DIY Motorola GP3688 Lithium Battery Charger Mod

If you're interested in DIY charging boards for Motorola GP3688/88S/328/338 lithium batteries, you might want to check out this article.

Both the Motorola GP88s and GP3688 can be modified to use lithium batteries. The original battery charger was designed for nickel-metal hydride (NiMH) batteries, so the charger needs to be modified to charge lithium batteries.

The original charger, which is controlled by a microcontroller, has several indicator lights during the charging process. It is commonly referred to by HAMs as an "intelligent charger," and many HAMs believe that this charger can handle both NiMH and lithium batteries. However, this charger can only charge NiMH batteries for the following reasons:

1. The original batteries were all NiMH.

2. The charger output voltage is 10V. Therefore, to properly charge a lithium battery, the circuit needs to be completely modified.

The original charging board uses a microcontroller + 494 scheme, and the charging current is relatively low. Modifying this circuit would take more time and effort than designing a new PCBA. If you're interested, let's get started on designing it together. The charging chip used is a 2A lithium battery-specific chip, and you can set the current as needed.

The design diagram is shown below.

If temperature protection is not required, disconnect RTS and short RTS1.C.

Once the design is complete, you can start producing the PCB.

Since the 3688 contact points are in the middle, you can design two versions of the board. Of course, if you remove the temperature detection, the two boards can be compatible.

Here's a comparison of the modified board with the original one:

After soldering:

After assembling and testing, it looks great.

When installed and tested, with no battery, the green and red lights flash.

While charging, the red light is on.

When fully charged, the green light is on.

With these two bases modified, you're done!