Some chargers will not charge a battery if the battery’s voltage is less than 10.5 volts. This is one of those chargers. One way to charge such a battery is to “fool” this charger by using jumper cables (negative to negative, positive to positive) connected from a good battery to the discharged battery. When the charger sees the “good” voltage level it will then charge. I prefer my Noco Genius 10 or my Ctek Multi 7002 chargers in this situation, both of which allow bypassing the low voltage check. On the other hand this Stanley charger offers higher charge rates than either the Noco or Ctek AND has a battery boost function to start a car/truck. The Stanley can also do a quick alternator check to see if your system is charging the battery.