When/How should I charge my iPad 2?
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"Use Your iPad Regularly
For proper reporting of the battery’s state of charge, be sure to go through at least one charge cycle per month (charging the battery to 100% and then completely running it down)."
Quoted from apple's site.
I personally recommend not leaving it charged over night, and at least once a month do a full recharge cycle to keep the battery life health. A full recharge cycle at 100% until it completely until it hits 0% (or in other worlds until it is in complete need for charge). This will help keep your iPad's battery healthy and/or good state.
To keep recycle charges low try to only charge your iPad only when really needed; maybe around 0% to 10% of battery life versus charging it at when battery life is 30% or above (from time-to-time it does not hurt to keep these tips on mind).
P.S: Use a trusted brand of charge or Apple's very own charger adapter to charge your iPad. I had a problem in the past with third party chargers, but just a tip to keep in mind,
— Hope this has helped you, and have a wonderful day.