Hi MCristina,
Thanks for your question. To extend the battery life of your Magic Trackpad, simply turn off the device when not in use.
Apple Wireless Keyboard, Mouse, and Trackpad: How to install batteries
Battery Types
Your Apple wireless device came with AA batteries installed. You can replace them with alkaline, lithium, or rechargeable AA batteries.
Warning: Don’t mix battery types (for example, alkaline and lithium batteries), and don’t mix old batteries with new batteries. Don’t open or puncture the batteries, install them backwards, or expose them to fire, high temperatures, or water. Don’t charge non-rechargeable AA batteries. Keep batteries out of the reach of children. Replace or recharge all batteries at the same time.
Important: Dispose of batteries according to your local environmental laws and guidelines.
Battery level indicator
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To check the battery level, choose Apple () > System Preferences then select the device you want to check from the View menu (mouse, keyboard, or trackpad).
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The battery level indicator is in the lower-left corner.
Apple wireless devices are designed to go to sleep to conserve battery power when you aren’t using them. If you won’t be using a wireless device for an extended period, turn it off. If you won't use the wireless device for more than three weeks, you should remove the batteries for long term storage.
Enjoy the rest of your evening.