We've never had to replace one Clive, and Coleman don't list them as a readily available part so I can't give you a price sorry. Lithium Ion behaves differently than the old SLA (Sealed Lead Acid) batteries in that they don't lose performance in storage, and they have a discharge/recharge cycle life 1000's times more than SLA batteries. It's more likely you'd be due for a new lantern before the battery dies, I'm sure there would be spare parts available in the unlikely event of failure though, and we would help you out with this.

Hi Gene, the Coleman Retro you have uses sealed lead acid batteries which need to be maintained and charged regularly in order to prevent leakage (which in turn can cause damage). The Coleman Lithium Ion LED lantern would be a far better upgrade as it uses rechargeable lithium batteries which should eliminate that issue. In my personal opinion, I would recommend upgrading your lantern to the Coleman Lithium.