If JS is not detecting the battery it is possible the voltage is too low to detect. JS is designed to jump start 13-volts lead-acid batteries down to 0-volt.But if your battery is below 2-volts, the LED of the smart clamp 【Only for the JS-Mini & JS-4】will be blank .This is an indication that JS can not detect a battery. If you need to jump start a battery below 2-volts there is a Manual Override feature, which allows you to force "On" the jump start function.