Android: If you use a Bluetooth headset with your phone, you'll want to know when its battery is about to die. BatON can monitor your accessories and give you a notification when you need to charge them.
BatON monitors Bluetooth handsets, as well as some other devices that support GATT profiles. So even if you don't use a headset, you can still try out BatON to see if it can keep track of your devices.
Once you connect to your accessories, you can see all their battery levels at a glance. You can configure the app to show you a notification with the battery level, and remove the notification when your device disconnects.