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The device has to be opened and partly disassembled. The battery has to be unsoldered from the logic board and can then be replaced.

  1. Braun Oral-B Sonic Complete 4729 Battery Replacement, Opening the bottom lid: step 1, image 1 of 3 Braun Oral-B Sonic Complete 4729 Battery Replacement, Opening the bottom lid: step 1, image 2 of 3 Braun Oral-B Sonic Complete 4729 Battery Replacement, Opening the bottom lid: step 1, image 3 of 3
    • Take the device and place it on top of its charger.

    • Now hold down firmly the charger and pull the top of the toothbrush backwards.

    • Just above the lid there is the induction coil, so don't pull too much, or it could get damaged!

    • When the lid at the bottom opens, stop pulling and take it off manually

  2. Braun Oral-B Sonic Complete 4729 Battery Replacement, Pull of the water protection ring: step 2, image 1 of 3 Braun Oral-B Sonic Complete 4729 Battery Replacement, Pull of the water protection ring: step 2, image 2 of 3 Braun Oral-B Sonic Complete 4729 Battery Replacement, Pull of the water protection ring: step 2, image 3 of 3
    • Take the screwdriver (or the plastic opening tool) and go under the ring at the top of the gummed area.

    • Now rotate it slightly so the ring comes loose.

    • Be careful not to damage the gum under the ring, as it assures that no water gets inside!

  3. Braun Oral-B Sonic Complete 4729 Battery Replacement, Take out the interior: step 3, image 1 of 3 Braun Oral-B Sonic Complete 4729 Battery Replacement, Take out the interior: step 3, image 2 of 3 Braun Oral-B Sonic Complete 4729 Battery Replacement, Take out the interior: step 3, image 3 of 3
    • Push the point of the device down onto the table and take out the inside.

  4. Braun Oral-B Sonic Complete 4729 Battery Replacement, Clearing the way for the battery (Part 1): step 4, image 1 of 3 Braun Oral-B Sonic Complete 4729 Battery Replacement, Clearing the way for the battery (Part 1): step 4, image 2 of 3 Braun Oral-B Sonic Complete 4729 Battery Replacement, Clearing the way for the battery (Part 1): step 4, image 3 of 3
    • First take away the metal sheet of the start/stop button.

    • Then pull the center of the black metal connection of the pressure sensor slightly upwards until the hook holding it to the sensor comes loose.

    • Now it has to be desoldered from the logic board.

  5. Braun Oral-B Sonic Complete 4729 Battery Replacement, Clearing the way for the battery (Part 2): step 5, image 1 of 3 Braun Oral-B Sonic Complete 4729 Battery Replacement, Clearing the way for the battery (Part 2): step 5, image 2 of 3 Braun Oral-B Sonic Complete 4729 Battery Replacement, Clearing the way for the battery (Part 2): step 5, image 3 of 3
    • Now you have to desolder the sensor connection and both the connections to the red led at the top.

    • The white plastic pins left and right to the led connections can easily melt!

    • Then desolder the connections to the charging coil.

    • Be careful not to touch nearby electronic components, as they could get damaged!

    • And keep the soldering time short and remove the soldering iron as soon as the solder has become liquid, so there is no overheating risk!

  6. Braun Oral-B Sonic Complete 4729 Battery Replacement, Clearing the way for the battery (Part 3): step 6, image 1 of 3 Braun Oral-B Sonic Complete 4729 Battery Replacement, Clearing the way for the battery (Part 3): step 6, image 2 of 3 Braun Oral-B Sonic Complete 4729 Battery Replacement, Clearing the way for the battery (Part 3): step 6, image 3 of 3
    • First pull off the white plastic support for the red led.

    • Then push the plastic casing of the battery together to pull of the coil.

  7. Braun Oral-B Sonic Complete 4729 Battery Replacement, Clearing the way for the battery (Part 3): step 7, image 1 of 1
    • Now the connections to the motor have to be desoldered to take the battery casing of.

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To reassemble your device, follow these instructions in reverse order.

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TheFixaholic

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A good guide but I found removing the bottom quite troublesome. Levering against the charging base felt like it would break the base. I needed to lever it in stages, wedge it and grab and pull it with mole grips. This damaged the plastic a little, but I got it off.

Cone - crwdns2853112:0crwdne2853112:0

Hmm, maybe they changed the construction. When I created the guide the bottoms (of the 2 toothbrushes I repaired) opened quite easily, but that's good to know.
Also I do not think you need to worry about breaking the base: as most chargers that you plug in the mains, it should be completely filled with a hard resin, and thus be almost indestructible.

TheFixaholic -

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