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Introduction
Old, not loading or weak battery removed and replaced by new 2S-Li-Ion battery
What you need
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To access the battery, open the Fitgun from below as follows:
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Lift the black adhesive foil covering the charging plug and ON/OFF switch with tweezers and peel it off.
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Loosen the 3 Phillips screws underneath.
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Lift off base plate and charging electronics
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Disconnect the 3-pin power supply plug from the main board of the charging electronics.
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Disconnect the 2-pin battery plug of the 2S Li-ion battery from the charging electronics.
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Now it gets tricky: the 2S-Li-ion battery is embedded in the base of the Fitgun and glued in place with foam rubber. It takes a lot of skill to loosen it and remove it past the screw mounts.
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The trick with the Allen key as a stable hook worked without causing any damage, as can be seen on the right-hand and undamaged side of the old battery.
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Obtain a standard replacement 2S-Li-Ion battery, i.e. 2 round Li-Ion batteries in series, i.e. 7.4 V voltage (charge with a maximum of 2*4.2 V = 8.4 V), e.g. a model battery pack LiIon 7.4 V 1500 mAh (many suppliers), if possible with the appropriate 2-pin plug for the Fitgun charging electronics.
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These battery packs often have both a 2-pin plug for 7.4 V power supply and a 3-pin charging plug (which can control and charge both batteries wired in series separately). If necessary, the 2-pin cable for power supply can be soldered to the old 2-pin plug resulting in 7.4 V across both cells of the 2S Li-ion battery.
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Alternatively, in the photo I have insulated the individual conductors of the 2-pin, very thick cable with heat-shrink tubing and connected the outer pins (red and black) of the 3-pin cable to the charging board as the + and - ends of the individual batteries connected in series. The middle white cable is also insulated with heat-shrink tubing.
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The battery socket on the charging board is not marked with + and - poles. In the photo, I painted the - pole black on the inside. The + pole is on the outside, at the bottom edge of the charging board in the photo.
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Insert the battery into the base of the Fitgun backwards.
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If a screw mount needs to be removed: Two remaining screw mounts are still in very good condition and stable, and the third can be replaced with a piece of cork that is pressed or glued into the gap. The third screw will then only have a loose hold, but it will still have some support and a counter surface.
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Reconnect the battery cable and power supply cable to the Fitgun's charging electronics.
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Secure the charging electronics and base plate to the bottom of the Fitgun using the three screws.
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Stick the black foil back onto the base section.
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Ready to charge
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The simple charging control means that the battery is not charged and discharged very carefully and, with frequent use and deep discharge, will unfortunately lose its capacity quickly and repeatedly. This is not very sustainable. A revival with a new replacement battery means you don't have to throw everything away.
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In my case, I was able to replace a defective boost converter with a standard MT3608 IC. The pin assignment for pins 1 to 5 and the design of the voltage divider for voltage control are identical. In the MT3608, pin 6 is not connected (marked NC).
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MT3608: https://datasheet.lcsc.com/lcsc/18111515... The internal limitation of the charging current only activates at 4 A.
Follow the steps in reverse order to reassemble your device.
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