Assuming nothing has exploded but the 12 volt battery eventually dropped in voltage to a point where the device cut off the supply you are left with a fairly flat 12 volt battery and a very flat 6 volt battery. Charging only some of the batteries will result in batteries attempting to equal out the power and charging each other.
Blog Charging With A Three Battery System
Charging 12v batteries in series. You need to overcome the total voltage of the series batteries 12 v 12 v 24 v to drive electrons. Then connect the negative end of the battery to the positive end of the next one and continue to do so for the rest of your batteries. One basic configuration for charging batteries in series is to connect the positive charger output in red to the positive end of one of the batteries. Note that a 12v battery will charge to approx. Charge more than five batteries by connecting one 12 volt battery charger across each battery in the series as if each battery were the only one being charged. Make sure that your 12v charger is rated for the type of batteries youre using like sealed vs.
After your 4 12v batteries are all topped off take them out of parallel and wire them in series. Charge all the batteries at the same time. Two 12 volt batteries wired in series produce 24 volts but the capacity remains the same as one for a 12 volt battery. 147v total and then float around 135v. For the final battery connect the negative end of the battery to the negative output in black of the charger. Charging 12 volt batteries wired in parallel requires a 12 volt battery charger.
As the batteries charge their voltages rise again and this time the smaller battery charges faster. Thats the pat answer. This means you have a fully charged 48v battery pack with each battery at the same state of charge. To charge two 12 volt batteries in series youd need a 24 volt charger. Batteries may be wired in parallel or in series. Two 12 volt batteries wired in parallel continue to produce 12 volts but the capacity doubles.