So the real question here is: how can a battery reverse polarity after it has been installed? That same previously discharged battery would then be vulnerable to reverse charging, either by connecting the battery charger backwards, or by a charging system that reversed polarity (very rare, but still possible).

In order to gain a negative charge, it would then necessitate being hooked up backwards, and charged that way. Remember, a completely discharged battery is nothing more than an empty vessel. After that happened it would appear to be a dead battery. The only way for this to happen would be to completely discharge the battery, either by leaving the key on, or by an unnoticed dead short that completely dissipated the charge over a few days. This is also rare, as it requires a sequence of errors to be present after the installation of the battery. The second possibility is reversing polarity after the activation process. This situation is possible, but not very likely. Notice there are a lot of ‘ands’ in the above scenario. If that was to happen, and a charger was hooked on backwards, or if it was installed in a kick start vehicle and hooked up backwards, then you can end up with a battery that has been charged, but backwards. You would not necessarily notice a spark because the battery gains voltage as the battery is being filled, and if it is being charged while you are filling it, the short would not be a strong enough discharge to create a spark. If you have a wet cell battery are filling it for the first time, and are using an old style battery charger, non smart charger, and short the terminals while you are filling it, yes it is possible to hook up the charger backward and reverse charge it.

Reversing the polarity on a battery can happen only a couple of ways.

Reverse polarity