I use a Coospo HRM which is about £30 from Amazon.
I bought it because I had misplaced my good quality one, but knew it was somewhere in the house and just wanted something cheap until I could be bothered to do the search through exercise kit bags (a combination of both a bit of laziness, plus a HUGE pile of sports equipment that was stacked into crates during a redecoration).
I found the old unit. I’m still using the Coospo, it’s been great and perfectly reliable.
As others have said, though, I’ve found it needs to be removed from the chest strap when not in use or it will pick up “phantom” connections and discharge far faster.
I use it pretty much daily and the strap/connections are fine two years on despite being connected and disconnected each time.
Sounds like a defective unit. The traditional electrical types have been around for a very long time and are generally very reliable. As I mentioned, poor contact with the skin happens in some conditions, and since they have a battery door, it is possible moisture can enter and cause problems when the battery is changed (~once per year for me) or if the seal is damaged or defective. If moisture has gotten in, leaving it open in a warm place will dry it out and usually gets it working again. In some cases, the device needs to be reset, which is done by leaving the battery out for a time, or putting the coin cell in upside down, which shorts the battery connections together. In the 12 years I’ve been using my Garmin strap, this has happened twice.
Optical detection is way more complicated and requires way more battery to power the LEDs, so have battery life measured in hours rather than years and use rechargeable batteries. A CR2032 coin cell that can last over a year in a chest strap wouldn’t have the oomph to run an optical sensor at all.