As the series-cell electrolyzer has very non-linear voltage vs current characteristic, it is important that some form of current limiting is used to prevent current runaway. While the electrolyzer is operating it will get hot and heat will decrease the required cell voltage to pass a certain number of Amps, thus increasing the electrolyzer current draw if the electrolyzer voltage remains the same. This is likely to increase the current draw very significantly, which may cause electrolyte boiling and other problems.
The best way to limit the current is to use PWM or pulsed DC and to adjust the duty cycle to maintain the average current. A fairly straight forward way is to use a Hall effect current transducer (such as LEM LTS25-NP), which outputs a voltage proportional to the current and use this as a feedback to a PWM controller chip (TL494) to adjust the PWM duty cycle. For the switch FET IRFZ44 (N-channel 17.5mO 49A, placed between electrolyzer negative and ground, switched on with positive voltage) or IRF9Z34 (P-channel 140mO 18A, placed between electrolyzer positive and battery positive, switched on with negative voltage) may be used.
For the PWM controller a ready made DC motor speed control unit will do fine. It usually has a potentiometer to adjust the duty cycle, which can be replaced by circuitry to automatically adjust the duty cycle based on measured current draw. If automatic set-and-forget operation is not required, the PWM controller may be used to control the electrolyzer current draw manually.
For most accurate current limiting the RMS current value may be calculated with for example MX536A or AD536A True RMS-to-DC converter chips.
For minimum parts count a microcontroller (e.g. Atmel AVR series) may be used with the Hall effect transducer output routed to the AD converter input and the duty cycle adjustment and RMS current calculation performed in software. The switch FET may be directly driven with the microcontroller.