Added documentation for peripheral battery monitoring

Co-authored-by: Cem Aksoylar <[email protected]>

docs/docs/config/battery.md

@@ -28,6 +28,17 @@ On macOS the BLE battery reporting packets can cause the computer to wakeup from
 
 :::
 
+### Peripheral battery monitoring
+
+You can [configure ZMK to allow support for peripheral battery monitoring over BLE](system.md#split-keyboards) (e.g. when having a split keyboard with two independent and wirelessly connected sides).
+If you want to report the battery levels of both sides of a split keyboard, you should have both `CONFIG_ZMK_SPLIT_BLE_CENTRAL_BATTERY_LEVEL_PROXY` and `CONFIG_ZMK_SPLIT_BLE_CENTRAL_BATTERY_LEVEL_FETCHING` set to `y`.
+
+:::note[Displaying both battery levels on your host]
+
+Host support for multiple battery levels is undefined. It appears that in most of the cases only the main battery is being reported. In order to correctly display all the battery values, you probably need a special application or script.
+
+:::
+
 ### Devicetree
 
 Applies to: [`/chosen` node](https://docs.zephyrproject.org/latest/guides/dts/intro.html#aliases-and-chosen-nodes)

docs/docs/features/battery.md

@@ -5,7 +5,7 @@ sidebar_label: Battery Level
 
 If your keyboard has a battery sensor, ZMK will report its battery level to the connected bluetooth host and show it on the keyboard's display, if it has one.
 
-For split keyboards, only the battery level of the central (usually left) side is reported over bluetooth.
+For split keyboards, only the battery level of the central (usually left) side is reported over bluetooth by default. ZMK can be [configured to report the battery levels for peripherals](../config/battery.md#peripheral-battery-monitoring), but virtually no host system will display this information without additional configuration or the use of third party utilities.
 
 :::note