Hello freeEnergy4me,
How about a $25 sensor? Yes runs on a 9v battery.
Check the data sheet for the home carbon monoxide (CO) tester.
CO detectors detect most any flammable gas, sensitivity is less for H2 than CO typically within 50% so the reading is in the ballpark. Remember if H2 is below 4% (4,000ppm) not a problem, as long as the max is below 4%. Way off the scale for most CO detectors. The CO detectors can trigger an alarm, use this to turn on the fan otherwise leave the fan off. Do not waste the heat.
Mine has a PPM reading.
The reason no body can tell you how much gas is given off when charging is because the amount of gas is dependent on internal battery voltage, charging current, state of charge, temperature, how long since the last equalizing charge and more. In other words, we don't know!-)
FYI
The battery little gassing until state of charge is above 80%. Still little gassing up to 90% or so. To charge above this is an equalizing charge with much gassing. Sealed lead acid batteries prevent this by never charging above 14.2 volts or so (14v for some).
Finishing charge - the battery voltage is high, current is limited to keep the gassing from going crazy. Most of the current is going into gas production. Finishing charge is not done frequently, added to the tail end of an equalizing charge, most don't even talk about it, may even consider it part of an equalizing charge.
Very important to make sure the battery is kept watered. (Flooded cell types!-)
Have fun,
Scott.