AVO 8 Battery Replacement BLR121 / B121

The BLR121, also called B121 is a 15 V dry cell battery used in some of the AVO range of analogue multimeters. The AVO 8 uses a commonly available 1.5 V U2 dry cell, and the less commonly available 15 V BLR121. When I was replacing mine, I found a twenty-year-old Ever Ready cell in the battery compartment!

Using without the 15 V battery

The 1.5 V U2 cell is for the "ohms" settings, whilst the 15 V cell is exclusively for the "Ω ×100" setting. The meter works perfectly fine without the 15 V cell, and you can still measure lower resistance values; however, the only setting that will not work is "Ω ×100" for high resistance values. All the other ranges for measuring current and voltage work fine without the 15 V cell.

Battery Replacement

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

Rebuilding a BLR121 Battery

There are many cowboy methods of building another 15 V battery. You could connect ten 1.5 V button cells in series. Alternatively, perhaps use two 9 V (PP3) batteries in series with a Zener diode and a resistor to drop the extra three volts.

As a professional who worked in avionics, I cannot possibly recommend such methods, and my advice is to buy the proper replacement component. There are many online shops selling the equivalent BLR121 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.

An electronic measuring instrument is a very delicate and critical piece of equipment, and it is not clever to fit non-standard components. Considering that these batteries last for many years, my recommendation is to buy a proper replacement battery, and keep your AVO 8 in its original condition.

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