Fitting Linecorders with
Rechargeable Batteries

A Telog Linecorder's data retention battery is a 9V, PP3 style battery. This battery also powers the unit during data downloads when no input is applied to the unit.

Although the data retention requires an infinitesimal current, downloads can require as high as 60mA. The fact that downloads are also required just after data is collected means it is highly suited that the battery be in tip-top shape after a recording, not before. With all these facts, a rechargeable battery starts appearing to be the ideal way to go.

Both the LC-812 and LC-836i can easily be converted to a rechargeable battery system with the aid of 5 relatively inexpensive components.

The principle of operation is simple. With battery technology having drastically improved over the last few years, it is now possible to constantly "top-up" rechargeable batteries without the danger of destroying them or, worse still, creating the dreaded "memory effect". This allows us to dispense with any timing circuits that would otherwise have been required.

Powered from the unregulated power supply rail, the two transistors, 100W, and 33k, form a constant current circuit that feeds the battery with the required current, which in this case is between C/10 and C/20. The 1N4148 is a blocking diode to ensure none of the current flows back into the power supply during no-mains situations. The 100k compensates for changes in the base current via the 33k with changes in supply voltage. It is a non-critical part and may be left out of the circuit.

The circuit is simply constructed on a tiny piece of Veroboard as shown. The circuit (viewed from the component side) has been laid out in such a fashion so as to keep it as simple as possible. It is suggested that the resistors and diodes be soldered in first, followed by the two transistors. When finished, remember to scrape between the tracks of the Veroboard to ensure there are no shorts.

  • There is a break in one track, located beneath the 100W resistor.

  • The fuse can alternatively be locataed in the 'PSU' wire.

  • The lower leg of the transistor to the right is bent out so as to miss the immediate track.

  • The 100k resistor shares a hole with the transistor.

Important notice: Remove the battery before proceeding with the modification.

Fitting the circuit requires that the unit be dismantled (not a job for the feint hearted!). The primary function is a connection to the positive side on the main power supply smoothing capacitor, a 220F/160V (812 = C4, 836 = C9). This can be achieved without actually taking the unit apart completely. Be careful not to lose any of the spacers that keep the boards apart.

812: The circuit is mounted on the top board in line with the battery holder using a piece of 'double-sided tape'.

836: The circuit is mounted on top of the data processor at right angles to the battery holder using a piece of 'double-sided tape'.

If you are certain you have done the task correctly, install a charged NiCd or NiMH battery. With the battery having been removed and the memory having lost power, it will be required to re-program the unit when powering the unit up for the first time after the modification.

You should get years of unbroken service from the unit. One word of warning. Unlike normal batteries it is not advised to allow rechargeable batteries to run flat. If the Linecorder is being stored for any period of time it is suggested that the unit be powered permanently. This has two advantages. The first is the battery is kept in a fully charged state ready for action at the drop of a hat. Second, the battery is not allowed to run flat and will therefore not be damaged by being fully discharged.

Parts List

R1          100 ohm
R2          33k
R3          100k

all resistors -watt metal film 1%

Q1, Q2      BF423 PNP general purpose transistor
            high voltage (300V), hfe = 50
            E-C-B pin-out
D1          12V 300mW zener diode
D2          1N4148 or other small signal diode
            (PIV>50V, PFI>50mA)

F1          'pico' fuse 100mA
Veroboard   piece 6 holes by 5 tracks

If there are any questions please email

NOTE: This web page does not form part of any official documentation.
Any information contained herein is used at own risk.

©  M.T.P. - 21.09.01