£18 - free postage to the UK

Battery Monitor BM2 Main Features:

Receive notification of battery conditions
Check starting system and charging system;
Alert mobile phone if some data abnormal;
Review voltage history in graph mode;
Short-circuit and reverse connection protection
Data stored in device for up to 35days if out of sync
Low power consumption, average current: 1 mA;
Free app for iOS & Android users;
Compatible with all 12-volt vehicle batteries;
Voltage: 9-16V