The HM-6 uses an ER 18505 3.6V battery which is a A-size lithium battery.
You may Google "ER 18505" and find several manufacturers that produce this type of battery. However, be sure that they include the cable and female terminal connection.
Replacing the Battery
- Peel back the corners of the label on the HM-6's electronic module. This will reveal four screws. The picture below shows one corner peeled back.
- Loosen the four screws with a few turns carefully with a Phillips screwdriver without stripping the screws until the module cover is loose (press down firmly on the screwdriver while turning to avoid screw head stripping). It is not necessary to loosen and remove the screws completely... a few turns alone will do.
- Carefully open the top cover.
- Carefully detach the existing battery by wiggling the terminal connector loose from the printed circuit board.
- Attach a new battery to the printed circuit board. Make sure you attach the connector with the correct polarity.
- Close the cover and tighten the screws.