Charging 6 volt battery with a 12 volt charger and small lightbulb

[John Sanford]

Connect a 12 volt taillight, turn signal or any small 12 volt lightbulb to your 6 volt battery. Now connect a 12 volt charger to the 6 volt battery. Voltage at battery will be 6.7 to 7.02 volts. Unscrew the three caps on the 6 volt battery and observe the bubbles of hydrogen gas leaving the electrolyte from each cell. The cell on the + side will stop bubbling first as the 6 volt battery reaches full charge, then the middle cell will stop bubbling, then the cell on the -- side will just barely be bubbling when fully charged. The small light creates a drain and the charger continues to charge that cell.
A 12 volt battery connected to a 12 volt charger will cut out at 14.6 volts. Then the 12 volt battery will settle to 12.7 volts after 30 minutes.
A 6 volt battery connected to a 12 volt charger with the small 12volt light WILL NOT cut out because you want a fully charged 6 volt battery to settle to 6.4 volts.
You must observe and manually disconnect the charger.
Do not connect the small lightbulb in series because the 12volt charger will sense too much resistance and then indicate a fault after two minutes.

 

[Alec_t]

Welcome to EP!
Whether or not it is SAFE to do that depends on the characteristics of the charger and the battery. An old-fashioned heavy-duty charger with nothing more than a bit of resistance wire as a crude current limiter could well drive excessive current through the battery and cause a fire, perhaps leading to an explosion if there were a hydrogen build-up!

 

[ChosunOne]

Your charging arrangement isn't charging your 12V and 6V batteries up to anywhere near their full rated capacity. That might be because of the charger itself, or the charging method; or it might be because the batteries are old or hard-used and just won't hold a full charge any more.

Whatever voltage the batteries settle down to after 30 min, they will probably settle to a lower voltage after ~5 days (of no electrical activity). That voltage will indicate roughly the percentage of charge, compared to the rated Ah capacity of the battery.

In the case of a "12V" Lead-Acid battery, an OCV reading of 12.7 volts indicates it has a stored charge of about 7/12, or 58% of its rated Ah capacity. A battery holding 100% of its rated capacity would read 13.2 V.

With the 6V battery, an OCV reading of 6.4 volts indicates it has a stored charge of ~4/6, or 67% of its rated Ah capacity. A battery holding 100% of its rated capacity would read 6.6 V.

However, as I mentioned before, an OCV reading 30 minutes after removing batteries from the charger won't give you an accurate figure. You need to let it settle (no electrical activity) for at least 2 days. Five days gives a more accurate reading.