What kind of battery is required for a Polar CS300 wrist computer?

Transmitter Battery
The estimated average battery life span of the transmitter is 2500 hours of use. If you suspect that the transmitter battery
has run out, contact your authorized Polar Service Center for a replacement transmitter. Polar recycles used transmitters;;;

this is f11 it may help you
http://timlaqua.com/2008/09/how-to-replace-the-battery-in-a-polar-f11-heart-rate-monitor-hrm/

A new battery fixed my issues on my polar 625x

check the date on the batteries you are buying.....they may be very old

Since your Cadence is working then there is a problem with the actual speed/ distance sensor. First make sure that in your settings you have speed turned on. If so then...
1) Your battery may be dead = Replace CS sensor
2) The distance of the sensor and magnet is not set correctly = make sure magnet on spokes and sensor are within 1-2mm of each other.

Polar can fix the strap, you need to send into an authorized service center. They replace the entire strap unit. If you send into Polar to get the strap fixed, then you can send in the sensor unit as well for diagnosis if you want. But if #2 doesn't fix, then it's #1 and they will just tell you to buy a new one anyway.

I don't know this watch in particular, but I know there are two main types of bands. Coded and non-coded. The S120 uses non-coded. That means you can use possibly any band on the market. Bands I know are compatible: Sigma and Ciclosport. The Polar bands are sealed, so you can use them while swimming. That's not true of the other two I mentioned - but the upside is you can replace the battery yourself once it wears down. You can probably get a replacement band from Polar directly, or buy a cheap knockoff elsewhere.

Follow this link:

http://www.polar.fi/en/support/product_support?product=125

This is original Polar support site.Here you will find all the info available.

Rate me, please.

Make sure you don't have the battery in upside down. The large outside case of a button cell is the +positive side.

Have you thought of taking it into a watch specialist.

I would advise you not to accept adresses on this site for your watch to get repaired .

Does it run on a lithium battery ?

try a new battery