LCR-T4 Transistor Tester Lithium Battery Mod

A modification for the LCR-T4 transistor tester that leverages its power-saving circuit to make a lithium cell last years on a charge

1. TP4056 lithium charger module with protection circuit
2. A boost converter module capable of producing 7.5V - 9V output from a single lithium cell (e.g. MT3608 or TPS63070 boost converter module). Setting the output to 7.5V improves efficiency and reduces heat from the LDO regulator. According to the source code (Battery_check.c), the tester starts warning at 7.3V, so 7.5V provides a safe margin while keeping the voltage as low as practical.
3. A single-cell lithium-ion or lithium-polymer battery
4. Wire and solder

Locate the 9015 PNP transistor on the bottom of the PCB. This transistor is part of the original power control circuit. Using a sharp knife, cut the traces running from its collector to both the LDO regulator and the voltage divider. This isolates the collector so the boost converter can be inserted in between.

Note: After cutting, verify the cuts with a multimeter in continuity mode. An incomplete cut will leave the boost converter's output connected back through the trace, which can cause back-voltage on the input and potentially damage the boost converter.

Wire the boost converter module as follows:

1. VIN to the 9015 collector
2. VOUT to the LDO regulator input and the voltage divider
3. GND to the 9V input negative rail

Pre-tune the boost converter output to 9V before installation by powering it from a bench supply and adjusting the trimmer potentiometer.

Solder the lithium cell's positive and negative leads to the B+ and B− pads on the TP4056 module respectively.

Wire the TP4056 module's OUT+ and OUT− pads to the tester's original 9V input positive and negative pads, replacing the 9V battery connection.

Press the button and the tester should boot up as normal, now running on lithium power with the original auto-shutoff behavior fully preserved.

Drawback: No Lithium Battery Voltage Monitoring

The voltage display will always show the regulated boost converter output near 9V, so there is no way to monitor the actual lithium cell charge level. Connecting the voltage detection point directly to the lithium cell was attempted, but the tester has a minimum voltage threshold for startup and will refuse to boot at lithium cell voltages, making this approach unworkable. The TP4056's protection circuit handles undervoltage by cutting power at the cutoff threshold. The only indication of a depleted battery is that the tester simply won't turn on.

For a more advanced modification, replacing the LDO with a lower-noise, lower-dropout alternative such as the ME6217C50 (0.1V dropout) and reprogramming the firmware's warning and shutdown thresholds to match a lithium cell's discharge curve would eliminate the need to cut the trace to the voltage divider, since the tester's power-saving circuit would then natively handle lithium voltage levels. This would also restore meaningful battery level indication on the display.