Like many people, I bought some NiMh (Nickel Metal Hydride) rechargeable batteries and a charger but was disappointed with the few weeks of time I would get before they needed recharging. Then I read something interesting about NiMh batteries - they have a rapid self-discharge time. No matter how little you use them, they will be discharged in 90 days.
That sent me looking through the junk drawer for our older-technology, rechargeable alkaline batteries from Pure Energy. We had moved away from them since they don't carry the amount of energy you can pack into an NiMh battery, but a wireless keyboard is perfectly suited to rechargeable alkalines - it draws little current, so a long self-discharge time is more important than stored energy density.
Anyway, the proof is in the pudding, it's been more than 6 months since I had to recharge the keyboard batteries...