The ETX-Series of batteries have over discharge protection and is designed to prevent a continuous active drain on the battery to the point of damaging the battery, such as leaving your key in the on position, master switch on your aircraft, or parasitic draw as examples.  If your battery is reading 0V, or near 0V, then this protection might be activated. To determine the actual voltage of the battery, remove the negative battery cable and measure the voltage at the terminals with a voltmeter.  If less than 8V, it will not accept a charge.  Once a lithium battery is approximately 8V, it can be permanently damaged and the EarthX BMS is designed to not allow  the recharging of the battery as this is no longer safe to do and can be dangerous and cause cell rupture.  Never force a charge into the battery. This is not a battery defect, nor a failed battery, but instead the protection working as it is designed. The rate of discharge depends on how many amp hours the battery has (less Ah = faster discharge) and the ambient temperature (warmer =faster discharge).  When the EarthX battery is about 95% drained, the BMS over discharge protection will disconnect the battery from the active drain so that you do not continue to discharge the battery to the point of damaging it. You know this has happened when you put a voltmeter on the battery installed in the vehicle and it reads close to 0V. We allow this much discharge of the battery to provide you the most energy possible if the drain is intentional, ie: your alternator fails in flight, and you are powering your electronics with the battery. The battery will continue to drain, at a much slower rate, with time as this is impossible to stop batteries from self-discharging.  Even “brand new” batteries left in a box will drain and if not recharged and maintained, will be permanently damaged. It is very important to recharge the battery immediately if it has been drained.  The longer the battery remains discharged will shorten the overall life span and increases the chance of permanent damage. Depending on the amp hour of the battery and the environmental temperature will dictate how much time can pass before it is permanently damaged.  Example, the ETX12A is a 4Ah battery and should be recharged within a week whereas the ETX900 is a 15.6Ah battery and should be recharged within a month of finding it in this state. Please click this link: How to Charge an EarthX Lithium Battery