Las "data batteries" son pilas o baterías que tienen un indicador de la duración/autonomía restante (de ahí que sean "data" batteries):

With the BNV-707 Data Battery the remaining battery and recording time can be easily viewed, by simply touching one button, even with the power turned off. ...
http://www.adorama.com/JVBNVF707.html

JVC BN-VF714U 3 Hour Recording Data Battery for GR Models
Technical Details
Replacement battery for select JVC GR camcorder models
1400mAh rechargeable lithium-ion cell
Battery is able to display its current charge condition at the touch of a button--even when the camcorder is off
Shows 5 levels of remaining battery and record time on camcorder LCD monitor
http://www.amazon.com/JVC-BN-VF714U-Recording-Battery-Models...

You can find these models on Ebay and Amazon for about £30 but they are not DATA batteries. The Non-data batteries literally work around the batteries secret coding by designing a connection to the Camera’s DC Power input port by a cable attachment to the battery. This seems to work quite well but you have to live with the limitation that you can not get remaining battery operating time information on the camera. Also you will need to buy a separate charger for this type of battery as the Camera charger will not charge the Non-data battery.
http://reviews.ebay.co.uk/Guide-to-JVC-BN-VF823-BN-VF815-BN-...

My 2 cents...