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.