The Philips HQ 7740 Coolskin is battery powered shaver. The built-in battery of my Philips HQ 7740 didn't charge properly any longer, so I replaced it. I didn't find instructions on how to change the battery, so this is how I did it...
Warning: The following instructions are not official. In fact Philips doesn't want you to replace the battery on your own. I'm sure you have heard of burning batteries in mobile phones and so on. This could happen here too e.g. by a faulty battery or because you do something wrong. Don't follow the instructions below if you don't know what you do. I don't take any responsibility for your actions.
Note: An Amazon customer noted in a battery review that you should discharge the battery before you replace it since the electronics will remember the remaining power and might otherwise get confused after you replaced the battery.
1. Open the transparent blue cover of the cartridge compartment first.
2. Remove the 5 torx screws. Once you've done that, you should be able to remove the white cover at the bottom easily. The top blue cover is a bit more tricky. I used a sharp knife to open it carefully from the side (a flat screwdriver might damage the case which was built to be waterproof). Once you lifted the cover, you will see the battery.
3. Though it looks as if you could just take the battery out, that's not possible. You can lift the battery on the bottom side, but you can't on the top, since the battery is connected on the top with a circuit board which is underneath the battery pack (via two solder tails). You need to carefully pull the connector on the bottom (see red marking on image) first, to disconnect the circuit board from the docking station pins.
4. Now you can lift the whole battery pack up on the side where you just pulled the connector. Be aware that the battery pack is still connected with the circuit board and the motor on the other side. If the battery pack poles touch the motor copper stripes, the motor will start.
I set up the shaver as you can see on the image in order to be able to unsolder the battery (the shaver is upside down on the table). A third hand is a very handy here. In order to place the third hand, I had to remove the transparent blue cover of the cartdrige compartment too.
Depending on how you want to do it, you might want to try to remove the battery by cutting the solder tails first (see step 5).
5. Now we need to unsolder the battery by removing the solder from the joints marked with a red dot (see image). Since the solder tails are bend on the pad, using a desoldering pump I wasn't able to remove the solder properly. So I removed the third hand again and carefully lifted the battery out of the battery pack on the docking station connector side and cut the solder tails (pliers) which connect the battery with the circuit board (don't short circuit the battery while doing that). Maybe desoldering wick is a be a better choice here. On the other hand that way you don't heat the battery up too much and you can just push or pull the remaining stripes through the circuit board more easily.
6. Put in the new battery. On the image you can see my replacement battery. There are several shops on the internet which sell them. The metal stripes on this model were a bit too long, so I shortened them first. Be careful to not rip them off the battery. After putting the battery in, I bend the stripes on the pad and finally soldered them them on the pad.
7. Check your soldering for any solder on the wrong place. Assemble the shaver again. Done!