The phone notifies you if the phone memory is getting low. To free phone memory you have the following options:
Use File Manager or the respective application in your phone to remove data or to move data to alternative memory as described in the user guide. Typically you first mark files to be removed or moved and then you select the corresponding option (Delete or Move to folder).
Use Nokia Suite to move data to a compatible computer.
To maximize free spaces to your phone memory, you can remove or move the following:
If you are deleting multiple items and any of the following notes are shown: Not enough memory to perform operation. Delete some data first or Memory low. Delete some data, try deleting items one by one (starting from the smallest item). After deleting data, restart (power off/on) your phone.
Tips: Use the memory card or mass memory instead of phone memory in the Camera or other applicable applications in case the selection is available (e.g. via Options > Settings > Memory in use).
Many features in your Nokia Lumia phone increase the demand on battery power and reduce the battery life time. For example if the signal strength of the mobile network varies much in your area, your phone must scan for the available network repeatedly. This increases the demand on battery power.
You can set your phone to automatically save power when the battery charge level is low. To check the battery status, and switch battery saver mode on, go to Settings > battery saver. Windows Phone 8: To choose when you want battery saver mode to be activated, switch Battery saver toggle to On and tap advanced.
Note: When Battery Saver is on, your phone will no longer: automatically receive email and calendar updates, update some Live Tiles and allow apps to run in the background.
If you are listening to music or otherwise using your phone, but do not want to make or receive calls, switch airplane (flight) mode on under the Settings menu.
To save battery power, note the following:
Use network connections selectively
Anything else I need to know?
The instructions to save power on your Lumia phone presented on this video (English) from Nokia Care.
If the battery drains faster even there is no active feature or applications, try to switch off the power and then switch back on. The battery may also be worn out and needs to be replacing with a new one.
SMS message may get converted to MMS message in the following situations:
1) When sending a long SMS message
In some devices the message type gets converted from SMS text message to MMS multimedia message if the message length exceeds the device-specific setting which is typically 5 - 10 SMS submessages (around 760-1520 characters). If the message is longer, it is converted to MMS to avoid potential issues in handling long text messages in the recipient device. The cost of a SMS message consisting of several concatenated submessages may also be higher than the cost of a single MMS message.
The conversion from SMS text message to MMS multimedia message occurs when the automatic message type is selected in the message options. This is the default mode where the device tries to automatically select the best mode for the message.
The conversion from SMS to MMS can be avoided by selecting Options > Sending options > Message type > Text when in the Text message window writing the message. The message is then always sent as SMS text message and no conversion to MMS occurs.
The maximum number of concatenated SMS submessages can be up to 10 (around 1520 characters). In multimedia mode the message length can be longer.
2) No phone number defined in user contact information
If sending SMS to one or more recipients and one of the potential recipients has only email address and no phone number, the SMS will be converted to MMS. This is independent of the number of the potential recipients.
In order to avoid the conversion to MMS, remove from the recipient list the contacts which don't have a phone number defined.