The Doc has been testing cheap battery monitors (full review now posted here). They attach to a 12-volt car battery and connect, via Bluetooth, to your phone or tablet. The better units store data and update your app when connected, so you have data over the whole day, not just when you are connected.

Commercial versions exist for 4X4s, caravans and RVs and can cost thousands of dollars, even cheaper units can cost several hundred dollars. They are made by REDARC, BMPRO, Enerdrive, Victron, etc.

For $33 you can get good functionality from the BLE Battery Monitor and your phone or tablet. OK the expensive systems are better quality and can offer more features, but if all you need is voltage monitoring (up to 30 days), graphing, warning alerts, battery charge testing and cracking tests the BLE Battery Monitor does the job for a fraction of the cost.

The Doc tested the BLE Battery Monitor, the CTEK Battery Sense and the Kickass Battery Guard. The BLE Battery Monitor won hands down, it was the cheapest, had the most features, was reliable and the app had less bugs than the other two. The Doc bought the BLE Battery Monitor on eBay here.