For this instructable, I'm going to purposely leave out detailed information. The missing information should be obvious for those experienced with electronics. Why am I leaving out information? Because you shouldn't be doing this.
DISMANTLING / ASSEMBLING LITHIUM ION BATTERIES ARE VERY DANGEROUS AND VERY STUPID ACTIVITIES. YOU ARE EITHER VERY EXPERIENCED OR VERY STUPID OR BOTH TO ATTEMPT THIS. IF YOU ARE NOT SURE WHICH CATEGORY YOU FALL INTO, DON'T TRY THIS. IS THE CHANCE OF BURNING DOWN YOUR HOUSE WORTH THE MONEY YOU WILL SAVE BY NOT JUST BUYING ANOTHER UNIT? THIS IS NOT AN EXAGGERATION.
We will start with the obvious. There is no battery replacement available for the Omnitech. The Omnitech takes a 3.7V Lithium Ion Battery. I am not sure of the original battery's capacity. When I dismantled my battery, all of the black lettering on the cell came off with the black insulation that was on it. I can see the outlines of some of the numbers, but I think it is just a serial number. This makes no difference as there is no place to buy that exact same cell. Li-ion batteries are never sold by the cell. They are always sold as assembled battery packs with protective circuitry boards (PCB). In order to add a new battery pack, we will have to buy a complete battery and strip out its cell. Then we will use the PCB from our old battery pack.
What you will need:
Your old battery pack
A new 3.7V li-ion battery pack that will fit inside your unit. (Nokia cell phone battery?)
Very good soldering equipment
Step 1: Safety
1. Don't cross your positive and negative terminals!
2. Please for the love of God, don't cross the terminals!
3. When soldering to a terminal, insulate the surrounding area well.
4. If while soldering to a terminal, you solder positive to negative, throw the battery as far away from you as possible and get a fire extinguisher. Do not try to remove the solder. These batteries will heat up and explode very quickly. You will have no time to fix the mistake.
5. When cutting through the insulation, don't cut. Use the razor to start a tear and peal the insulation by hand.
6. When your cell is "naked", treat it like a live hand grenade. Don't just set it down anywhere.
7. If you are 18 years or younger, first get permission from your parents, your local fire-marshal, the Pope, and God him/herself before proceeding. Never mind. Just don't do it!
8. Never leave your new battery charging and unattended.
9. Never leave your new battery discharging and unattended.
10. Until you verify that your new battery is safe, don't leave it unattended!
11. Wear every type of protection you can: eye, hand, ear, breathing...etc.
12. If you don't follow these rules, you and those around you will be in great danger.
You don't believe me?
Most examples of batteries exploding are ones where the battery is sound and straight from the factory. You are going to try to do what the factory does but with substantially lower quality equipment and knowledge. If you have an ounce of intelligence in your body, you will not do this mod. You will get hurt.
Step 2: Get Your PCB
Remember, no Omnitech PCB means no battery pack. I tried looking up a replacement, but the PCB has no markings. The few PCB's that I did try on this unit failed to initiate charging.
Step 3: Prep Your New Cell
You will need to strip the PCB off of your new battery. Use the same cautious approach you used when dismantling your original battery pack. Make sure the new cell retains a good amount of the metal strips. This will allow you to avoid soldering directly to the cell itself later. Verify the positive and negative terminals using a voltmeter. Do not assume the polarity is the same as your old battery just because they look the same. Throw away the PCB and thermal switch of the new cell and continue.
Step 4: Solder
You will want to insulate the area surrounding where you will solder to the battery. I highly recommend using this tape. Go ahead and solder the PCB to the new battery cell without mixing up the polarity. Don't be afraid to use some muscle to quickly join the metal strips from the PCB to the metal strips from your new cell. Avoid excessively heating your li-ion cell. If the solder does not melt instantly, your soldering equipment is under powered. Stop! Holding the soldering iron on the cell for longer will only increase your chances of hurting yourself and will not get you any closer to soldering the PCB to the cell. If at any point you feel your cell heating up rapidly, throw the battery away from you and get a fire extinguisher.
Now fold over the excess metal strips if you have any. Place tape under the thermal switch and the portion of the metal strip which runs from the PCB to the thermal switch. The thermal switch should be running down the side of your cell. Then tape over the thermal switch such that it is pressed tightly against the cell. You are sandwiching the thermal switch with tape to electrically isolate it from the cell. This will ensure that the thermal switch will disconnect the cell if it gets too hot and that there is no chance of the metal strips bypassing the thermal switch. Again, if you are confused by this, stop! The thermal switch is very very important. It will stop the cell from overheating and exploding while being charged. Your car will thank you.
Next, wrap your new naked cell in some protective tape. You should not use regular electrical tape because the adhesive fails when it is heated up. You need to cover every square inch of the cell. This is the most important step to not screw up. If you insulate your battery poorly and it shorts when you are not around, you will have a very dangerous situation on your hands. You should add enough insulation that you would feel safe leaving this under your pillow overnight.
Do not leave the battery under your pillow overnight!!!
Step 5: Testing
Once you see that the battery discharges, you will try charging it. While the unit is on, connect the external power. The light on the Omnitech should change from green to orange. If the light stays green or if the light turns off, you did something wrong. Did you use the PCB from your original battery? If the light flickers different colors, you did a poor job soldering the PCB to the cell. You will have to dismantle it again and redo the soldering work.
If you pass those first two tests, try charging the pack to full. Do not leave the pack unattended! If you have equipment to monitor the temperature of the battery, use it. Do not touch the cell to see if it is hot. Keep your distance while it is charging. If all goes well, the orange light should turn green. If you feel at any point that something is wrong, immediately disconnect the power. If you see smoke, get the fire extinguisher. There will be a fire soon.
If you have gone this far down the rabbit hole and have all of your fingers, you are in good shape. The last step will be to discharge your battery pack and recharge it again. Once again, don't leave it alone. If all goes well, close up your unit and redo the charging test. I would keep an eye on the unit for the next few days of use. If all goes well, you can safely enjoy your new battery pack.
My new cell was a few mm too large for my case. So I used a dremel to dig out a space for it in the back cover. Even though the new cell was advertised as 1800 mAh on the outer case, the cell itself had 1400 mAh printed on it. Expect the same from any $5 battery. In the end, I had over three and a half hours of battery life. That is a good improvement on my original 30 min battery. Below, I have my final unit with all three instructable mods on it.
If you want a GPS with voice recognition, automatic power on capabilities, and a large battery, I'd suggest just buying a unit that has all of those features stock.
If you are unemployed and sick with the flu like me, you can go ahead and spend your time modding this GPS unit.
I hope you've enjoyed my mods. Please be safe.