I bought the Polar T31 as a replacement for one whose battery was depleted. While the product works well I downgrade it because of the manufacturer's designing it so that depleted batteries cannot simply be replaced. The company does make other models (more expensive, naturally) whose batteries can be changed but it is very "un-green" to make any products that need to be thrown away simply because a battery cannot be replaced. As a protest, I will replace the Polar receiver with another company's product when my current Polar transmitter's battery is depleted -- unless Polar changes its policy with regard to non-replaceable batteries.
I should add that the battery to the receiver of the model that comes with the T31 can be changed but the manufacturer recommends that the product be taken to an authorized repairer to effect the change. I managed to do it myself but it wasn't easy and there were no instructions in the product manual. Again, this is very bad policy and quite anti-consumer. Again, more expensive models are describe as having batteries that can be replaced by the owner. This is bad. I expect better of a Scandinavian manufacturer.