Can you use a charger with 2000 mAh on a battery 1200 mAh What effect would it have?

Yes, you can use a charger with a capacity of 2000 mAh on a 1200 mAh battery, as the charger’s capacity indicates how much current it can provide, not the amount it will force into the battery. The battery will draw only the amount of current it needs, so it will charge safely. However, ensure that the voltage is compatible; using a charger with a higher voltage than the battery's rating can damage it. Always check manufacturer specifications to avoid potential issues.