DO NOT use a resistor for coin battery solutions. Sorry for yelling, but that could cause the battery to EXPLODE.
What you need is a diode instead, for performance reasons a Germanium one. It'll allow the battery to power the clock while disallowing the mainboard circuit to charge it. Coin batteries are not usually rechargeable and might (will) explode when charged.
The diode needs to be oriented correctly as well. Point the stripe away from (+) or towards (-).
Don't want to spoil any business here, but you can get the required parts in any electronics shop for <2 €.
