Tag Archives: electronics

Get 16v out of a MagSafe2 Apple Charger

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If you read the specs for a MagSafe2 Apple Charger, it says output 16.6V 3.5A, which can be useful when you are looking for a 12V source power for i.e. a battery.

But, if you just cut the Magsafe2 cable and measure the voltage, you’ll read only around 0.3V.

What’s the trick?

This charger has an internal 16-bit microprocessor. You need to put a resistance between both terminals (i.e. 22K Ohm), to tell the microprocessor that there is a resistive load between terminals (same does your mac when you plug it).

When the microprocessor registers this load, after 1 second, it will deliver the 16V.

Now you can use this charger to regulate the output (maybe using an lm2596) and charge a 12V battery…

More information: http://www.righto.com/2013/06/teardown-and-exploration-of-magsafe.html

12V Battery Charging

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Measuring current to detect when the battery is fully charged:

In normal (old age) battery chargers there will be the Ammeter connected in series between the battery and the battery charger.

When the battery is almost full. The current will gradually descend near zero. Because both voltages are almost the same. Until the current flows less.

Due to the V = IR rule, if the charging voltage V increases. It will cause the charging current (I)to decrease.

In modern battery charging circuits, The battery voltage will be measured. When the voltage is indicated that the battery is fully charged. The system will automatically cut off the power more convenient.

Reference: https://www.eleccircuit.com/how-many-amps-hours-to-charging-battery-full/