Bluetooth CTEK Battery Sense Monitor.
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Did you know that in most states (definitely in NSW) it is a legal requirement for the driver to be able to monitor the condition of the break-a-way battery on the boat or caravan trailer from their drivers seat. Simply connect the two terminals and small sending unit to your battery and download the free app to any smartphone and you are good to go. No only does it monitor the condition of the trailers battery but can notify you if the Battery's sate of charge falls to a critical level.