How to tell if a 12v battery is charged

Written by g.k. bayne
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Only 12-volt, lead acid, batteries can be recharged by an electrical battery charging device. There are two basic physical types of the lead acid battery, an SLA (sealed lead acid), and an open top maintainable battery. The SLA is just as the name applies; it allows no access to check the level of battery fluid inside each individual battery cell. The open top or maintainable battery allows the user to check and add fluid to the battery. Both of these types of batteries can be easily tested for a full charge.

Skill level:
Moderate

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Things you need

  • 12-volt car headlamp
  • Voltmeter

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Instructions

  1. 1

    Attach the 12-volt car headlamp to the positive(+) and negative(-) terminals of the battery. Keep the lamp connected for approximately three minutes. This will remove any surface charge to the battery if it was freshly charged. A surface charge occurs through the charging process and can give a false result to the following test.

  2. 2

    Insert the red lead for the voltmeter into the connector identified as "Volts." Place the black lead for the meter into the connector marked as "com." Turn the switch on the meter to the position of "DC Volts."

  3. 3

    Touch the red lead to the positive terminal on the battery. Connect the black lead to the negative terminal.

  4. 4

    Read the value in DC volts on the face of the meter.

  5. 5

    Observe the following values to assess the condition of the battery through the meter reading. A voltage value of 12.7 volts and above indicates a 100 per cent charged battery. The voltage reading of 12.4 volts will indicate that the battery is only 75 per cent charged. A meter reading of 12.2 volts or 12.0 volts will indicate a charge rate of 50 per cent and 25 per cent, respectively. Any readings below 11.9 volts will indicate a fully discharged battery, and damage may have occurred to the storage device. Voltage readings that are 10.5 volts and below show that the battery is severely damaged and needs to be replaced.

Tips and warnings

  • Follow the battery manufacturer's specifications and maintenance details whenever servicing any lead acid battery. Special precautions may apply for some types of batteries.
  • Always charge any lead acid battery in a well ventilated area. The charging process emits a highly flammable gas that can be ignited by any open flames.

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