a Charger and Port
Since there is no charging circuitry left in the battery pack, the external charger must be specifically made for charging TWO Li-Ion cells. DO NOT TRY TO CHARGE USING A STANDARD ADAPTER. FIRE MAY RESULT. The adapter MUST be made specifically for charging Li-Ion cells and contain circuitry for this purpose.
I found one on Amazon that comes with a nice plug. I also found a matching jack at a local electronics dealer (HSC) for 40 cents.
Search for the charger on Amazon
"Intsun® 8.4V Li-ion Charger for 4 Mode 1200 Lumen CREE XML T6 Bulb LED Bicycle bike HeadLight Lamp Flashlight Light Headlamp"
Finding the jack is the hard part. It might be easier to find a matching plug and jack and just replace the plug on the charger.
I specifically found a jack that cuts off the load when the plug is inserted. This is generally a feature used the opposite way... to remove the battery in order to power the load with the adapter. In this case I want to remove the load so as not to have to remove the battery from the NXT brick to charge it. I also do NOT want to try running the NXT brick from this adapter as I don't want that variable in the charge. Nor do I want to risk damaging the NXT brick.
The jack has three terminals. One is the tip, the other two only connect when the plug is absent.
Step 6: Glue the Port in Place
The port should fit just inside the original charging port hole.
Glue the port into the proper position using hot-melt glue (or your preference). Be sure to not get any glue in the port or on the contacts.
Insert the plug into the adapter to make sure it is the correct position while gluing it in place.
Step 7: Wire the Batteries and Install Into the Case
I found a thermal fuse in an old laptop battery pack. I used this between the positive of one cell and the negative of the other cell. If the battery should overheat during use or charging, this will disconnect the cells to avoid fire. This is not necessary, but probably a good idea.
Follow the notes on the picture to complete the wiring.
Step 8: Test It.
The cells should already have some charge. Use a multimeter to check if you have about 7.8 volts across the terminals and that the positive is truly on the correct terminal.
Plug the charger into the jack with the charger unplugged from the AC. Check to make sure that there is now no voltage on the terminals (if you used the cutoff type jack).
Plug the charger into the AC and check that the red light comes on as it should. Let the pack fully charge while monitoring it for safe charging. There should be no smoke and not much heat if any. The green light should come on when it is fully charged.
Lastly, unplug the charger from the pack and test the voltage again to be SURE. It will probably read about 8 volts or more just after a full charge. The positive terminal is the one with the springy button like terminal.
Step 9: Reassemble the Pack
If all goes well, the cover should fit just nicely.
Find a computer keyboard in your house and remove a couple screws. If you want to use all five holes, steal a couple from a few keyboards so no one will notice they are missing when the keyboard falls apart.
Seriously, find some screws of the appropriate size. I used a keyboard. I only needed two since the other three holes were messed up. Two seems to work fine.
Put the screws in the old rivet holes.
I suppose you could just glue it together, but that's just not as cool.
Use with NXT and enjoy, cheap and plentiful power for your robot.
There are two types of LEGO robot battery chargers. The original grey battery charger has a larger plug. It won't fit in this reconditioned pack and that is good. You don't want to do that or you will have a fire.
The other type is for both the EV3 and the White NXT batteries. It WILL fit in this reconditioned pack. This is unfortunate since the wall wart also looks amazingly similar. The main difference being the red/green LED on the charger's wall wart. DO NOT plug an EV3/White charger into this pack. Fire may (probably will) result. I tried plugging the Li-Ion charger into a white pack. It seemed to work, but was not testing that. I was only testing if it would harm the battery. Just don't use the wrong charger on the wrong battery.
Somehow keep the charger and modified battery segregated from original equipment. Or Label them VERY well.
Don't tell the judges what you have done, or they will probably disqualify your robot. Or better, don't use this pack at a tournament if you are worried. The only advantage is not having to charge your battery at the tournament. If your programming is good and you are properly prepared, that shouldn't matter anyway since you shouldn't be practicing, programming, or running your robot much between rounds at a tournament. Go socialize and see what others are doing.
White NXT and EV3 battery note:
I'm not about to rip apart a working battery to see if this works with the other types of batteries. I only have bad grey NXT batteries. If I ever take one of the others apart, I will update.