Avo 8 Battery BLR121

Battery compartment.

The 15 V Avometer Model 8 Batteries can be purchased online from most reputable stores. The BLR121 is also sometimes also called B121.

There are two batteries. The commonly available 1.5 V U2 Cell, and a less commonly available 15 V BLR121 Cell. My one contains the original factory fitted Ever Ready Cell which is around 20 years old! And as a sign of respect, for the unit having given me good service over the years, I fitted it with a genuine replacement BLR121 Battery.

The 1.5 V battery is used for the ohms setting when measuring resistance. The ohms setting is generally good enough for most resistor values and this is the battery I have changed on a few occasions.

The 15 V battery is used for the Ω × 100 setting when measuring very large resistance values and often resistance of insulators.

BLR121 Battery Compartment.

The positive end of the BLR121 goes to the terminal clip with a red painted bolt.

Original BLR121 Battery.

BLR121: You should note that this cell is used for the Ω × 100 resistance measurement setting alone. Without the BLR121 battery you still have the resistance measuring capability available to you from the 1.5 V battery.

If you you can live without using ohm × 100 setting then you don't need to replace this as your meter can still measure amps, volts, and Resistance (Standard Setting) as normal without this battery. The ohm × 100 setting is used to measure  resistances up to 200 MΩ.

Rebuilding a BLR121 Avo 8 Battery

Blue battery - Made in Britain. B121

There are many crazy-fool methods of building another 15 V battery. You could connect ten 1.5 V watch batteries in series. Or perhaps use two PP3 (9 V) batteries in series with a zener diode and a resistor to drop the extra 3 V...

As a professional who works in avionics I cannot possibly recommend such methods. My advice is to use the proper replacement component. There are many online shops selling the BLR121 equivalent battery for almost nothing. At a glance I found two firms selling an equivalent battery for as little as £5, and these prices are set to drop further.

As a professional I would only use the original proper component that has been tested properly. On something as critical as a measuring instrument, you really should not be cutting corners by fitting non-standard components.

Considering that these batteries last for many years, my recommendation as an expert is to buy a proper  replacement battery, and keep your AVO 8 in its original condition.

Digital & Analog Multimeters

Author: Peter J. Vis