If the connection between the battery and the charger is not normal, in addition to the line or plug problem, it does not rule out that the battery is welded off somewhere inside the battery, you can use a multimeter to directly measure the output of the battery to see if there is voltage, if there is no voltage, it is this problem, if simple disassembly can be obviously seen, You can find a large power Luo iron welding back, if it is more troublesome, you can find repair battery to deal with.
If the polarity of the lead-acid battery is reversed when charging, it will form the phenomenon of excessive discharge of the battery, which will cause the serious consequences of the plate lead powder to fall off, and even the battery is disconnected, directly leading to the battery scrap. If there is no electricity when charging, and the reverse charging time is short, you can charge with a small current of the charger for 24 hours, and then charge with normal current.