A bit late to the discussion, but batteries bubbling and getting hot...  The first thing that jumps to mind is one or more dead (as in shorted) cell(s).  Disconnect the "front" string, let them rest for an hour or two and measure the voltage of each battery.  Report back your findings.

Hi Daniel

We see it that when current flows in any circuit that has resistance then volt drop will be experienced over that resistor. (That is the reason why you can use a shunt resistor to determine current flow (V =  I.R)) If a shunt resistor is used to determine current flow in a DC circuit you keep the resistance of the Shunt as low as possible to have very low volt drop over the resistor when current is flowing through it. Usually in milliVolts output to keep the Wattage or heating component as low as possible. The wattage is determined by the Volt drop over the Shunt multiplied by the current flowing through it (P=V x I) The Volt drop over the resistor is directly proportional to the current that is flowing through it. (A 600 Amp 50 mV shunt will give out 25mv when 300 Amps flows through it)

The same thing will happen when batteries are charged in series. There will be volt drop over each cell/Battery. Volt drop over each cell or Battery is determined by the current flowing through the string in series and the resistance of each cell. If the resistance of the batteries are different then different volt drops over each of the cells/Batteries will be experienced. This out of step condition causes some of the cells/Batteries to overcharge and some to undercharge. Differences between cell/Battery voltages indicates this condition. Obviously all cells/Batteries will not have exactly the same resistances but it should be kept as near as possible to each other. This is where the use of battery balancers come in.  Overcharging will definitely cause overheating of these cells/Batteries with the higher resistances and voltages cells/Battery as the Wattage component (P=V x I) becomes more on these cells. Unfortunately if the cells/Batteries are kept in this state for too long damage might occur.