Modern chargers have protective circuits that are provided to ensure the safety of operators and equipment. Most safety standards require that proper DC connections to the battery are made first, then the AC connection. So, why is this important? For two important reasons :
1- They minimize the possibility of a large spark occurring if the charger is powered when the battery connection is made.
2- If battery and charger connections are reversed in polarity, the charger will not turn on. This protects the battery from being destroyed by reversing a cell or two.
Both protective features work by sensing the polarity of the battery. But, what if the battery is so drained that no voltage can be measured? Very simply put, the charger will not start.
Now what? The battery is dead, a charger is on hand and AC is available. The easiest way to get the charger started is to parallel the dead battery with another battery with the same voltage range. For example, a dead 12-volt battery should be connected in parallel with a charged 12-volt battery. After the charger's DC connections are made and AC is connected, the charger will start charging. Of course, it actually senses the second battery and starts charging. After a few minutes, carefully disconnect the second battery and the charger will continue to operate.
No matter the condition of the battery to be charged, always make the DC connections, first, then the AC connection. After the battery is charged, disconnect AC first then DC connections.
So how long does it take to fully charge a totally “dead” lead-acid battery? Sometimes it can take weeks. Yes, weeks.
After two weeks of charging, disconnect the battery charger AC and measure the battery after a few minutes. If the battery shows some small voltage, immediately reconnect the AC and continue charging for two more weeks. Continue charging and testing until the battery shows full voltage after AC is disconnected and at least an hour has elapsed.
A lead-acid battery's useful life is generally considered to be over when it cannot support the DC load for half the time it did when it was new and fully charged.
For charging flooded lead-acid batteries in vehicles, follow standard safe connection and disconnection procedures. Be aware that batteries in vehicles that have been recently depleted by cranking the engine have volumes of explosive gasses surrounding the battery. Never make the second DC connection to a battery terminal. Always bring the spark hazard away from the battery by making connection to the vehicle frame. Then connect the charger to an AC outlet.
After charging is complete, disconnect AC first, and then frame connection, then battery connection.
Some small battery chargers with plug in connections do not sense if a battery is connected. These can be first connected to the battery, usually a sealed lead acid or lithium battery and then to AC. There is no need to be concerned about explosive battery gas.
As with all lead acid batteries, total battery draining is to be avoided because the battery life is drastically reduced. Battery “health” is ensured by frequent charging to full level.